Formation of bile acids in man: conversion of cholesterol into 5-beta-cholestane-3-alpha, 7-alpha, 12-alpha-triol in liver homogenates

CYP8B1 Catalyzes 12alpha-Hydroxylation of C27 Bile Acid: In Vitro Conversion of Dihydroxycoprostanic Acid into Trihydroxycoprostanic Acid

Sterol 12α-hydroxylase (CYP8B1) is the unique P450 enzyme with sterol 12-oxidation activity, playing an exclusive role in 12α-hydroxylating intermediates along the bile acid (BA) synthesis pathway. Despite the long history of BA metabolism studies, it is unclear whether CYP8B1 catalyzes 12α-hydroxylation of C27 BAs, the key intermediates shuttling between mitochondria and peroxisomes. This work provides robust in vitro evidence that both microsomal and recombinant CYP8B1 enzymes catalyze the 12α-hydroxylation of dihydroxycoprostanic acid (DHCA) into trihydroxycoprostanic acid (THCA). On the one hand, DHCA 12α-hydroxylation reactivity is conservatively detected in liver microsomes of both human and preclinical animals. The reactivity of human tissue fractions conforms well with the selectivity of CYP8B1 mRNA expression, while the contribution of P450 enzymes other than CYP8B1 is excluded by reaction phenotyping in commercial recombinant enzymes. On the other hand, we prepared functional recombinant human CYP8B1 proteins according to a recently published protocol. Titration of the purified CYP8B1 proteins with either C4 (7α-hydroxy-4-cholesten-3-one) or DHCA yields expected blue shifts of the heme Soret peak (type I binding). The recombinant CYP8B1 proteins efficiently catalyze 12α-hydroxylation of both DHCA and C4, with substrate concentration occupying half of the binding sites of 3.0 and 1.9 μM and kcat of 3.2 and 2.6 minutes-1, respectively. In summary, the confirmed role of CYP8B1 in 12α-hydroxylation of C27 BAs has furnished the forgotten passageway in the BA synthesis pathway. The present finding might have opened a new window to consider the biology of CYP8B1 in glucolipid metabolism and to evaluate CYP8B1 inhibition as a therapeutic approach of crucial interest for metabolic diseases. SIGNIFICANCE STATEMENT: The academic community has spent approximately 90 years interpreting the synthesis of bile acids. However, the 12α-hydroxylation of intermediates catalyzed by CYP8B1 is not completely mapped on the classic pathway, particularly for the C27 bile acids, the pivotal intermediates shuttling between mitochondria and peroxisomes. This work discloses the forgotten 12α-hydroxylation pathway from dihydroxycoprostanic acid into trihydroxycoprostanic acid. The present finding may facilitate evaluating CYP8B1 inhibition as a therapeutic approach of crucial interest for metabolic diseases.

Human and murine steroid 5β-reductases (AKR1D1 and AKR1D4): insights into the role of the catalytic glutamic acid

Mammalian steroid 5β-reductases belong to the Aldo-Keto Reductase 1D sub-family and are essential for the formation of A-ring 5β-reduced steroids. Steroid 5β-reduction is required for the biosynthesis of bile-acids and the metabolism of all steroid hormones that contain a Δ⁴-3-ketosteroid functionally to yield the 5β-reduced metabolites. In mammalian AKR1D enzymes the conserved catalytic tetrad found in all AKRs (Y55, H117, K84 and D50) has changed in that the conserved H117 is replaced with a glutamic acid (E120). E120 may act as a "superacid" to facilitate enolization of the Δ⁴-ketosteroid. In addition, the absence of the bulky imidazole side chain of histidine in E120 permits the steroid to penetrate deeper into the active site so that hydride transfer can occur to the steroid C5 position. In murine steroid 5β-reductase AKR1D4, we find that there is a long-form, with an 18 amino-acid extension at the N-terminus (AKR1D4L) and a short-form (AKR1D4S), where the latter is recognized as AKR1D4 by the major data-bases. Both enzymes were purified to homogeneity and product profiling was performed. With progesterone and cortisol, AKR1D4L and AKR1D4S catalyzed smooth conversion to the 5β-dihydrosteroids. However, with Δ⁴-androstene-3,17-dione as substrate, a mixture of products was observed which included, 5β-androstane-3,17-dione (expected) but 3α-hydroxy-5β- androstan-17-one was also formed. The latter compound was distinguished from its isomeric 3β-hydroxy-5β-androstan-17-one by forming picolinic acid derivatives followed by LC-MS. These data show that AKR1D4L and AKR1D4S also act as 3α-hydroxysteroid dehydrogenases when presented with Δ⁴-androstene-3,17-dione and suggest that E120 alters the position the steroid to enable a correct trajectory for hydride transfer and may not act as a "superacid".

Impact of an enriched-cholesterol diet on enzymatic cholesterol metabolism during rabbit gestation

An appropriate cholesterol homeostasis is vital for the maintenance and the optimal fetal development. The cholesterol is essential for the synthesis of progesterone and 17beta-estradiol, hormones that actively participate to sustain gestation. However, the administration of 0.2% enriched cholesterol diet (ECD) during rabbit gestation significantly increased the cholesterol blood profile (total-cholesterol, LDL, HDL, esterified-cholesterol and free-cholesterol) of dams and offspring, and induced a reduction of the offspring weight of 15% as compared to the control group. Enzymes involved in cholesterol metabolism (ACAT, HMG-CoA-reductase and cholesterol-7alpha-hydroxylase) are greatly influenced by cholesterol profile. We hypothesized that the administration of an ECD during rabbit gestation modifies the activity of those enzymes. Female rabbits (pregnant or not) were fed with a standard diet or an ECD. At term, livers (dams and offspring) and placentas were collected and ACAT, HMG-CoA-reductase and cholesterol-7alpha-hydroxylase activities were assayed. Our results demonstrate that gestation induced a reduction of ACAT activity (48.9%) in dam's liver and, an augmentation of HMG-CoA-reductase activity (142.4%) whereas it has no effect on cholesterol-7alpha-hydroxylase activity. The administration of the ECD has no additive effect on ACAT, but significantly reduced the HMG-CoA-reductase activity and cholesterol-7alpha-hydroxylase activity as compared with the pregnant control group. In placentas the ECD supplementation has an influence for HMG-CoA-reductase activity, where a 43% increased in observed. Any ACAT activity was detected in placenta and the ECD has no influence on the cholesterol-7alpha-hydroxylase activity. Whereas their offspring's liver present a reduction of ACAT and HMG-CoA-reductase activity. Gestation associated with ECD reduces significantly the HMG-CoA-reductase activity, decreasing the cholesterol synthesis, but placenta seems to compensate this effect by increasing its HMG-CoA-reductase activity.

Rabbit liver contains one major sterol 12alpha-hydroxylase with broad substrate specificity

Conversion of cholesterol into cholic acid in mammalian liver requires a 12alpha-hydroxylation step. Results have been presented suggesting that two different enzymes are involved in this hydroxylation with different activities towards the two steroids believed to be the physiological substrates for the enzyme, 7alpha-hydroxy-4-cholesten-3-one and 5beta-cholestane-3alpha,7alpha-diol. It is shown here that rabbit liver microsomes and partly purified sterol 12alpha-hydroxylase as well as COS cells transfected with a cDNA coding for this enzyme are able to catalyze 12alpha-hydroxylation of the two substrates at similar relative rates. Also 7alpha-hydroxycholesterol and 3alpha,7alpha-dihydroxy-5beta-cholestanoic acid are 12alpha-hydroxylated by the three systems. It is concluded that rabbit liver contains one major sterol 12alpha-hydroxylase with a broad substrate specificity.

Bile acid synthesis in humans: regulation of hepatic microsomal cholesterol 7 alpha-hydroxylase activity

The present work tested the hypothesis that portal venous bile acids regulate the activity of the cholesterol 7 alpha-hydroxylase and studied the influence of hepatic microsomal free cholesterol concentration on the enzyme activity. Operative liver biopsies and samples of portal venous blood were obtained from a total of 61 patients with gallstones who were undergoing cholecystectomy. Fifteen of the patients were treated with cholestyramine (16 g/day) for 2-3 wk before operation and 23 patients with chenodeoxycholic acid (15 mg/kg.day) or ursodeoxycholic acid (15 mg/kg.day) for 3-4 wk before operation. Highly accurate methods based on isotope dilution-mass spectrometry were used for assay of the cholesterol 7 alpha-hydroxylase activity, the concentration of free cholesterol in the microsomes, and the levels of individual bile acids in portal venous blood. Cholestyramine treatment increased the cholesterol 7 alpha-hydroxylase activity about sixfold, from 7.6 +/- 1.1 (mean +/- SEM) to 45.7 +/- 6.7 pmol/min.mg protein. Administration of chenodeoxycholic acid reduced the enzyme activity considerably to 1.0 +/- 0.3 pmol/min.mg protein, whereas ursodeoxycholic acid did not significantly affect the enzyme activity (7.9 +/- 2.2 pmol/min.mg protein). The concentration of microsomal free cholesterol remained essentially unchanged in spite of a 45-fold variation in enzyme activity. There was a negative correlation between the absolute as well as the relative concentration of chenodeoxycholic acid in portal blood and the activity of the cholesterol 7 alpha-hydroxylase, whereas there was no correlation between the total concentration of bile acids and the enzyme activity. It is concluded that the composition of individual bile acids may be more important than the total concentration of bile acids in the portal vein for the regulation of the cholesterol 7 alpha-hydroxylase activity in humans. It is further concluded that chenodeoxycholic acid is a considerably stronger suppressor of bile acid synthesis than ursodeoxycholic acid.

In vivo and vitro studies on formation of bile acids in patients with Zellweger syndrome. Evidence that peroxisomes are of importance in the normal biosynthesis of both cholic and chenodeoxycholic acid

The last step in bile acid formation involves conversion of 3 alpha,7 alpha,12 alpha-trihydroxy-5 beta-cholestanoic acid (THCA) into cholic acid and 3 alpha,7 alpha-dihydroxy-5 beta-cholestanoic acid (DHCA) into chenodeoxycholic acid. The peroxisomal fraction of rat and human liver has the highest capacity to catalyze these reactions. Infants with Zellweger syndrome lack liver peroxisomes, and accumulate 5 beta-cholestanoic acids in bile and serum. We recently showed that such an infant had reduced capacity to convert a cholic acid precursor, 5 beta-cholestane-3 alpha,7 alpha,12 alpha-triol into cholic acid. 7 alpha-Hydroxy-4-cholesten-3-one is a common precursor for both cholic acid and chenodeoxycholic acid. Intravenous administration of [3H]7 alpha-hydroxy-4-cholesten-3-one to an infant with Zellweger syndrome led to a rapid incorporation of 3H into biliary THCA but only 10% of 3H was incorporated into cholic acid after 48 h. The incorporation of 3H into DHCA was only 25% of that into THCA and the incorporation into chenodeoxycholic acid approximately 50% of that in cholic acid. The conversion of intravenously administered [3H]THCA into cholic acid in another infant with Zellweger syndrome was only 7%. There was a slow conversion of THCA into 3 alpha,7 alpha,12 alpha-trihydroxy-5 beta-C29-dicarboxylic acid. The pool size of both cholic- and chenodeoxycholic acid was markedly reduced. Preparations of liver from two patients with Zellweger syndrome had no capacity to catalyze conversion of THCA into cholic acid. There was, however, a small conversion of DHCA into chenodeoxycholic acid and into THCA. It is concluded that liver peroxisomes are important both for the conversion of THCA into cholic acid and DHCA into chenodeoxycholic acid.

Cerebrotendinous xanthomatosis: a defect in mitochondrial 26-hydroxylation required for normal biosynthesis of cholic acid

Oxidation of side chain of 5 beta-cholestane-3 alpha,7 alpha,12 alpha-triol was studied in a patient with cerebrotendinous xanthomatosis (CTX) and in control subjects, using various subcellular fractions of liver homogenate and a method based on isotope dilution-mass spectrometry. In the control, 5 beta-cholestane-3 alpha,7 alpha,12 alpha-triol was converted into 5 beta-cholestane-3 alpha,7 alpha,12 alpha,26-tetrol and 3 alpha,7 alpha,12 alpha-trihydroxy-5 beta-cholestanoic acid by the mitochondrial fraction, and into 5 beta-cholestane-3 alpha,7 alpha,12 alpha,-25-tetrol by the microsomal fraction. In the CTX patient, liver mitochondria were completely devoid of 26-hydroxylase activity. The same mitochondrial fraction catalyzed 25-hydroxylation of vitamin D3. The microsomal fraction of liver of the subject with CTX contained more than 50-fold the normal amount of 5 beta-cholestane-3 alpha,7 alpha,12 alpha-triol. The basic metabolid defect in CTX appears to be a lack of the mitochondrial 26-hydroxylase. The excretion in the bile of 5 beta-cholestane-3 alpha,7 alpha,12 alpha,25-tetrol and 5 beta-cholestane-3 alpha,7 alpha,12 alpha,24 alpha,25-pentol observed in CTX patients may be secondary to the accumulation of the major substrate for the 26-hydroxylase, i. e., 5 beta-cholestane-3 alpha,7 alpha,12 alpha-triol, and exposure of this substrate to the normally less active microsomal 25-and 24-hydroxylases. It is concluded that the major pathway in the biosynthesis of cholic acid in human liver involves a mitochondrial C27-steroid 26-hydroxylation.

Biosynthesis of chenodeoxycholic acid in man: stereospecific side-chain hydroxylations of 5beta-cholestane-3alpha,7alpha-diol

Stereospecific side-chain hydroxylations of 5beta-cholestane-3alpha, 7alpha-diol were studied in mitochondrial and microsomal fractions of human liver. Incubation of 5beta-cholestane-3alpha, 7alpha-diol resulted in hydroxylations at C-12, C-24, C-25, and C-26. Hydroxylations at C-24 and C-26 were accompanied by the introduction of additional asymmetric carbon atoms at C-24 and C-25 respectively, that led to the formation of two distinct pairs of diastereoisomers, namely 5beta-cholestane-3alpha, 7alpha,24-triols (24R and 24S) and 5beta-cholestane-3alpha, 7alpha,26-triols (25R and 25S). A sensitive and reproducible radioactive assay to measure the formation of the different biosynthetic 5beta-cholestanetriols was developed. Optimal assay conditions for human mitochondrial and microsomal systems were tentatively established.The mitochondrial fraction was found to predominantly catalyze the 26-hydroxylation of 5beta-cholestane-3alpha, 7alpha-diol with the formation of the 25R-diastereoisomer of 5beta-cholestane-3alpha, 7alpha,26-triol as the major product. In the microsomal fraction, on the other hand, 25-hydroxylation was more efficient than 26-hydroxylation and accounted for 6.4% of the total hydroxylations. The microsomes catalyzed the formation of both diastereoisomers of 5beta-cholestane-3alpha, 7alpha,26-triol (25R and 25S, 4.2 and 1.6% respectively). These experiments suggest that the initial step in the degradation of the steroid side chain during the biosynthesis of chenodeoxycholic acid in man is mediated by the mitochondria, and involves the formation of the 25R-diastereoisomer of 5beta-cholestane-3alpha, 7alpha,26-triol. The role of the microsomal 25- and 26-hydroxylated intermediates requires further exploration.

Metabolism of bile alcohols in the perfused rabbit liver - C26 bile alcohols

The metabolism of a C26 bile alcohol (I, 24-nor-5beta-cho-lestane-3alpha, 7alpha,25-triol) was studied in the isolated perfused rabbit liver. The new bile alcohol and bile acid metabolites secreted into the bile were isolated and identified by a combination of TLC, GLC and GLC-MS. The following bile alcohols were found: II, 24-nor-5beta-cholestane-3alpha,7alpha,12alpha,25-tetrol, III, 24-nor-5beta-cholestane-3alpha,7alpha,12alpha,25,26-pentol; IV, 24-nor-5beta-cholest-23-ene-3alpha,7alpha,12alpha-triol; and V, 24-nor-5beta-cholest-23-ene-3alpha,7alpha-diol. In the bile acid fraction, 24-nor-cholic acid and 3alpha,7alpha,12alpha-trihydroxy-24-nor-5beta-cholest-23-en-26-oic acid were present. The perfused nor-triol was not resistant to 12alpha-hydroxylation.

Formation of bile acids in man. Metabolism of 7alpha-hydroxy-4-cholesten-3-one in normal subjects with an intact enterohepatic circulation

The formation of bile acids in man is thought to involve a series of reactions in which the initial steps are the same for both cholic acid and chenodeoxycholic acid. The point of bifurcation of the pathway is postulated to occur after the formation of 7alpha-hydroxy-4-cholesten-3-one. To test the hypothesis that the entire synthesis of both bile acids proceeds through this intermediate we studied the metabolism of labeled 7alpha-hydroxy-4-cholesten-3-one in eight normal subjects with an intact enterohepatic circulation. If all the production of cholic acid and chenodeoxycholic acid takes place via 7alpha-hydroxy-4-cholesten-3-one, the areas under the specific decay curves of cholic acid and chenodeoxycholic acid should be identical following a single injection of this labeled intermediate. However, in 6 of the 8 subjects studied the area under the cholic acid specific activity decay curve was significantly less than the area under the chenodeoxycholic acid specific activity decay curve. These results that the production of cholic acid in man may not always involve the intermediate 7alpha-hydroxy-4-cholesten-3-one.

Effects of clofibrate on some microsomal hydroxylations involved in the formation and metabolism of bile acids in rat liver

1. The liver microsomal metabolism of [4-14C]cholesterol, endogenous cholesterol, 7 alpha-hydroxy-4-[6 beta-3H]cholesten-3-one, 5-beta-[7 beta-3H]cholestane-3 alpha, 7 alpha-diol and [3H]lithocholic acid was studdied in control and clofibrate (ethyl p-chlorophenoxyisobutyrate)-treated rats. 2. The extent of 7 alpha-hydroxylation of exogenous [414C]cholesterol and endogenous cholesterol, the latter determined with a mass fragmentographic technique, was the same in the two groups of rats. The extent of 12 alpha-hydroxylation of 7 alpha-hydroxy-4-cholesten-3-one and 5 beta-cholestane-3 alpha, 7 alpha-diol was increased by about 60 and 120% respectively by clofibrate treatment. The 26-hydroxylation of 5 beta-cholestane-3 alpha, 7 alpha-diol was not significantly affected by clofibrate. The 6 beta-hydroxylation of lithocholic acid was about 80% higher in the clofibrate-treated animals than in the controls. 3. The results are discussed in the context of present knowledge about the liver microsomal hydroxylating system and bile acid formation in patients with hypercholesterolaemia, treated with clofibrate.

Biosynthesis of bile acids in man. Hydroxylation of the C27-steroid side chain

The first step in the degradation of the steroid side chain during biosynthesis of bile acids from cholesterol in man was studied in microsomal and mitochondrial fraction of homogenate of livers from 14 patients. The microsomal fraction was found to catalyze an efficient 25-hydroxylation of 5,8-cholestane-3a,7a,12atriol. A small extent of 23-, 24-, and 26-hydroxylation of the same substrate was observed. 53-Cholestane-3a,7adiol was hydroxylated in the 25-position only to a very small extent. The mitochondrial fraction was found to catalyze 26-hydroxylation of cholesterol, 5-cholestene-3P,7a-diol, 5P-cholestane-3a,7a-diol, 7a-hydroxy-4-cholesten-3-one, and 5,0-cholestane-3a,7a,12a-triol. Addition of Mg++ stimulated the 26-hydroxylation of cholesterol but had no effect or an inhibitory effect on 26-hydroxylation of the other substrates, indicating a heterogeneity of the mitochondrial 26-hydroxylating system. The level of 26-hydroxylase activity towards different substrates varied considerably with different mitochondrial preparations. The roles of the microsomal and mitochondrial 26- hydroxylations as well as the microsomal 25-hydroxylation in biosynthesis of bile acids in man are discussed. The results indicate that microsomal 26-hydroxylation is less important than mitochondrial 26-hydroxylation under normal conditions. The possibility that microsomal 25-hydroxylation is important cannot be ruled out.

Normal antipyrine metabolism in patients with cholesterol cholelithiasis. Evidence that the disease is not due to generalized hepatic microsomal dysfunction

Plasma antipyrine half-lives and metabolic clearances were measured after a single oral dose of antipyrine in 10 control subjects, 12 patients with gallstones, and 9 patients having undergone cholecystectomy for cholesterol cholelithiasis, to determine whether impairment of hepatic antipyrine metabolism occurs in patients with cholesterol cholelithiasis. The plasma antipyrine half-life and metabolic clearances in the control subjects were 11.7 plus or minus 1.3 hours and 42.5 plus or minus 3.3 ml/min, respectively; in patients with gallstones, 12.3 plus or minus 1.3 hours and 36.0 plus or minus 3.2 ml/min, respectively; and in patients having undergone cholecystectomy, 13.2 plus or minus 1.8 hours and 33.8 plus or minus 4.2 ml/min, respectively. Values for antipyrine half-life and metabolic clearance were not statistically different in these three groups. This study suggests the presence of normal hepatic antipyrine metablosim in patients with cholesterolcholelithiasis.

Bile acid synthesis in man: metabolism of 7 -hydroxycholesterol- 14 C and 26-hydroxycholesterol- 3 H

The pathways of bile acid synthesis in man were evaluated by studying the metabolism of 7alpha-hydroxycholesterol-4-(14)C and 26-hydroxycholesterol-16, 22-(3)H administered parenterally to individuals requiring external biliary drainage. Techniques for the identification of metabolites were thin-layer chromatography, column chromatography, gas-liquid chromatography with stream splitting, and crystallization to constant specific activity. It was found that both compounds were rapidly metabolized to bile acids and excreted in bile. Of the total radioactivity recovered in bile as bile acids, 87% of the 26-hydroxycholesterol-(3)H and 90% of the 7alpha-hydroxycholesterol-(14)C was found to be metabolized to both chenodeoxycholate and cholate. Compared to 7alpha-hydroxycholesterol, a greater proportion of 26-hydroxycholesterol was found to be metabolized to chenodeoxycholate. These findings indicate that both 7alpha-hydroxycholesterol and 26-hydroxycholesterol can be intermediates in the metabolism of cholesterol to bile acids in man. The observation that conversion to cholate takes place less readily after C-26 hydroxylation is consistent with previous findings in other species.