Tyrosinase In Crustacean Blood

Tyrosinase is an enzyme system which is capable of oxidising the amino-acid tyrosine with the utilisation of atmospheric oxygen, with the ultimate production of a black pigment. It will also oxidise various phenols to coloured compounds, and form oxidation products with pyrogallol and dihydroxyphenylalanine, producing from the latter a melanin identical with that formed from tyrosine. The way in which the enzyme brings about these oxidations is not yet clear. A discussion of the mechanism of the oxidation can be found in Raper’s review in Physiological Reviews, 1928. It is sufficient here to summarise certain of the results of Raper and his collaborators which are pertinent to the present investigation.

When tyrosinase acts on tyrosine, 3-4 dihydroxyphenylalanine is formed, utilising oxygen from the air. This compound is readily oxidised by the enzyme to an orthoquinone, which gives rise to an indole derivative by intramolecular change. Further oxidation produces the corresponding quinone, which is a red compound, and the first visible product of the oxidation of tyrosine.

For all the above reactions the presence of the enzyme is essential. Further reactions in the formation of the melanin proceed in the absence of the enzyme, but require oxygen. The melanins derived from different sources may or may not have the same chemical constitution. Available analyses differ. The name is generally applied to any naturally occurring black or deep brown biological pigment. The series of colours produced in the course of the reaction depend on the relative proportions of red and black pigments present at any particular time.

The present investigation started with the purpose of finding a satisfactory method of preparing a blood solution which would keep without discoloration, without adding to it reagents whose later removal would present difficulties. Other problems presented themselves : granted that the system in question was a tyrosinase, what was its substrate in the blood? No blackening of the blood appears in the body of the animals, but only in clots at wounds, and when the blood is shed. What keeps the enzyme and its substrate apart in the circulating blood? Why are some preparations entirely free from this discoloration, and why does this variation in blackening occur?

Blood from Maia squinado and Cancer pagurus was used. The enzyme preparation used in the first experiments was the clear serum solution with the lipochrome and some proteins removed, as prepared for investigations on haemocyanin. Some preparations were dialysed for three days against running water, others were used without dialysis. These solutions were made by bleeding the animals at room temperature, extracting the lipochrome pigment by shaking with chloroform, and allowing the precipitate to settle. The clear blue serum is then poured off, and kept over an excess of chloroform. This treatment frequently fails to prevent subsequent blackening. Before experiments on the tyrosinase activity of the preparation were commenced, the chloroform in the solutions was removed by aeration. When a small quantity of such a solution is added to a tyrosine solution, and air supplied, a red colour develops, turning violet and finally black as the reaction proceeds. These are the characteristic colour changes produced during the action of a tyrosinase on tyrosine. The presence of this enzyme was further confirmed by the production of an orange colour with paracresol. Other oxidase systems do not produce this colour, but a milky cloudiness, nor will they oxidise tyrosine. Blood solutions which do not blacken on keeping will not give these reactions, and therefore cannot contain the complete enzyme system. But if to some of the blood solution which does not show the presence of the enzyme be added a small amount of blood from a solution which does show blackening, the blood which failed to give the tyrosinase reaction will now discolour, showing that it must contain the substrate, as the blackening is very much more intense than that which could be produced from the action of the added enzyme on the small amount of substrate necessarily added with it. Thus the variations in discoloration of various bloods are due to differences in the amount of enzyme present, the substrate being present in both cases.

At this stage arose the necessity of estimating the strength of the enzyme in the blood. Raper and Wormall (1923) used an accurate method of allowing the enzyme to act on the tyrosine, and at intervals estimating the remaining tyrosine in the solution by bromination.

Chodat (1910) had used a colorimetric method, and estimated the complex colour produced by the mixture of red and black pigments, using standards of varying quantities of Bismarck brown and corallin.

If crustacean tyrosinase preparations are allowed to act on a solution of tyrosine at pH 8-8 (or any more alkaline pH) and a constant current of air is bubbled through, no trace of red colour appears, as the red compound is oxidised as fast as it is formed. This is due partly to the fact that the enzyme is less active at this low hydrogen-ion concentration, and partly because oxidation of the red quinone to the later products of the reaction proceeds more rapidly in alkaline solutions. Under these conditions, the first visible sign of the oxidation of the tyrosine is a grey colour, which becomes more intense as the reaction proceeds, and more quinone is formed, till the solution finally becomes inky black. As the red colour requires tyrosinase for its formation, and obviously the grey colour can be formed only subsequently to the preliminary formation of the red quinone, the activity of the enzyme becomes the limiting factor in the reaction, and the estimation of the greyness of the solution gives a quantitative measure of the amount of tyrosine converted. There are objections to this method: the amount of blackness may change with aggregation of molecules in the solution, and this would not necessarily be influenced by the activity of the enzyme. A measure of the oxygen uptake would give a more accurate idea of the degree of oxidation of the tyrosine, but this would include the autoxidation of the quinone as well as the preceding oxidation due to the activity of the.enzyme.

The concentration of tyrosinase varies with every preparation. If 0·5 to 1·0 c.c. of fluid obtained from grinding potatoes is used (the amount of blood preparation used as enzyme) the activity of this preparation at pH 8·8 will not be depressed sufficiently to ensure the instantaneous oxidation of the red compound, which will be formed faster than it can be oxidised under the conditions in my experiments. With less concentrated tyrosinase preparations the conditions will be fulfilled at greater hydrogen-ion concentrations. The value of pH 8·8 was chosen to cover the most active blood preparations, and all reacting mixtures used in the following experiments were strongly buffered to this pH.

A set of standards made from the black solution produced by the action on tyrosine of the enzyme was found to be impracticable, as the melanin tends to precipitate in the course of a few days. Accordingly a set of standards was made from a suitable dilution of india ink. Test tubes of equal bore were selected for the colorimeter. The colorimeter tubes were sealed to prevent evaporation, and the same colorimeter used throughout all the experiments.

The tyrosine-tyrosinase reactions were carried out in similar tubes. Air was supplied through a system of rubber tubing, glass tipped, leading from the compressed air main. The air was passed through a soda lime tube and washed in water. The rate of bubbling through the reacting mixture was regulated by screw clips. Frothing was prevented by a thick layer of medicinal paraffin. Bubbling of the paraffin was checked by a drop or two of capryl alcohol, which is prevented from precipitating the proteins in the blood by the separating paraffin layer.

The importance of carefully controlled pH in comparative experiments with enzymes cannot be overestimated. Except in experiments on the limiting and optimal pH of the enzyme action, every reacting mixture has been buffered with half saturated carbonate-bicarbonate buffer to pH 8·8, i c.c. of buffer to 5 to 7 c.c. of solution, according to the experiment. At this hydrogen-ion concentration there is no appreciable change in pH during an experiment due to blowing off the CO_a from the buffer.

Tyrosinase is a catalyst that disappears during the course of the reaction. This is easily demonstrated : if to a tyrosinase-tyrosine mixture which shows no further blackening more blood is added, the reaction will proceed again, showing that the cessation of oxidation was due to failure of the enzyme. The amount of tyrosine which can be oxidised by any blood sample will therefore vary greatly with the varying enzyme content in different animals. In all experiments the amount of tyrosine present was in excess of that which could be oxidised by the tyrosinase, so that in every case the enzyme was the limiting factor in the reaction. It should be pointed out that when, in the curves which follow, 100 per cent, oxidation is shown, this does not mean that all the tyrosine present is oxidised, but that the amount oxidised gives a colour corresponding to that point on the arbitrary scale of the colorimeter. All the values are purely relative.

The limits of pH at which the crustacean tyrosinase is active are not significantly different from those found by Raper for potato tyrosinase and the enzyme from the mealworm, or those found by Venn (1920) for a bacterial enzyme. These Raper, from a study of the products of their oxidations when acting on tyrosine, considers identical systems, and the findings of this investigation suggest the identity of the Crustacean tyrosinase with the same group.

To determine the optimum pH for the action of the enzyme, the following series was set up : 4 c.c. of a 0·05 per cent, solution of tyrosine, 1 c.c. of Maia blood with the lipochrome removed by chloroform, and either HC1 or NaOH to the desired pH. The buffering power of such tyrosine-blood mixtures is sufficient to keep the pH constant during an experiment. The volumes in the various tubes were equalised by distilled water. The pH values, determined by the Clark and Lubs series of indicators, were as follows: pH 4·4, 4·6, 5·5, 6·9, 7·6, 8·0, 8·4, 9·0, 9·6, 10·0, 10·6. As it was not intended to determine the optimum within narrower limits than the intervals between the above hydrogen-ion concentrations, no correction is made for possible protein or salt errors. The solution at 5·5 showed slight precipitation, and those at 4·6 and 4·4 heavier precipitates, as these approached the isoelectric point of the blood proteins. Air was blown through as described, and the experiment carried out at room temperature (17·0° C.). The series showed the characteristic colour range of the tyrosinase reaction. At the end of an hour the pH 8·0 tube was a rose colour, followed in the next hour by the 7·6 tube, while in the same time the 8·4 tube became violet coloured. At the end of four hours the oxidation was proceeding in all those between 5·5 and 10·0, rose at the acid end of the range and grey at the alkaline, where more rapid oxidation of the red quinone to melanin takes place. The optimum pH is 8·0 ; thus the enzyme is most active at a hydrogen-ion concentration near to that of the blood, which is 7·8. It is inhibited below 5·5 and above10·0. Although the experiment was continued for two days, no reaction occurred in the solutions at 4·4, 4·6, or10·6.

An experiment using Cancer pagurus instead of Maia squinado as the source of the enzyme gave similar results.

The crustacean enzyme, like the tyrosinases from other sources, is thermolabile. Strong preparations will show activity for two hours at 48°C. At 52° the blood proteins in the reacting mixture coagulate in the first five minutes of exposure to this temperature, but if this coagulated mixture is removed from the thermostat and aeration continued at room temperature, it will slowly darken, showing that this exposure to the higher temperature has not entirely destroyed the enzyme. If the same procedure is carried out at 6o°, no activity of the enzyme can be demonstrated.

Each series of experiments was carried out simultaneously, with the same sample of blood as enzyme. The reacting mixture was 4 c.c. of 0·05 per cent, tyrosine in distilled water, buffered with 1 c.c. buffer solution, and with 2 c.c. of lipochrome free blood as enzyme. Air was supplied as described, and the tubes immersed in thermostats at the required temperatures.

The curves illustrate an important point in the interpretation of the “temperature optimum?’ This is commonly understood to be the temperature at which the increase in velocity of the reaction due to heat more than compensates the decrease in velocity due to the destruction of the enzyme. Typical of the statements in this connection appearing in many textbooks is the following from Waksman and Davison’s book on enzymes (1926): “The velocity of enzyme reactions is accelerated as the temperature is increased until a certain optimum is reached. On further increasing the temperature the reaction velocity begins to diminish until it ceases completely.” These authors do point out that this optimum is influenced by the concentration of the enzyme and the nature and concentration of the substrate, and so cannot be considered a constant for any particular enzyme, but merely for any given set of conditions. But they ignore the time factor. Instances of this could be multiplied from the literature; a common procedure is to allow the enzyme to act at a series of temperatures for some arbitrary time, plot the temperature against the amount of substrate transformed, and consider the highest point of the curve the optimum temperature. What such an “optimum temperature” really means is the temperature producing the greatest resultant velocity for a particular enzyme preparation acting on a particular substrate under certain given conditions for a given time.

Blackman, as long ago as 1905, emphasised the importance of this time factor. Working on the carbon assimilation of green leaves, he found that at high temperatures the rate of decrease in the velocity of the reaction was so great that it was practically impossible to measure the original rate, as the reaction had to proceed for a certain time before sufficient products could be obtained for quantitative estimation. By calculation and experiment he came to the conclusion that rip to a certain hypothetical “extinction temperature,” where destruction of the enzyme could be considered as instantaneous, the initial velocity of the reaction increases the higher the temperature. Bayliss (1925), who quotes Blackman’s results, defines the “so-called optimum temperature” as “merely an expression of the fact that at a certain temperature the increased velocity due to this raised temperature is more than sufficient, for a time only, to counteract the rapid destruction of the enzyme,” and rightly concludes that it is negligible, practically and theoretically.

The phenomenon is clearly illustrated below, in the action of tyrosinase on tyrosine. At the end of two hours, the apparent optimum is 44°C., at the end of four, 33°, at the end of ten, 26°, and so on (Fig. 1). This same experiment is plotted in Fig. 2 in the usual way for the demonstration of “temperature optima.” The shift in this optimum is obvious.

Though it is true that in all enzyme systems the initial velocity of the reaction is greater the nearer it approaches the temperature of instantaneous extinction, the amount of shift of the optimum will vary greatly. In an enzyme system where destruction of the enzyme is not significantly affected by a rise in temperature till a certain value is reached, and then is rapidly and increasingly destroyed by further rises in temperature, becoming totally destroyed within the limits of a few degrees, the temperature optimum can shift only within the limiting temperatures of that restricted range where destruction of the enzyme is going on. In tyrosinase, and other enzyme systems where destruction starts in at a relatively low temperature and increases steadily and slowly over a wide range of temperature, the effect is much greater, and the time factor must be taken into account in any discussion of the effect of temperature on the action of these enzymes.

It has been emphasised that the “optimum temperature” depends on the concentration of the enzyme and the conditions of the substrate, and is therefore variable. But this further variation, which may be very great, that the optimum is also dependent on the time factor, is too often ignored.

The rate of destruction of tyrosinase increases with rising temperature to such an extent that no exact estimation of the temperature coefficient can legitimately be made under the conditions of my experiments.

Blood collected from Cancer or Maia during the spring discolours less than that collected in the same way in the autumn and winter. It was thought that a seasonal variation in the tyrosinase content might be demonstrable, but experiments carried on over eight months led to no result more definite than the above general observation.

The experimental animals were allowed to walk about for about 20 minutes to get rid of the sea water in the gills, and then bled through a cut in the lower joint of one of the walking legs. A few c.c. of blood were collected in a small test-tube, then haemorrhage stopped by breaking off the leg at its natural shedding point, where there is a membrane which prevents further bleeding. The blood was allowed to stand half an hour at room temperature for the clot of leucocytes to settle, and i c.c. of the clear fluid used for the enzyme. There was no chloroform extraction of the lipochrome. All the samples of blood were treated in exactly the same way, and an arbitrary standard for comparison chosen. Even strong tyrosinase bloods have ceased to show activity after four hours’ reaction with tyrosine at 31°C. The standard chosen was the amount of oxidation (measured by the standard colorimeter described) of 4 c.c. of 0·05 per cent, tyrosine, buffered with 1 c.c. buffer, by i c.c. blood, in four hours.

The enormous differences in different animals, and in the same animal at different times, are exemplified in the following table:

It is not improbable that there exist shorter cycles related to the variation in the number of the leucocytes, upon whose cytolysis, as will be shown later, the tyrosinase activity of the blood depends.

Filtration suggested itself as a possible means of removing the enzyme. If the colloidal particles were of sufficient size, filtration through filter paper might b an effective means of checking the discoloration. Ultrafiltration is too slow a process for the preparation of large quantities of blood.

The blood of a Maia m treated with chloroform as described, and the clear solution poured off. The bulky chloroform precipitate was poured on to a filter paper, and some of the fluid filtered through it, while a third portion was filtered through the paper alone. When 2 c.c. of each of these solutions—the decanted solution, the same filtered through filter paper, and another portion of the same through the chloroform precipitate on the filter paper—were tested for tyrosinase activity in the usual way, they showed striking differences (Fig. 3).

A similar experiment on a different Maia, in which treatment of some of the original decanted solution by shaking with charcoal, and then filtering off the charcoal before using the blood for the experiment was included, gave confirmatory results. Filtering the blood through the chloroform precipitate always depresses the enzyme activity, but does not always, as in the above experiments, remove it entirely. A sure method of keeping blood solutions from discoloration is discussed in the next section.

The blood circulating in the animal does not discolour, therefore both the substrate and the enzyme cannot be free in the blood, unless one postulates the presence of some anti-enzyme, or the necessity for some activator which comes into action when the blood is shed. The main, or rather the first change in the shed blood is the bursting of the explosive corpuscles (Hardy, 1892). These explosive corpuscles are not present in the Arachnids or the Mollusca, and the fact that the haemocyanin containing blood of Limulus, the king crab, and Helix, the snail, do not give a tyrosinase reaction with tyrosine supports the thesis developed below that the enzyme is freed from these corpuscles on their bursting when exposed to air. Qualitative experiments with the blood of Homarus vulgaris, Cancer pagurus (which was used in a pH experiment and in some early temperature experiments), Carcinus maenas and Portunus púber, proved the presence of a tyrosinase in these species as well.

The effect of hindering the cytolysis of the corpuscles is to inhibit the tyrosinase activity of the blood, as is shown in the following experiment. A Maia, packed in ice for three hours to lower the temperature of the blood before it was drawn, was partly bled at o° C., and the corpuscles allowed to settle at o°. The same Maia was allowed to finish bleeding at room temperature−18·5° C. In both cases most of the serum was poured off as soon as the clot had settled. A small quantity was left on the clots for about fifteen minutes at room temperature.

A series of five tubes was set up, in all 4 c.c. of 0·05 per cent, tyrosine solution, 1 c.c. buffer, and 2 c.c. of the following portions of blood as enzyme :

In test-tube 1 : serum poured off the clot at o° ;

2 : filtrate through filter paper of the above serum ;

3 : serum left soaking with the clot from the blood at o° ; from the blood drawn at o°.

In test-tube 4 : serum poured off the clot at room temperature ;

5 : serum which had soaked with the clot formed at room temperature;

from the blood drawn at 18·5°.

The portions were allowed to come to room temperature before adding them to the rest of the reacting mixture, and the series was then aerated and the oxidation estimated at this temperature.

The results, plotted in Fig. 5, show clearly the increase in tyrosinase activity associated with cytolysis of the corpuscles. The amount of oxidation produced by the serum poured off the clot in the cold is very slight, not amounting to more than 7·5 per cent, on the colorimetric scale. The tyrosinase activity is obviously associated with the leucocytes, and the longer the serum is left with the clot, the more tyrosinase escapes into the serum. The slight oxidation produced by the serum poured off cold is due to the inclusion in it of a few leucocytes, which cytolyse during the course of the experiment carried out at room temperature. If these are immediately filtered off while the serum is still near a temperature of o°, the tyrosinase is entirely removed, and no oxidation of tyrosine results, as in curve 2, Fig. 5. Filtration after the serum has been associated with the corpuscles at room temperature is not successful, as in this case the enzyme has escaped from these on cytolysis.

The leucocytes are not discoloured in the animal, so it seemed probable that the substrate was to be looked for in the serum. Early attempts to demonstrate the presence of tyrosine in filtrates from heat coagulums of whole blood, using Millon’s of Momer’s reagents, were unsuccessful. But free tyrosine may be demonstrated quantitatively in Maia blood by the use of the Folin-Denis (1912) phenol reagent (sodium tungstate, phosphomolybdic acid and phosphoric acid). If the animal is cooled to o°, and bled at this temperature, the blood proteins precipitated rapidly with Folin and Wu’s tungstic acid reagent (1919) (sodium tungstate and sulphuric acid), there is little chance of any breakdown of the blood proteins. The filtrate tested with the phenol reagent gives a positive reaction, the colour can be estimated against a suitable tyrosine standard, and the percentage of tyrosine calculated. The average amount present in the blood is 0·004 percent. It is unlikely that other phenols are present in the blood stream to interfere with the estimation, and Folin and Denis (1912) have shown that tyrosine is the only amino-acid which will give a positive reaction with the reagent. Further, its presence in the blood would be expected as a normal product of protein digestion. Folin and Denis (1912) got positive tests for the presence of tyrosine by this method from the blood of a normal cat, but did not estimate the small quantity present. 0·004 Percent, of tyrosine is quite sufficient to account for the degree of blackening in the blood. Later experiments with the blood of animals bled at room temperature, and with small quantities of serum left soaking with the clot for a short time, did not give a higher percentage, so it is unlikely that the tyrosine found in the cold-bled blood is a breakdown product.

The tyrosine content of the blood will vary with the usual increase in aminoacids after digestion of food ; there are no available figures for the changes in aminoacid content of Crustacean blood, but this change cannot account for the large differences in enzyme activity of the various preparations, as the amount of substrate added with the blood used as enzyme is negligible in the quantity of tyrosine supplied as substrate. The quantity of enzyme present per c.c. of blood must vary. In the comparative experiments reported earlier, where the enzyme was allowed to escape from the corpuscles through cytolysis of the latter, varying quantities of enzyme must have been present in the blood used. As the enzyme is contained in the leucocytes, either the quantity of enzyme present per leucocyte must change, or the number of leucocytes, or both these vary. What the true facts are must be left for further experiments when the leucocyte count is done simultaneously with the test for tyrosinase activity. It may be mentioned here that experiments on the comparative tyrosinase activity of different bloods showed the same large variation even when the clot was allowed to cytolyse several days in the serum.which was later used for the experiment, indicating that the differences formerly obtained are real differences, and not due merely to unequal cytolysis of the samples in the short time during which the clot was allowed to settle out.

This explanation of the tyrosinase system explains why blackening of the blood occurs only at wounds or when the blood is shed, as it is only when the corpuscles burst that the tyrosinase can come into contact with its substrate, the tyrosine which is always present in the blood serum.

Warburg (1928) has shown that the reagents inhibiting oxidative processes may be divided into two classes, those that depress the oxidation when present in very dilute concentrations, and those requiring relatively larger concentrations to be effective. In the first class are those reagents which are known to have an affinity for metals, such as the cyanides, sodium pyrophosphate and carbon monoxide; in the second those substances such as the alcohols, the urethanes, various ketones, vanillin and thymol, which have no special affinity for Fe or Cu. Using charcoal as a respiratory model he was able to establish an analogy between the action of the charcoal as a catalyst and that of an enzyme. The surface of the charcoal particle may be considered a mosaic of charcoal and iron, with the latter occupying a small proportion only of the surface area. If the reacting substances be considered as acting through cutting off the charcoal (or enzyme) from its surrounding substrate through becoming themselves adsorbed on the active surface, the following striking facts present themselves: the members of each series in the second class are effective.in smaller molecular concentrations the greater their molecular weights. Thus amyl alcohol is more effective than propyl, propyl than butyl, and so on; phenyl urethane is more effective than methyl urethane, methyl than ethyl. By calculation the effective concentrations can be shown to be proportional to the area covered if the reagent be considered to be adsorbed in a mono-molecular layer on the surface of the charcoal, and by analogy the enzyme.

But the cyanides and sodium pyrophosphate are effective in concentrations too dilute to be considered as covering the whole surface of the particle, and so are to be considered as combining selectively with the metallic portion of the surface. And as these small concentrations can inhibit the oxidation, Warburg comes to the conclusion that a metal is an essential component of any respiratory or oxidation process. This conclusion is reinforced by the fact that small traces of added iron will accelerate the oxidation, and indirectly by the known inclusion of metals in the molecules of respiratory pigments. There are, however, oxidation processes brought about by enzymes which are not inhibited by KCN, such as the aldehyde oxidases (Bernheim, 1928), indicating that there may exist enzymes in which a metal is not necessary to the reactive part of the molecule.

Haldane (1927) has corroborated the inhibitory effect of CO on respiration in the wax-moth, and Keilin (1927) has shown that a polyphenolase present in yeast cells is acted on in the same way by CO and KCN as Warburg’s respiratory model. This enzyme takes part in the respiration of the cell, and is probably the same enzyme as the indophenol oxidase long recognised in the tissues of higher animals, and lately demonstrated in plants. This enzyme is extremely sensitive toKCN.

Crustacean blood will accelerate the oxidation of the “Nadi” reagent (dimethylphenylenediamine and a napthol) to a blue indophenol derivative. But the indophenol reaction of the blood is inhibited only by much higher concentrations of NaCN than those required to inhibit the tyrosinase reaction, and in fact at concentrations too high to suggest that the NaCN is combining selectively with what must be a very small portion of the molecule. In a typical experiment with blood from the same animal, the tyrosinase reaction was inhibited by M/3500 NaCN, and the indophenol reaction only at a concentration of M/70 NaCN. The latter is an extremely high concentration. Recently Szent-Györgyi (1925) has shown that ortho-quinones will autoxidise in air, and will therefore give positive tests with guiacum, guiacol, diphenylenediamine or other oxidisable substances. In the shedblood traces of ortho-quinones will be present as a result of the action of tyrosinase on tyrosine, and may account for the indophenol action of the blood, so that no importance is attached to the presence of this reaction till further experiments have been carried out.

The amount of any narcotic required to inhibit the reaction will of course be dependent on the strength of the particular enzyme preparation used in the experiment, and so will vary from blood to blood—those showing great tyrosinase activity requiring higher concentration of narcotics for any given effect than those of weaker enzyme activity.

The tyrosinase of the blood may be depressed and inhibited by narcotics of both Warburg’s classes. Among the alcohols, ethyl is effective in lower molecular concentration than methyl, producing a distinct depression of the oxidation of tyrosine in 2·7 molar concentration, while methyl alcohol is ineffective in concentrations of 3·5 molar, as are the low concentrations of higher alcohols which it is possible to get into watery solution; and as tyrosine is almost insoluble in alcohol, the effects of higher concentrations cannot be investigated, using this substrate.

With thymol, an unexpected phenomenon occurred. This reagent depresses the oxidation of the tyrosine in small concentrations, the inhibition increasing with the concentration, while in higher concentrations it increases the oxidation to an astonishing extent, affecting not only the rate of the reaction but the actual amount of tyrosine oxidised. Phenyl urethane and urethane also increase the oxidation in some concentrations while depressing it in others, but the results have not been as regular as those obtained with thymol. In a reacting mixture M/7 for thymol, not only was the inhibition due to the ethyl alcohol in which the thymol was dissolved removed, but the reaction was more powerful than in a watery solution. To eliminate the possibility that thymol was acting through causing further cytolysis of leucocytes that were free in the blood used as enzyme, the experiment was repeated on filtered blood, with the same results (Fig. 6).

Thymol and phenyl urethane are necessarily introduced into the reacting mixture in alcoholic solution. The higher concentrations produce an obvious emulsion. It is suggested that in the dilute solutions the thymol becomes adsorbed on the surface of the colloidal enzyme particles, separating them from their substrate; but when the reagent forms an emulsion, the enzyme and the substrate become adsorbed on to the surface of the thymol globules, and their contact so assisted. The velocity of the reaction is changed, as estimated colorimetrically, and so is the ultimate end-point of the oxidation, as can be seen in Fig. 6 by comparing the 24-hour values for the various curves. There is no further change in the depth ot colour of the solutions even when kept for several days. The enzyme in contact with its substrate tyrosine does not retain its activity for longer than 30 hours at room temperature, so that any means of hastening the association of the enzyme with its substrate, and thus changing the velocity of the reaction, would also alter the end-point. A word of caution is necessary in considering this theory of the increase in velocity of melanin formation ; what is measured colorimetrically is the depth of colour, and not the oxygen uptake of the system. If this should prove to be altered in a different way than the melanin formation, the possibility that the thymol was causing a change in the aggregation of the coloured molecules, so that the product of the reaction was visible more easily, would have to be considered. But the fact that the end-point of the reaction is affected, as well as the velocity, supports the idea that this is a real alteration in the oxidative process.

If such a change in the physical properties of the solution as that suggested above were in fact taking place, a change might be expected in the viscosity of the reacting mixture. This possibility was tested, with negative results recorded below. The determination of the viscosity was of course carried out, using the same sample of filtered blood as that used in the oxidation experiment with thymol, and in the same reacting mixture.

There is here no confirmation of the adsorption on thymol globules hypothesis. In view of the crudity of the enzyme system used, the tyrosinase reaction representing but one of the possible enzyme reactions of the blood, no further interpretation is possible till experiments are continued, using a more purified preparation of tyrosinase. Experiments on the effects of thymol on the oxidation of tyrosine by a crude preparation of tyrosinase from the potato showed the same phenomenon. It seems worth while to record the anomalous effect above, as if the above interpretation be the true one, it supports Warburg’s thesis that this type of reagent acts by surface adsorption rather than by chemical combination with the iron or some other group in the enzyme molecule.

Sodium fluoride in M/13 concentration depresses the oxidation of tyrosine by the enzyme, but complete inhibition is not effected by stronger concentrations.

Sodium pyrophosphate inhibits the reaction in concentrations of M/150.

The effects of NaCN and H₂S are plotted above, and require no explanation. The effectiveness of cyanide in small concentration indicates the presence of a metal as the active group in the molecule. The reversibility of these inhibitions can be demonstrated, in the case of NaCN, by acidifying the solution and blowing off the HCN formed ; and in the case of H₂S, the aeration of the reacting mixture ultimately removes the H₂S, and the reaction takes its normal course, as can be seen in the curve for M/3900 H₂S, Fig. 8. In the case of the latter reagent, the concentrations are of course the initial concentrations of NaSH in the reacting mixture, the blowing off of the reagent changing these as the experiment proceeds.

So far, no acceleration of the reaction has been observed on adding small traces of Fe, as FeCl₃, or Cu, as CuSO₄. Both these reagents inhibit the oxidation in greater concentrations, copper sulphate at a molar concentration of 1/1400 (the rate of oxidation was depressed to one-third by M/14,000: no intermediate values were used). FeCl₃ depresses the reaction velocity to one-half at M/700.

Tyrosinase is a widely distributed enzyme. First noticed in fungi, by Bourquelot and Bertrand (1896), it is now known to be a component of the enzyme systems of all plants which turn brown on injury, and is widely distributed among the Invertebrata. Roques (1903) found varying tyrosinase activity in the body fluids of Limnophilus, which rose to a maximum just before the beginning of pigmentation in the nymph. Gortner (1910), in an investigation of the tyrosinase system in the larvae of the mealworm, Tenebrio, established the oxidative nature of the enzyme, and its thermolability. The system was most active at stages of development during which darkening took place, but the presence of the enzyme could always be demonstrated by the addition of tyrosine, variations in the darkening of the animals being due to variations in the amount of an unidentified chromogen present. Krukenberg (1908) reported a similar darkening in Hydrophilus, with similar variations in the activity of the system. In view of the recent work of Schmalfuss and Müller (1927) who identified dihydroxyphenylalanine in the cockchafer, Melolontha, and considered it to be the chromogen from which the dark coloration in these animals is produced, it seems probable that both Roque’s and Gortner’s chromogens were this substance.

In the blood of Maia, the spider crab, and Cancer pagurus, the edible crab, the chromogen has been shown to be tyrosine. The chromogen which gives rise to the black pigmentation of the eyes, legs, and carapace is not yet identified, but is known to be an amino-acid (Jean Verne, 1923), and is possibly dihydroxyphenylalanine.

Among vertebrates, the presence of a true tyrosinase has not been established. But there are enzymes which share something of the nature of a tyrosinase. One of these is H. Onslow’s “tyrosinase” from the skin of rabbits, which will oxidise tyrosine in the presence of hydrogen peroxide. The other is the “dopa” oxidase of Bloch and Schaaf (1925) which acts on dihydroxyphenylalanine (“dopa”) as tyrosine does, but will not oxidise tyrosine. Neither of these is a true tyrosinase, as neither will oxidise tyrosine in the presence of atmospheric oxygen. But both produce melanins, and the similarity of their action tempts one to suggest that further investigation will show them to be components of the same system as that recognised as tyrosinase.

The blackening of crustacean blood when it is shed, or in clots at wounds, is caused by an enzyme similar to, if not identical with, the tyrosinase systems previously described in various invertebrates, fungi, and in the higher plants. The components of the system are an enzyme contained in the blood corpuscles, from which it is freed on cytolysis, and its substrate tyrosine, which is free in the blood stream. The enzyme is by definition a tyrosinase, since it will bring about the oxidation of tyrosine with the ultimate production of melanin, deriving the oxygen necessary for the reaction from the air.

The tyrosinase content of the blood is not constant, nor does it undergo a seasonal variation. The possibility of shorter cycles in tyrosinase activity has not been investigated.

The blood will accelerate the oxidation of diphenylenediamine and a napthol to the blue indophenol derivative ; but as this reaction is comparatively insensitive to NaCN, it is unlikely that it is due to an indophenol oxidase¹, and is probably caused by the autoxidation of some substance in the blood such as an orthoquinone.

The tyrosinase action is inhibited by low molecular concentrations of NaCN, indicating the presence of a metallic group as the active part of the enzyme molecule.

The activity of the enzyme can be depressed by H₂S, CuSO₄, FeCl₃, sodium fluoride, sodium pyrophosphate, and the alcohols. Of the latter, ethyl alcohol is effective in concentrations of 2·7 molar, while methyl alcohol tested on the same blood had no effect in concentrations of 3·5 molar. This is the expected result from Warburg’s hypothesis, but as tyrosine is insoluble in alcohols, and the amounts of the higher alcohols which could be introduced into the watery solution were too small to have any effect on the oxidation, a series could not be investigated.

Thymol, phenyl urethane and urethane will depress the oxidation of tyrosine by the enzyme. The anomalous effect of certain concentrations of these reagents in increasing the rate and amount of oxidation is discussed.

In conclusion, I would like to thank Dr Allen for the hospitality and facilities of the Marine Biological Station at Plymouth, and Mr C. F. A. Pantin for his interest and criticism during the investigation.