OXYGEN DEPENDENCE OF REDOX STATE OF COPPER IN CYTOCHROME-OXIDASE INVITRO

To obtain quantitative information about tissue oxygenation from near-infrared signals, the oxygen dependencies of the redox states of both heme a+a3 and copper in cytochrome oxidase of isolated mitochondria were determined at low oxygen concentrations (10(-6)-10(-9) M) using leghemoglobin as an oxygen indicator. The maximum absorbance changes caused by the aerobic-anaerobic transition measured at 830-760 nm of copper in state 3, state 4, and the uncoupled state were 10, 17, and 5% of those at 605-620 nm of heme a+a3, respectively. Thus the relative absorbance ratio of copper to heme a+a3 could be used as a sensitive indicator for the mitochondrial energy state. The oxygen concentrations required for the half-maximal reduction of heme a+a3 varied with the energy state and the respiratory rate and were 7.8 X 10(-8) and 1.6 X 10(-7) M in state 4 and state 3, respectively. In contrast, that of copper was 7.5 X 10(-8) M and was independent of both the energy state and the respiratory rate. The relationship between the percent oxidation of heme a+a3 and copper in the aerobic-anaerobic transition did not show a straight-line relationship. This was referred to as the difference in oxygen affinity between these two chromophores. The deviation from the straight line was larger in state 3 than in state 4. There was the smallest deviation in the uncoupled state. On the basis of these in vitro data, simultaneous measurement of the redox states of heme a+a3 and copper in cytochrome oxidase makes it possible to determine the metabolic state in living tissue as well as oxygen concentrations in mitochondria.