Fig. 4. Mitochondrial oxygen uptake as functions of myoglobin or leghemoglobin concentration. Closed symbols, myoglobin; open symbols, leghemoglobin c. Different symbols represent different experiments. With the exception of the two lowest points, oxygen uptake rates are those reported spectrophotometrically by the near-steady-state rates of deoxygenation of myoglobin or leghemoglobin, as in Fig. 3. Each point is the average of measurements on seven (myoglobin) or two (leghemoglobin) preparations of isolated mitochondria. The two lowest points were determined using trace concentrations of myoglobin or leghemoglobin as a reporter of oxygen pressure. Oxygen uptakes are normalized to a plateau value determined by the polarographically determined specific activity. The ratios of the rates of oxygen uptake at the plateau of this figure to those determined polarographically were 0.91, 0.77 and 1.30 in experiments using myoglobin and 0.76, 0.96 and 0.92 in experiments using leghemoglobin.