Human liver microsomal steroid metabolism: identification of the major microsomal steroid hormone 6 beta-hydroxylase cytochrome P-450 enzyme

Arch Biochem Biophys. 1988 Jun;263(2):424-36. doi: 10.1016/0003-9861(88)90655-8.


Cytochrome P-450-dependent steroid hormone metabolism was studied in isolated human liver microsomal fractions. 6 beta hydroxylation was shown to be the major route of NADPH-dependent oxidative metabolism (greater than or equal to 75% of total hydroxylated metabolites) with each of three steroid substrates, testosterone, androstenedione, and progesterone. With testosterone, 2 beta and 15 beta hydroxylation also occurred, proceeding at approximately 10% and 3-4% the rate of microsomal 6 beta hydroxylation, respectively, in each of the liver samples examined. Rates for the three steroid 6 beta-hydroxylase activities were highly correlated with each other (r = 0.95-0.97 for 25 individual microsomal preparations), suggesting that a single human liver P-450 enzyme is the principal microsomal 6 beta-hydroxylase catalyst with all three steroid substrates. Steroid 6 beta-hydroxylase rates correlated well with the specific content of human P-450NF (r = 0.69-0.83) and with its associated nifedipine oxidase activity (r = 0.80), but not with the rates for debrisoquine 4-hydroxylase, phenacetin O-deethylase, or S-mephenytoin 4-hydroxylase activities or the specific contents of their respective associated P-450 forms in these same liver microsomes (r less than 0.2). These correlative observations were supported by the selective inhibition of human liver microsomal 6 beta hydroxylation by antibody raised to either human P-450NF or a rat homolog, P-450 PB-2a. Anti-P-450NF also inhibited human microsomal testosterone 2 beta and 15 beta hydroxylation in parallel to the 6 beta-hydroxylation reaction. This antibody also inhibited rat P-450 2a-dependent steroid hormone 6 beta hydroxylation in uninduced adult male rat liver microsomes but not the steroid 2 alpha, 16 alpha, or 7 alpha hydroxylation reactions catalyzed by other rat P-450 forms. Finally, steroid 6 beta hydroxylation catalyzed by either human or rat liver microsomes was selectively inhibited by NADPH-dependent complexation of the macrolide antibiotic triacetyloleandomycin, a reaction that is characteristic of members of the P-450NF gene subfamily (P-450 IIIA subfamily). These observations establish that P-450NF or a closely related enzyme is the major catalyst of steroid hormone 6 beta hydroxylation in human liver microsomes, and furthermore suggest that steroid 6 beta hydroxylation may provide a useful, noninvasive monitor for the monooxygenase activity of this hepatic P-450 form.

Publication types

  • Comparative Study
  • Research Support, U.S. Gov't, P.H.S.

MeSH terms

  • Androstenedione / metabolism
  • Animals
  • Cross Reactions
  • Cytochrome P-450 CYP3A
  • Cytochrome P-450 Enzyme System*
  • Female
  • Humans
  • Male
  • Microsomes, Liver / enzymology*
  • NADP / metabolism
  • Progesterone / metabolism
  • Rats
  • Steroid Hydroxylases / antagonists & inhibitors
  • Steroid Hydroxylases / immunology
  • Steroid Hydroxylases / metabolism*
  • Substrate Specificity
  • Testosterone / metabolism
  • Troleandomycin / metabolism


  • Testosterone
  • Androstenedione
  • Progesterone
  • NADP
  • Cytochrome P-450 Enzyme System
  • Troleandomycin
  • Steroid Hydroxylases
  • CYP3A protein, human
  • Cytochrome P-450 CYP3A
  • steroid hormone 6-beta-hydroxylase