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Abstract
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.
Highlights
The present work describes an accurate assay of the rate-limiting enzyme in bile acid synthesis, the cholesterol 7ahydroxylase, in human liver
In view of the great impact of abnormal cholesterol and bile acid metabolism in various human diseases, we considered it important to develop a more accurate method than those previously used for assay of the 7a-hydroxylase activity in human liver
The development of the present assay of cholesterol 7ahydroxylase activity in human liver was initiated due to our failure to obtain reproducible results with previously published methods based on conversion of labeled substrate
Summary
The present work describes an accurate assay of the rate-limiting enzyme in bile acid synthesis, the cholesterol 7ahydroxylase, 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. 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 fourfold higher enzyme activity. - ing the concept that the cholesterol 7a-hydroxylase is modulated by phosphorylation-dephosphorylation. Bile acid synthesis in man: assay of hepatic microsomal cholesterol 7ahydroxylase activity by isotope dilution-mass spectrometry, J.
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