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Nutritive metal uptake in teleost fish

Nicolas R. Bury^*, Paul A. Walker and Chris N. Glover

King's College London, School of Health and Life Sciences, Franklin-Wilkins Building, 150 Stamford Street, London, SE1 9NN, UK

View larger version (26K): [in a new window]   Fig. 1. Hypothetical representation of cellular iron uptake pathways in fish combining data from gill and intestine. See text for more details. Briefly, ferric iron (Fe3+) is reduced via an apical membrane bound ferric reductase (FR). Ferrous iron (Fe2+) enters the cell via a Fe2+/H+ symporter (DMT1). Basolateral Fe2+ export occurs via an iron regulated transporter (IREG1), also known as ferroportin. IREG1 is linked to a membrane-bound copper-containing oxidase, termed hephaestin (HP), that oxidizes Fe2+ to Fe3+. Fe3+ binds to transferrin (TF) in the blood. Li, aquatic ligand.

View larger version (20K): [in a new window]   Fig. 2. (A) Inhibition of rainbow trout gill basolateral membrane vesicle (BLMV) Ag(I) transport at 50 µmol l-1 AgNO3 by 500 µmol l-1 of non-radioactive Ag(I), Cu(II), Pb(II), Cd(II), Zn(II) and Fe(III). Values are means ± S.E.M., numbers indicate N values. Asterisks indicate significant differences from control values (Student t-test performed on arcsine transformed data, P<0.05). (B) Concentration-dependent Ag(I) transport (solid circles), and in the presence of 800 µmol l-1 Cu(II) (open circles). Values are means ± S.E.M., N=6, taken from two separate experiments. Both sets of data best-fitted a sigmoidal curve with regression equations: for the control vesicle Ag(I) transport, y=3.5±2.1/(1+e-(x-15.1±3/17.5±2.4)), r2=0.992; for Ag(I) transport in Cu(II)-treated vesicles, y=9.8±1.6/(1+e-(x-17.4±2.9/6.4±3.5)), r2=0.993. The Cu(II)-treated vesicle Ag(I) transport is significantly reduced (two-way analysis of variance, P=0.0052). The protocol for BLMV preparation and transport buffer media were taken from Bury et al. (1999).

View larger version (31K): [in a new window]   Fig. 3. Hypothetical representation of cellular copper uptake pathways in fish, combining data from gill and intestine. See text for more details. Briefly, in the gills, cupric copper is probably reduced to Cu+ and enters via either a putative epithelial sodium channel (EnaC) or copper transporter 1 (CTR1). Metallochaperones (MC) bind Cu+ and guide it to the Golgi network (GN), where it is transported into the Golgi lumen via a MNK-`like' Cu+-ATPase. Cu+ is incorporated into metal binding proteins (MBP) within the GN. GN vesicles then traffic the copper to the basolateral membrane for release via exocytosis. Other ATPases on the basolateral membrane exporting copper (i.e. Ag+/Cu+-ATPase) may also be present. In the intestine, apical entry is presumed to be passive. Intestinal export may be via a Cu/Cl- symporter, or via the MNK pathway described above. Excess copper is bound to low molecular mass proteins, such as metallothionein (MT). Li, aquatic ligand; MNK, Menkes Cu+-ATPase; CR, copper reductase.

View larger version (27K): [in a new window]   Fig. 4. Hypothetical representation of cellular zinc uptake pathways in fish combining data from gill and intestine. See text for more details. Li, aquatic ligand; ZIP1, Zrt, Irt-like protein; EcaC, putative lanthanum-sensitive epithelial calcium channel; ZTL, zinc-regulated zinc transporter; AA, amino acid; AAT, amino acid transporter; ZNT1, zinc transporter 1. Briefly, zinc enters via either a putative calcium channel, a ZIP-`like' transporter, ZTL a zinc transporter similar to zinc transporter 1 (ZnT-1), or bound to an amino acid (i.e. histidine), via an amino acid transporter. Excess cytoplasmic zinc is bound to metallothionein (MT). Basolateral transfer is via a Znt-1.