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Conversion of Cholesterol Into 5Β-Cholestane-3Α,7Α,12Α-Triol in Liver Homogenates

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Conversion of Cholesterol Into 5Β-Cholestane-3Α,7Α,12Α-Triol in Liver Homogenates Formation of bile acids in man: conversion of cholesterol into 5β-cholestane-3α,7α,12α-triol in liver homogenates Ingemar Björkhem, … , Kurt Einarsson, Gunnar Johansson J Clin Invest. 1968;47(7):1573-1582. https://doi.org/10.1172/JCI105849. Research Article The mechanisms of the conversion of cholesterol into bile acids in man were studied by examining the metabolism of cholesterol-1,2-3H, cholest-5-ene-3β,7α-diol-7β-3H, tritiumlabeled 7α-hydroxycholest-4-en-3-one, 7α,12α- dihydroxycholest-4-en-3-one, and cholest-5-ene-3β,7α,12α-triol in fractions of liver homogenates. The 20,000 g supernatant fluid catalyzed the conversion of cholesterol into cholest-5-ene-3β,7α-diol, 7α-hydroxycholest-4-en-3-one, 7α- 12α-dihydroxycholest-4-en-3-one, and 5β-cholestane-3α,7α,12α-triol. In the presence of microsomal fraction fortified with NAD+, cholest-5-ene-3β,7α-diol was converted into 7α-hydroxycholest-4-en-3-one, and when this fraction was fortified with NADPH small amounts of cholest-5-ene-3β-7α,12α-triol were formed. 7α-Hydroxycholest-4-en-3-one was metabolized into 7α-12α-dihydroxycholest-4-en-3-one in the presence of microsomal fraction fortified with NADPH and into 5β-cholestane-3α,7α-diol in the presence of 100,000 g supernatant fluid. Cholest-5-ene-3β,7α,12α-triol was converted into 7α,12α-dihydroxycholest-4-en-3-one in the presence of microsomal fraction fortified with NAD+. The 100,000 g supernatant fluid catalyzed the conversion of 7α,12α-dihydroxycholest-4-en-3-one into 5β-cholestane- 3α,7α,12α-triol. The sequence of reactions in the conversion of cholesterol into 5β-cholestane-3α,7α-diol and 5β- cholestane-3α,7α,12α-triol, the subcellular localization of the enzymes, and the cofactor requirements were found to be the same as those described for rat liver. Find the latest version: https://jci.me/105849/pdf Fonnation of Bile Acids in Man: Conversion of Cholesterol into 53-Cholestane-3a,7a, 12a-triol in Liver Homogenates * INGEMAR BJORKHEM, HENRY DANIELSSON, KuRT EINARSSON, and GUNNAR JOHANSSON From the Department of Chemistry, Karolinska Institutet, Stockholm, Sweden A B S T R A C T The mechanisms of the conversion 5,1-cholestane-3a,7a-diol and 5,8-cholestane-3a,7a, of cholesterol into bile acids in man were studied 12a-triol, the subcellular localization of the en- by examining the metabolism of cholesterol- zymes, and the cofactor requirements were found 1,2-3H, cholest-5-ene-3,8,7a-diol-7,8-3H, tritium- to be the same as those described for rat liver. labeled 7a-hydroxycholest-4-en-3-one, 7a,12a-di- hydroxycholest-4-en-3-one, and cholest-5-ene-3,8, INTRODUCTION 7a,12a-triol in fractions of liver homogenates. The 20,000 g supernatant fluid catalyzed the conversion Bile acids are major end products of cholesterol of cholesterol into cholest-5-ene-3,8,7a-diol, 7a- metabolism in vertebrates. (For a review, see hydroxycholest4-en-3-one, 7a-12a-dihydroxycho- reference 1.) The conversion of cholesterol into lest4-en-3-one, and 5,8-cholestane-3a,7al2a-triol. bile acids takes place in the liver, and the main In the presence of microsomal fraction fortified bile acids formed are, in most mammalian species, with NAD+, cholest-5-ene-3,3,7a-diol was con- cholic acid 1 and chenodeoxycholic acid (1). In verted into 7a-hydroxycholest-4-en-3-one, and bile, bile acids are present as conjugates with when this fraction was fortified with NADPH taurine and glycine. Bile acids are excreted with small amounts of cholest-5-ene-3f8-7a,12a-triol bile into the intestine, reabsorbed in the ileum, were formed. 7a-Hydroxycholest-4-en-3-one was and again excreted with bile. A small amount of metabolized into 7a-12a-dihydroxycholest-4-en-3- bile acids escapes reabsorption in each cycle of one in the presence of microsomal fraction fortified enterohepatic circulation, and these bile acids are with NADPH and into 5,8-cholestane-3a,7a-diol in excreted with the feces. During their enterohepatic the presence of 100,000 g supernatant fluid. circulation bile acids are subjected to the action Cholest-5-ene-3,/,7cv,12a-triol was converted into of intestinal microorganisms. Major reactions in- 7a,12a-dihydroxycholest-4-en-3-one in the pres- volve the hydrolysis of the conjugates and the ence of microsomal fraction fortified with NAD+. removal of the 7a-hydroxyl group yielding deoxy- The 100,000 g supernatant fluid catalyzed the cholic acid from cholic acid and lithocholic acid conversion of 7a,12a-dihydroxycholest-4-en-3-one from chenodeoxycholic acid (1). Further struc- into 5f3-cholestane-3a,7a,12a-triol. The sequence 'The following systematic names are given to bile of reactions in the conversion of cholesterol into acids referred to by trivial names: cholic acid, 3a,7a, 12a- trihydroxy-5f-cholanoic acid; chenodeoxycholic acid, 3a,- * Bile acids and steroids 197. 7a-dihydroxy-5#-cholanoic acid; deoxycholic acid, 3a,12a- Received for publication 5 February 1968 and in re- dihydroxy-5,B-cholanoic acid; lithocholic acid, 3a-hy- vised form 7 March 1968. droxy-5fi-cholanoic acid. The Journal of Clinical Investigation Volume 47 1968 1573 tural modifications of the bile acids in the intestine of atropine and 1 ml of Ketogin.2 The general anaesthesia lead to the complex mixture of bile acids excreted consisted of sodium hexobarbital, nitrous oxide, succinyl with the feces. The bile acids formed by microbial choline, and fluothane and was initiated 15 min or less action on cholic acid and chenodeoxycholic acid before operation. can be reabsorbed, but of all Preparation of homogenates and analysis of incuba- the different metabo- tions. Liver biopsies (3-5 g) were taken within 15 min lites formed only deoxycholic acid appears to be after the abdomen had been opened and were immediately efficiently reabsorbed, and deoxycholic acid is a put in cold homogenizing medium. Homogenates were major bile acid in many species. Thus, the main prepared within 30 min after the biopsy. The homogenates, bile acids in human bile are cholic acid, cheno- 20% (liver wet weight per volume), were prepared in a modified Bucher medium (5), pH 7.4, using a Potter- deoxycholic acid, and deoxycholic acid. Elvehjem homogenizer equipped with a loosely fitting The mechanisms of the conversion of cholesterol pestle. The homogenate was centrifuged at 800 g for 10 into cholic acid and chenodeoxycholic acid have min and then at 20,000 g for 10 min. The microsomal and been extensively studied in recent years (1). The soluble fractions were obtained by centrifuging the 20,- results of 000 g supernatant fluid at 100,000 g for 2 hr. The microso- investigations on the metabolism of mal fraction was suspended in the same volume of homog- cholesterol and other oxygenated C27 steroids in enizing medium as the volume of the 20,000 g supernatant bile fistula rats and in homogenates of rat liver fluid from which it had been isolated. The labeled com- have led to the elucidation of the major pathways pounds were added to the incubation mixtures dissolved for the formation of bile acids in the rat (1). The in 50 ,ul of acetone, and incubations were run at 37°C. In- cubations with labeled cholesterol were run for 60 min, conversion of cholesterol into bile acids in man has and those with the other substrates for 10 or 20 min. not been studied in any detail. Staple and Rabino- (See legends to figures.) Incubations were terminated witz (2) and Carey (3) have isolated labeled by the addition of 20 volumes of chloroform-methanol 3a,7a,12ca-trihydroxy-5,8-cholestanoic acid from (2:1, v/v). The precipitate was filtered off, and the bile of patients treated with injection of chloroform-methanol solution was washed with 0.2 vol- labeled umes of a 0.9%o (w/v) solution of sodium chloride. cholesterol. Herman, Weinstein, Staple, and Ra- The chloroform phase was collected, and the solvent was binowitz (4) have recently reported the isolation evaporated under reduced pressure. The methanol-saline from human bile of small amounts of 5,8-choles- phase, which in all experiments contained less than 10% tane-3a,7a-diol, another probable intermediate in of the total radioactivity, was discarded. The residue of the conversion of cholesterol into bile acids. the chloroform extract together with appropriate reference These compounds was subjected to thin-layer chromatography results indicate that the pathways for the forma- with Kieselgel G 3 as adsorbent. The solvent systems used tion of bile acids in man are analogous to those in to develop the chromatoplates are given in the legends rat. However, it appeared of interest to attempt to the figures. The internal standards were visualized by to define more closely the sequence of reactions exposing the chromatoplates to iodine vapor. The ap- propriate zones were scraped into test tubes and were in the conversion of cholesterol into bile acids in extracted with methanol by vigorous stirring. The silica man by studying the metabolism of cholesterol and gel was allowed to settle by gravity, and an aliquot of the cholesterol metabolites in homogenates of human methanol solution was assayed for radioactivity. Further liver. identification of the metabolites formed was done by crys- tallizing representative samples together with authentic METHODS material to constant specific radioactivity. The reference compounds were material synthesized in this laboratory Subjects. The patients in this investigation were: and used in earlier studies in this series of investigations. A.R., female, aged 60; I.-L.J., female, aged 41; and H.S., Labeled compounds.
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