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 7a- hydroxylase, in human liver. The assay is based on isotope dilu- tion-mass spectrometry, and endogenous microsomal choles- terol is used as the only substrate for the enzyme. Operative liver biopsies were obtained from patients undergoing elective cho- lecystectomy under highly standardized conditions. In ten gall- stone patients, the enzyme activity of the microsomal fraction averaged 9.6 * 1.4 (mean * SEM) pmol. min-' . mg protein-' 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 support- ing the concept that the cholesterol 7a-hydroxylase is modulated by phosphorylation-dephosphorylation. - Einarsson, K., B. Angelin, S. Ewerth, K. Nilsell, and I. Bjorkhem. Bile acid synthesis in man: assay of hepatic microsomal cholesterol 7a- hydroxylase activity by isotope dilution-mass spectrometry, J. Lipid&, 1986. 27: 82-88.