Fig. 3. Myoglobin and leghemoglobin deoxygenation as functions of time in hemoglobin-containing suspensions of mitochondria. Initially the hemoglobins are fully oxygenated. Deoxygenation is down. (A) Myoglobin, 50 µmol l^–1; mitochondria (as cytochrome aa₃) 170 nmol l^–1, 10 mm light path. (A, inset) Myoglobin 500 µmol l^–1; mitochondria (as cytochrome aa₃) 700 nmol l^–1, 2 mm light path. (B) Leghemoglobin c, 50 µmol l^–1; mitochondria (as cytochrome aa₃) 65 nmol l^–1, 10 mm light path. The broken lines are drawn at a tangent to the nearly linear portions of the curves. The arrows indicate the points at which the rates of mitochondrial oxygen uptakes are half that during the near-linear portion of the progress curves. The quasi-steady state oxygen uptakes calculated from the slopes of the broken lines are 204 and 358 mol O₂ (mol cytochrome aa₃)^–1 min^–1 for myoglobin and leghemoglobin, respectively. The oxygen pressure at half-maximal oxygen uptake is found to be the same for two proteins for which affinities differ 10-fold (see Table 2). This shows that the P_O2 for half-maximal mitochondrial oxygen uptake is not related to the oxygen affinity of the hemeprotein supplying oxygen. AU, absorbance units.