Bile acid synthesis in man: assay of hepatic microsomal cholesterol 7 alpha-hydroxylase activity by isotope dilution-mass spectrometry.

The present work describes an accurate assay of the rate-limiting enzyme in bile acid synthesis, the cholesterol 7 alpha-hydroxylase, in human liver. The assay is based on isotope dilution-mass spectrometry, and endogenous microsomal cholesterol is used as the only substrate for the enzyme. Operative liver biopsies were obtained from patients undergoing elective cholecystectomy under highly standardized conditions. In ten gallstone patients, the enzyme activity of the microsomal fraction averaged 9.6 +/- 1.4 (mean +/- SEM) pmol X min-1 X mg protein-1 corresponding to a daily synthesis of about 0.5 mmol of bile acids. Three cholestyramine-treated patients displayed a four-fold higher enzyme activity. No evidence was obtained supporting the concept that the cholesterol 7 alpha-hydroxylase is modulated by phosphorylation-dephosphorylation.