Cholesterol 7 alpha-hydroxylase from human liver: partial purification and reconstruction into defined phospholipid-cholesterol vesicles.

Cholesterol 7 alpha-hydroxylase, the rate-limiting enzyme for bile acid synthesis, was shown to be copurified with human liver microsomal cytochrome P-450. When these cytochrome P-450 species were reconstituted in phospholipid-cholesterol vesicles together with NADPH-cytochrome P-450 reductase, high cholesterol 7 alpha-hydroxylase activity was obtained in the presence of NADPH. The activity represented a twofold enrichment relative to cytochrome P-450 and 43-fold enrichment relative to total microsomal protein. Availability of such a preparation will allow further characterization of the enzyme and will also allow studies of its mechanisms of regulation.