1. The actions of various inhibitors of the microsomal NADPH–cytochrome P-450 electron-transport chain have been studied on the stimulatory effect of carbon tetrachloride on malonaldehyde production. 2. Carbon monoxide, p-chloromercuribenzoate, β-diethylaminoethyl-3,3′-diphenylpropyl acetate (SKF 525A) and nicotinamide did not decrease the stimulatory action of carbon tetrachloride on malonaldehyde production when present in concentrations shown to be capable of strongly inhibiting the demethylation of aminopyrine. 3. In contrast with the effects of the substances mentioned above, low concentrations of cytochrome c strongly depressed the stimulatory action of carbon tetrachloride on malonaldehyde production while increasing the endogenous rate of peroxidation. 4. Aging the microsomal suspensions at 0°C caused a rapid decrease in aminopyrine demethylation activity and in lipid peroxidation catalysed by ADP and Fe2+. The stimulation of malonaldehyde production by carbon tetrachloride was relatively unaffected, however, by aging the microsomes at 0°C for 3 days; during this period cytochrome P-450 decreased by more than 30%. 5. The conclusion is reached that the interaction between carbon tetrachloride and the NADPH–cytochrome P-450 electron-transport chain necessary for the stimulation of malonaldehyde production involves a locus near to if not identical with the NADPH–cytochrome c reductase flavoprotein.

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