Mild proteolysis of rat liver microsomes with increasing concentrations of proteinase K caused a marked decrease in the levels of microsomal cytochrome P-450 reductase (Fp) without having any significant effect on the cytochrome P-450s. About 20% of the microsomal cytochrome b5 was susceptible to proteolysis at low concentrations of proteinase K, while the remaining 80% was resistant to proteolysis, even at significantly higher proteinase K concentrations. Low concentrations of the proteases released about 30% of Fp from microsomes isolated from both uninduced and phenobarbital-induced rats, but did not affect the rates of benzphetamine bital-induced rats, but did not affect the rates of benzphetamine demethylation significantly. Further depletion of microsomal Fp at higher concentrations of proteinase K resulted in reductions of the rates of benzphetamine demethylation. However, even at higher protease concentrations, the decrease in the rate of the demethylation reaction was significantly less than the loss of Fp. Similar results were observed for the metabolism of two other substrates, 7-ethoxycoumarin and p-nitroanisole, suggesting that the P-450s, not the Fp, were the rate-limiting components in the metabolism of these xenobiotics by microsomes. It is clear that the decreases in the P-450-dependent oxidations were due to depletion of the NADPH-cytochrome P-450 reductase since reconstituting the protease-treated microsomes with native Fp restored the oxidation reactions. The amount of Fp required to completely restore the oxidation of benzphetamine only partially restored the oxidation of 7-ethoxycoumarin and p-nitroanisole.(ABSTRACT TRUNCATED AT 250 WORDS)