Inhibition of microsomal drug metabolism by mitochondria and cytochrome c oxidation of extramitochondrial NADPH.

Microsomal aniline p-hydroxylase and aminopyrine N-demethylase activities were inhibited by mitochondria. The magnitude of the inhibition increased in parallel with the amount of added mitochondria. The inhibition was reverted by 0.2 mM KCN. Marked inhibition of these microsomal enzyme activities was observed also in the presence of cytochrome c and low amounts of mitochondria causing negligible inhibition in themselves. The inhibition increased with the concentration of cytochrome c and it was reverted by KCN. Microsome-free mitochondria did not oxidize NADPH even in the presence of cytochrome c, although NADH oxidation has been demonstrated under these circumstances [Sottocasa et al., J. cell Biol. 32, 415, (1967)]. However, completion of the system by addition of microsomes resulted in the oxidation of NADPH, which was inhibited by KCN. These findings may indicate the cooperation of the microsomal and mitochondrial compartments in the regulation of drug metabolism.