Cytochromes P-450 LM3b and LM4 in biosynthesis of bile acids.

Cytochromes P-450 LM3b and LM4 were prepared from untreated and cholestyramine-treated rabbits. The catalytic properties of these cytochrome P-450 fractions towards substrates in bile acid biosynthesis were studied in reconstituted systems containing NADPH -- cytochrome P-450 reductase and phospholipid. Cytochrome P-450 LM3b showed no hydroxylase activity towards cholesterol and only low activity towards some other C27-steroids whereas it catalyzed efficient hydroxylation of testosterone and demethylation of ethylmorphine. Preparations of cytochrome P-450 LM4 catalyzed hydroxylation of cholesterol and other C27-steroids more efficiently than microsomes. Cytochrome b5 had no stimulatory effect on the C27-steroid hydroxylase activities.