The catalysis of heme degradation by purified NADPH-cytochrome C reductase in the absence of other microsomal proteins.

The report by Schacter et al. (J Biol Chem 247: 3601, 1972) that an antibody to NADPH-cytochrome c oxidoreductase inhibited NADPH-cytochrome c reductase and heme oxygenase activities in rat and pig liver and spleen microsomes demonstrated the role of this flavoprotein in microsomal heme oxygenation. Recent studies from other laboratories (Yoshida et al., J Biochem 75, 1187: 1974 and Bissell et al., Fed Proc 33: 1246, 1974) have strongly suggested that cytochrome P-450 is not involved in heme oxygenation. The availability of a homogeneous preparation of NADPH-cytochrome c reductase prompted us to test heme oxygenase activity in a system devoid of hemoprotein contamination. NADPH-cytochrome c reductase catalyzed biliverdin formation at a rate of 8.26 +/- 0.5 SEM nmole min-1mg-1 in the absence of biliverdin reductase. The rate of bilirubin formation in the presence of biliverdin reductase was less than 10% of the rate of biliverdin formation, suggesting that mixture of biliverdin isomers may be produced. Biliverdin production was potently (70--80%) inhibited by catalase, but was unaffected by superoxide dismutase. Epinephrine also inhibited heme oxygenation, presumably by utilizing O2. required for the formation of H2O2 by the reductase. By extrapolation, the NADPH oxidase activity due to NADPH-cytochrome c reductase can account for heme degradation occurring in microsomes. However, the specificity of ring scission at the IXalpha position must be due to another microsomal protein, perhaps the heme oxygenase of Yoshida et al., and not cytochrome P-450.