Incorporation of cytochrome b5 into rat liver microsomal membranes. Impairment of cytochrome P-450-dependent mixed function oxidase activity.
Cytochrome b5 was purified to electrophoretic homogeneity from the liver microsomes of untreated rats and reincorporated into liver microsomes from phenobarbital-treated rats, resulting in an approximate three-fold enrichment of the cytochrome b5 specific content (1.5 nmol haemoprotein X mg-1 protein). Our results have shown that the N-demethylation of benzphetamine was progressively inhibited in cytochrome b5-fortified microsomal preparations. Using stopped flow, visible difference spectrophotometry, the NADPH-driven reduction kinetics of cytochrome P-450 were examined in the modified microsomes over the first few seconds of reaction. Increasing the amount of incorporated cytochrome b5 resulted in a progressive inhibition of the initial, fast phase reduction rate constant of microsomal cytochrome P-450, both in the absence and presence of the type I substrate benzphetamine. Although the initial rate of NADPH-driven cytochrome b5 reduction was the same for both native and cytochrome b5-fortified microsomes, the extent of cytochrome b5 reduction was greater in the fortified microsomes. If cytochrome b5 has a positive role to play in cytochrome P-450-dependent mixed function oxidase activity either as an effector or in electron transfer or both, the former haemoprotein must be already present in sufficient concentrations in the native microsomes.
D000818:Animals; D003577:Cytochrome P-450 Enzyme System / Q000378:metabolism*; D003573:Cytochrome b Group / Q000378:metabolism* / Q000494:pharmacology; D015786:Cytochromes b5; D007700:Kinetics; D008297:Male; D008862:Microsomes, Liver / Q000378:metabolism*; D010089:Oxidoreductases, N-Demethylating / Q000037:antagonists & inhibitors / Q000378:metabolism*; D051381:Rats; D011919:Rats, Inbred Strains
D003573:Cytochrome b Group; D015786:Cytochromes b5; D003577:Cytochrome P-450 Enzyme System; C019781:benzphetamine N-demethylase; D010089:Oxidoreductases, N-Demethylating