Metabolism of Non-Enzymatically Derived Oxysterols: Clues from sterol metabolic disorders

Cholestane-3β,5α,6β-triol (3β,5α,6β-triol) is formed from cholestan-5,6-epoxide (5,6-EC) in a reaction catalysed by cholesterol epoxide hydrolase, following formation of 5,6-EC through free radical oxidation of cholesterol. 7-Oxocholesterol (7-OC) and 7β-hydroxycholesterol (7β-HC) can also be formed by free radical oxidation of cholesterol. Here we investigate how 3β,5α,6β-triol, 7-OC and 7β-HC are metabolised to bile acids. We show, by monitoring oxysterol metabolites in plasma samples rich in 3β,5α,6β-triol, 7-OC and 7β-HC, that these three oxysterols fall into novel branches of the acidic pathway of bile acid biosynthesis becoming (25R)26-hydroxylated then carboxylated, 24-hydroxylated and side-chain shortened to give the final products 3β,5α,6β-trihydroxycholanoic, 3β-hydroxy-7-oxochol-5-enoic and 3β,7β-dihydroxychol-5-enoic acids, respectively. The intermediates in these pathways may be causative of some phenotypical features of, and/or have diagnostic value for, the lysosomal storage diseases, Niemann Pick types C and B and lysosomal acid lipase deficiency. Free radical derived oxysterols are metabolised in human to unusual bile acids via novel branches of the acidic pathway, intermediates in these pathways are observed in plasma.

Cyp2c70 is responsible for the species difference in bile acid metabolism between mice and humans

Bile acids are synthesized from cholesterol in the liver and subjected to multiple metabolic biotransformations in hepatocytes, including oxidation by cytochromes P450 (CYPs) and conjugation with taurine, glycine, glucuronic acid, and sulfate. Mice and rats can hydroxylate chenodeoxycholic acid (CDCA) at the 6β-position to form α-muricholic acid (MCA) and ursodeoxycholic acid (UDCA) to form β-MCA. However, MCA is not formed in humans to any appreciable degree and the mechanism for this species difference is not known. Comparison of several Cyp-null mouse lines revealed that α-MCA and β-MCA were not detected in the liver samples from Cyp2c-cluster null (Cyp2c-null) mice. Global bile acid analysis further revealed the absence of MCAs and their conjugated derivatives, and high concentrations of CDCA and UDCA in Cyp2c-null mouse cecum and feces. Analysis of recombinant CYPs revealed that α-MCA and β-MCA were produced by oxidation of CDCA and UDCA by Cyp2c70, respectively. CYP2C9-humanized mice have similar bile acid metabolites as the Cyp2c-null mice, indicating that human CYP2C9 does not oxidize CDCA and UDCA, thus explaining the species differences in production of MCA. Because humans do not produce MCA, they lack tauro-β-MCA, a farnesoid X receptor antagonist in mouse that modulates obesity, insulin resistance, and hepatosteatosis.

Characterization of a novel bile alcohol sulfate released by sexually mature male sea lamprey (Petromyzon marinus)

A sulphate-conjugated bile alcohol, 3,12-diketo-4,6-petromyzonene-24-sulfate (DKPES), was identified using bioassay-guided fractionation from water conditioned with sexually mature male sea lamprey (Petromyzon marinus). The structure and relative stereochemistry of DKPES was established using spectroscopic data. The electro-olfactogram (EOG) response threshold of DKPES was 10⁻⁷ Molar (M) and that of 3-keto petromyzonol sulfate (3 KPZS; a known component of the male sea lamprey sex pheromone) was 10⁻¹⁰ M. Behavioural studies indicated that DKPES can be detected at low concentrations by attracting sexually mature females to nests when combined with 3 KPZS. Nests baited with a mixture of DKPES and 3 KPZS (ratio 1∶29.8) attracted equal numbers of sexually mature females compared to an adjacent nest baited with 3 KPZS alone. When DKPES and 3 KPZS mixtures were applied at ratios of 2∶29.8 and 10∶29.8, the proportion of sexually mature females that entered baited nests increased to 73% and 70%, respectively. None of the sexually mature females released were attracted to nests baited with DKPES alone. These results indicated that DKPES is a component of the sex pheromone released by sexually mature male sea lamprey, and is the second biologically active compound identified from this pheromone. DKPES represents the first example that a minor component of a vertebrate pheromone can be combined with a major component to elicit critical sexual behaviors. DKPES holds considerable promise for increasing the effectiveness of pheromone-baited trapping as a means of sea lamprey control in the Laurentian Great Lakes.

Biosynthesis of bile acids in a variety of marine bacterial taxa

Several marine bacterial strains, which were isolated from seawater off the island Dokdo, Korea, were screened to find new bioactive compounds such as antibiotics. Among them, Donghaeana dokdonensis strain DSW-6 was found to produce antibacterial agents, and the agents were then purified and analyzed by LC-MS/MS and 1D- and 2D-NMR spectrometries. The bioactive compounds were successfully identified as cholic acid and glycine-conjugated glycocholic acid, the 7alpha-dehydroxylated derivatives (deoxycholic acid and glycodeoxycholic acid) of which were also detected in relatively small amounts. Other masine isolates, taxonomically different from DSW-6, were also able to produce the compounds in a quite different production ratio from DSW-6. As far as we are aware of, these bile acids are produced by specific members of the genus Streptomyces and Myroides, and thought to be general secondary metabolites produced by a variety of bacterial taxa that are widely distributed in the sea.

Formation of DNA adducts with cholyl adenylate, a putative intermediate for biosynthesis of cholyl-CoA

Cholyl adenylate is a putative intermediate for biosynthesis of cholic acid-coenzyme A (CoA) thioester conjugates by acyl-CoA synthetase. Early studies showed the conjugated acid anhydride moiety of cholyl adenylate to be reactive, attacking proteins to form protein-cholic acid adducts. In the present study, to clarify reactions of cholyl adenylate with DNA under physiological conditions, products with nucleosides were analyzed. HPLC-MS analyses indicated cholyl adenylate to primarily attack hydroxy groups of ribose moieties of nucleosides. Moreover, as speculated from UV and MS studies, exocyclic amino groups of 2'-deoxycytidine and 2'-deoxyadenosine were found to serve as targets of cholyl adenylate; the corresponding cholic amides, N4-cholyl-2'-deoxycytidine and N6-cholyl-2'-deoxyadenosine, were formed at yields of 0.32 and 0.06%, respectively. Structures of these base modified adducts were confirmed by direct comparison with synthetic compounds obtained from coupling reactions of cholic acid with each nucleoside in the presence of dicyclohexylcarbodiimide in pyridine at 70 degrees C. N4-Cholyl-2'-deoxycytidine was also obtained at a level of 1.6 adducts per 10(5) nucleosides from enzymatic hydrolysates of calf thymus DNA reacted with cholyl adenylate. These results suggest that cholyl adenylate, released from CoA synthetase, may have some possibility as a DNA modifier in vivo.

Preparation of (25R)- and (25S)-26-functionalized steroids as tools for biosynthetic studies of cholic acids

A new synthesis of both epimeric forms of 26-cholestanoic acids and 26-alcohols containing a 3beta-hydroxy-Delta(5)- or a Delta(4)-3-keto-functionality in ring A is described starting from stigmasterol or (20S)-3beta-acetoxy-pregn-5-en-20-carboxylic acid. The obtained compounds are useful as standards for studies of cholic acids. Construction of the side chain was achieved by linkage of steroidal 23-iodides to sulfones prepared from (2R)- and (2S)-3-hydroxy-2-methylpropanoates. Oxidation of intermediate 26-alcohols into the corresponding carboxylic acids ensuring preservation of stereochemistry at C-25 and functional groups in the cyclic part was achieved with sodium chlorite catalyzed by TEMPO and bleach.

Gene structure of pig sterol 12alpha-hydroxylase (CYP8B1) and expression in fetal liver: comparison with expression of taurochenodeoxycholic acid 6alpha-hydroxylase (CYP4A21)

Cholic acid is the major trihydroxy bile acid formed in most mammals. The domestic pig (Sus scrofa) is an exception. The bile of adult pig is devoid of cholic acid whereas hyocholic acid is found in amounts equal to that of cholic acid in humans. The pathway leading to formation of hyocholic acid is believed to be species-specific and to have evolved in the pig to compensate for a nonexistent or deficient cholic acid biosynthesis. However, a high level of cholic acid has recently been found in the bile of fetal pig. Here we describe that a gene encoding the key enzyme in cholic acid biosynthesis, the sterol 12alpha-hydroxylase (CYP8B1), is in fact present in the pig genome. The deduced amino acid sequence shows 81% identity to the human and rabbit orthologues. CYP8B1 mRNA is expressed at significant levels in fetal pig liver. Both CYP8B1 and the key enzyme in hyocholic acid formation, taurochenodeoxycholic acid 6alpha-hydroxylase (CYP4A21), were found to be expressed in pig liver in a developmental-dependent but opposite fashion.

Bile acid synthesis in primary cultures of rat and human hepatocytes

The regulation of hepatic bile acid formation is incompletely understood. Primary cultures of mammalian hepatocytes offer an opportunity to examine putative regulatory factors in relative isolation. Using rat and human hepatocytes in primary culture, we examined bile acid composition and the expression of the rate-limiting enzyme of formation, cholesterol 7alpha-hydroxylase. Control rat hepatocytes showed a declining bile acid production over 4 days, from 156 +/- 24 ng/mL (67% cholic acid) on day 1 to 55 +/- 11 ng/mL (55% cholic acid) on day 4. In addition to cholic acid, chenodeoxycholic acid, alpha-muricholic acid, and beta-muricholic acid were formed. Treatment with triidothyronine (T3) or dexamethasone alone had no significant effect on bile acid production. A combination of T3 and dexamethasone significantly increased the total bile acid production on day 4 (224 +/- 54 ng/mL) and resulted in a marked change in composition to 23% cholic acid and 77% non-12alpha-hydroxylated bile acids. Control rat hepatocytes had a cholesterol 7alpha-hydroxylase activity of 3.3 +/- 0.6 pmol/mg protein/min after 4 days in culture. Cells treated with the combination of dexamethasone and T3 had an activity of 16.4 +/- 3.6 pmol/mg protein/min. The cholesterol 7alpha-hydroxylase messenger RNA (mRNA) levels, determined by solution hybridization after 4 days of culture, showed results similar to those for the activity data; control cells had 5.3 +/- 0.9 cpm/microg total nucleic acids (tNAs). T3 or dexamethasone-treated cells did not differ from control cells, whereas the combination of T3 and dexamethasone increased the mRNA levels to 20.6 +/- 2.8 cpm/microg tNAs. In human hepatocytes, isolated from donor liver, bile acid formation increased from 206 +/- 79 ng/mL on day 2 to 1490 +/- 594 ng/mL on day 6 and then declined slightly. Cholic acid and chenodeoxycholic acid were formed, constituting about 80% and 20%, respectively. The combined addition of T3 and dexamethasone had a tendency to decrease rather than increase bile acid formation. Also, mRNA levels of the cholesterol 7alpha-hydroxylase increased severalfold in the human hepatocytes from day 2 to day 4 and then declined. The addition of T3 or dexamethasone did not effect the mRNA levels in any consistent way. It is noteworthy that the capacity of the cultured human hepatocytes to produce bile acids was higher than that of cultured rat hepatocytes, in spite of the fact that the production of bile acids in rat liver is 3- to 5-fold higher than that in human liver in vivo. It is also evident that while hormonal factors appear to regulate bile acid synthesis in the rat, no evidence for this was found in human hepatocytes. As the composition of bile acids secreted by human hepatocytes in primary culture closely resembles that found in vivo, this represents a useful model for further studies of the synthesis and regulation of bile acids.

Cholic acid synthesis is reduced in pediatric liver recipients during graft dysfunction due to ischemic injury and allograft rejection

BACKGROUND

Bile acids are synthesized and secreted by the liver. During liver failure and hepatic dysfunction, a marked reduction of bile acid synthesis has been shown. The purpose of this study was to determine whether the biliary bile acid pattern was affected by preservation injury and rejection and whether it was a reliable marker for graft function in pediatric liver recipients after liver transplantation.

METHODS

We prospectively measured the biliary bile acid pattern in 126 serial bile samples obtained from 15 consecutive pediatric liver recipients by reversed phase high pressure liquid chromatography and correlated our results with clinical findings: preservation injury, no rejection, rejection, or infection.

RESULTS

There was a significant change of the bile acid pattern during the first 3 days after transplant. Total biliary bile acids, cholic acid (CA), and CA/chenodeoxycholic acid (CDCA) ratio increased in 12 of 15 patients with mild preservation injury. These changes of the bile acid pattern were markedly delayed in patients with severe preservation injury. During 16 rejection episodes, total biliary bile acid, CA, and CA/CDCA ratio decreased significantly, but returned to normal after successful treatment of rejection. Bacterial infection, observed in nine children, and cyclosporine toxicity, observed in three children, seemed to have no affect on the biliary bile acids.

CONCLUSIONS

Liver cell damage as a result of preservation injury or rejection leads to a reduction of biliary CA, resulting in a decrease of total biliary bile acids and the CA/CDCA ratio in pediatric liver recipients. This might be caused by a diminished secretion of bile acids and by a decreased synthesis of bile acids.

Recombinant 2-enoyl-CoA hydratase derived from rat peroxisomal multifunctional enzyme 2: role of the hydratase reaction in bile acid synthesis

Rat liver peroxisomes contain two multifunctional enzymes: (1) perMFE-1 [2-enoyl-CoA hydratase 1/Delta3,Delta2-enoyl-CoA isomerase/(S)-3-hydroxyacyl-CoA dehydrogenase] and (2) perMFE-2 [2-enoyl-CoA hydratase 2/(R)-3-hydroxyacyl-CoA dehydrogenase]. To investigate the role of the hydratase activity of perMFE-2 in beta-oxidation, a truncated version of perMFE-2 was expressed in Escherichia coli as a recombinant protein. The protein catalyses the hydration of straight-chain (2E)-enoyl-CoAs to (3R)-hydroxyacyl-CoAs, but it is devoid of hydratase 1 [(2E)-enoyl-CoA to (3S)-hydroxyacyl-CoA] and (3R)-hydroxyacyl-CoA dehydrogenase activities. The purified enzyme (46 kDa hydratase 2) can be stored as an active enzyme for at least half a year. The recombinant enzyme hydrates (24E)-3alpha,7alpha,12alpha-trihydroxy- 5beta-cholest-24-enoyl-CoA to (24R,25R)-3alpha,7alpha,12alpha, 24-tetrahydroxy-5beta-cholestanoyl-CoA, which has previously been characterized as a physiological intermediate in bile acid synthesis. The stereochemistry of the products indicates that the hydration reaction catalysed by the enzyme proceeds via a syn mechanism. A monofunctional 2-enoyl-CoA hydratase 2 has not been observed as a wild-type protein. The recombinant 46 kDa hydratase 2 described here survives in a purified form under storage, thus being the first protein of this type amenable to application as a tool in metabolic studies.

Effect of the side-chain structure on the specificity of beta-oxidation in bile acid biosynthesis in rat liver homogenates

3Alpha, 7alpha, 12alpha-trihydroxy-5beta-cholestan-26-oic acid (C27-5beta-cholestanoic acid) derivatives with different carbon-number side chains were incubated with rat liver 800 g supernatant to study the effect of the side-chain length on the beta-oxidation system in bile acid biosynthesis. The intermediate alpha, beta-unsaturated and beta-hydroxylated bile acids, and the corresponding degradation products, were quantitatively determined by gas chromatography. The longer side-chained derivatives (C28- and C29-5beta-cholestanoic acids) were converted into corresponding sidechain degradation products, and the alpha,beta-unsaturated and beta-hydroxylated intermediates were also produced. On the other hand, the shorter side-chained derivative (C26-5beta-cholestanoic acid) only gave alpha,beta-unsaturated intermediate. The total formation of intermediates and degradation products from corresponding substrates was in the order of C27- > C28- > C29- > C26-5beta-cholestanoic acids. In the case of clofibrate-treated rat liver 800 g supernatant, the formation of intermediates and final degradation products from C28- and C29-5beta-cholestanoic acids increased significantly. These longer side-chained analogues seemed to be subjected to beta-oxidation system(s) induced with clofibrate treatment. The effect of a terminal methyl group in the side chain of 5beta-cholestanoic acid on the oxidation system was also investigated using 3alpha, 7alpha, 12alpha-trihydroxy-27-nor-5beta-cholestanoic acid derivatives as enzymatic substrates. These derivatives gave corresponding side chain degradation products, but the formation of intermediates was not detected. The formation of side chain cleavage products from 27-nor-5beta-cholestanoic acid derivatives increased to 10 to 25-fold that of the controls by treatment with clofibrate. The results suggested that the beta-oxidation system for 27-nor-5beta-cholestanoic acid derivatives was different from that for C27-5beta-cholestanoic acid, despite their bile acid steroidal structure.

Bile acid metabolism in young-old parabiotic rats

Serum cholesterol, triglyceride and phospholipid levels, liver cholesterol concentration, bile flow, biliary cholesterol, phospholipid and bile acid secretion rates, fecal sterol and bile acid levels and their bile acid compositions were examined in young-old parabiotic rats and compared with those in young and old control rats and young-young parabiotic rats. Bile acid composition was expressed in terms of the cholic acid group/chenodeoxycholic acid group (CA/CDCA) ratio. Body weight (BW) gain decreased after parabiosis especially in old rats, but the liver weight (g/100 g BW), diet-intake, feces dry weight, liver cholesterol concentration and fecal sterol level were almost the same in all the groups. The biliary bile acid secretion rate was higher and the fecal bile acid level was lower in old rats than those in young rats but both the levels became comparable with those in young rats after parabiosis of old rats with young rats. Young rats, however, showed no changes in these levels after parabiosis. The serum cholesterol level and the biliary and fecal CA/CDCA ratios in old rats were higher than those in young rats but decreased after parabiosis with young rats, although they were still higher than those in young rats. The serum cholesterol level in young rats increased after parabiosis with old rats, but not after parabiosis with young rats, and the fecal bile acid level and the CA/CDCA ratio were not changed in either case. It is concluded from these findings that the serum cholesterol level and the CA/CDCA ratio increased with age and that these increases were prevented after parabiosis with young rats while young rats, although their serum cholesterol level was increased, showed no increase in the CA/CDCA ratio after parabiosis with old rats.

Disruption of cholesterol 7alpha-hydroxylase gene in mice. I. Postnatal lethality reversed by bile acid and vitamin supplementation

Mice deficient in cholesterol 7alpha-hydroxylase, the rate-limiting enzyme of bile acid biosynthesis, were constructed by targeted disruption of the Cyp7 gene. The introduced mutation removed exons 3-5 of the gene and gave rise to a null allele that encoded no immunoreactive or enzymatically active protein. Heterozygous carriers of the disrupted gene (Cyp7+/-) were phenotypically normal. Homozygous animals (Cyp7-/-) appeared normal at birth, but died within the first 18 days of life. Approximately 40% of the animals died between postnatal days 1 and 4 and 45% between days 11 and 18. The addition of vitamins to the water of nursing mothers prevented deaths in the early period, whereas the addition of cholic acid to chow prevented deaths in the later period. Newborn Cyp7-/- mice whose mothers were maintained on unsupplemented chow failed to gain weight at a normal rate and developed oily coats, hyperkeratosis, and apparent vision defects. These symptoms waned at 3 weeks of life, and their disappearance was accompanied by a marked increase in survival. In the accompanying study, the induction of an alternate pathway of bile acid biosynthesis is shown to underlie this unusual time course (Schwarz, M., Lund, E. G., Setchell, K. D. R., Kayden, H. J., Zerwekh, J. E., Björkhem, I., Herz, J., and Russell, D. W. (1996) J. Biol. Chem. 271, 18024-18031). We conclude that cholesterol 7alpha-hydroxylase is an essential enzyme for normal postnatal development.

Differential effects of chylomicron remnants derived from corn oil or palm oil on bile acid synthesis and very low density lipoprotein secretion in cultured rat hepatocytes

The effects of chylomicron remnants derived from corn oil (rich in n-6 polyunsaturated fatty acids) and palm oil (rich in long chain saturated fatty acids) on bile acid synthesis and very low density lipoprotein secretion in cultured rat hepatocytes were studied. Incubation of the cells with corn oil remnants led to increased bile acid production, while the secretion of lipid in very low density lipoprotein remained unchanged. In contrast, addition of palm oil remnants to the medium did not affect bile acid synthesis, but resulted in the secretion of cholesterol-rich very low density lipoprotein. These findings show that chylomicron remnants of different fatty acid composition have differential effects on cholesterol metabolism in liver cells, and provide part of the explanation for the hyper- and hypocholesterolaemic effects of saturated and polyunsaturated fatty acids.

Formation of cholic acid and chenodeoxycholic acid from 7 alpha-hydroxycholesterol and 27-hydroxycholesterol by primary cultures of human hepatocytes

It has been suggested that chenodeoxycholic acid is preferentially formed by the alternative or 'acidic' pathway of bile acid biosynthesis starting with 27-hydroxylation of cholesterol, while cholic acid is derived from 7 alpha-hydroxycholesterol which initiates the 'neutral' pathway. We have studied bile acid formation from each of these precursors using human hepatocytes cultured in a novel sandwich collagen configuration. Culture supernatants were analyzed using capillary gas chromatography and gas chromatography-mass spectrometry. 27-Hydroxycholesterol and 7 alpha-hydroxycholesterol were both found to be efficiently converted to cholic acid as well as chenodeoxycholic acid. Analysis of acidic intermediates after addition of 7 alpha-hydroxycholesterol to the cultures revealed a significant increase of side-chain oxygenated C24- and C27-steroids with a 3-oxo-7 alpha-hydroxy-delta 4-ring structure. These data indicate that (i) the 'neutral' pathway is connected to the 'acidic' pathway by side-chain oxidation of C27-steroids with a 3-oxo-7 alpha-hydroxy-delta 4-ring structure and that (ii) the relative formation of cholic acid and chenodeoxycholic acid is regulated by metabolic events distal to the initial hydroxylation at either position 7 or position 27 of the cholesterol molecule.

Antibacterial substance produced by Streptococcus faecium under anaerobic culture

A facultative anaerobe isolated from Korean domestic soil produced an antibacterial substance under strict anaerobic conditions. Based on the morphological and biochemical tests, and cellular fatty acid profiles, the anaerobe was identified as Streptococcus faecium. An antimicrobial compound produced from the S. faecium was identified as 3,7,12-trihydroxy-24-cholanic acid methylester on the basis of its physico-chemical analysis. This substance had potent antibacterial activities against a test organism harboring multiple antibiotic resistance markers, and a variety of pathogenic bacteria. The isolated S. faecium produced lactic acid as well as the antibiotic compound under the anaerobic conditions.

Cytochrome P450 CYP27-catalyzed oxidation of C27-steroid into C27-acid

Rabbit liver cytochrome P450 CYP27 has been previously shown to catalyze the complete conversion of 5 beta-cholestane-3 alpha,7 alpha,12 alpha-triol into 3 alpha,7 alpha,12 alpha-trihydroxy-5 beta-cholestanoic acid. This study compares some properties of the reactions involved, the 27-hydroxylation of 5 beta-cholestane-3 alpha,7 alpha,12 alpha-triol and the further oxidation of 5 beta-cholestane-3 alpha,7 alpha,12 alpha,27-tetrol. The Km was the same for the two substrates, whereas the Vmax was three times higher for 27-hydroxylation than for the oxidation of 5 beta-cholestane-3 alpha,7 alpha,12 alpha,27-tetrol. Ketoconazole inhibited both reactions, whereas disulfiram did not. Carbon monoxide inhibited the 27-hydroxylation of 5 beta-cholestane-3 alpha,7 alpha,12 alpha-triol but not the further oxidation of 5 beta-cholestane-3 alpha,7 alpha,12 alpha,27-tetrol. There was no difference in sensitivity to varying oxygen concentrations between the two reactions. The present study shows that CYP27 also converts, although less efficiently, 5 beta-cholestane-3 alpha,7 alpha-diol into 3 alpha,7 alpha-dihydroxy-5 beta-cholestanoic acid and cholesterol into 3 beta-hydroxy-5-cholestanoic acid. The rate of oxidation of cholesterol into C27-acid was very low--less than 1% of that with the other C27-steroids.

Changes in biliary lipid secretion and cholic acid kinetics induced by diet, diet plus simvastatin and diet plus ursodeoxycholic acid in obese subjects

The aim of this work was to evaluate and compare the effects of a low calorie diet (1026 kcal), simvastatin and ursodeoxycholic acid administration on biliary lipid secretion and cholic acid kinetics in dieting obese subjects. We studied 6 obese subjects before and after four weeks of a hypocaloric diet alone, after four weeks of diet plus ursodeoxycholic acid (900 mg/day) and after four weeks of diet plus simvastatin (40 mg/day), according to a Latin square design. The cholesterol saturation index was increased after diet alone, significantly reduced with diet plus ursodeoxycholic acid (p < 0.01), and unchanged during simvastatin administration. While the cholesterol output was reduced by all three regimens, diet plus ursodeoxycholic acid caused a significantly greater decrease than diet alone (p < 0.01). Cholic acid synthesis and bile acid secretion were decreased by diet and diet plus simvastatin (p < 0.05), but neither was affected by ursodeoxycholic acid. For cholic acid, all three treatments, but especially diet alone and diet plus simvastatin (p < 0.05), reduced the pool size; all three regimens also increased the turnover rate, but this was significant only for ursodeoxycholic acid (p < 0.01). Our study shows that, in obese patients, a hypocaloric diet reduces cholesterol-holding biliary lipid output and consequently increases the cholesterol saturation index. The addition of simvastatin to a hypocaloric dietary regimen reduces cholesterol secretion, but without variation in bile acid and phospholipid output thus the cholesterol saturation index remains unchanged. When ursodeoxycholic acid is added to the dietary regimen, it reduces cholesterol secretion, while maintaining bile acid output and, thus, lowers the cholesterol saturation index. Unlike simvastatin, ursodeoxycholic acid prevents the drop in cholic acid synthesis induced by a low calorie diet.

Deoxycholate and cholate modulate the source of cholesterol substrate for bile acid synthesis in the rat

In the current study, the role of the supply of preformed and newly synthesized cholesterol for the feedback control of the synthesis of different bile acids and the secretion of biliary cholesterol was investigated. To define these cholesterol fluxes and the possibility of a different modulation by bile acids with different suppressive capacities, a continuous labeling with tritiated water was used in rats with an extracorporeal bile duct receiving intraduodenal infusions of taurocholate or taurocholate plus deoxycholate. After bile acid pool depletion (6 to 9 hours) total muricholate, cholate, and chenodeoxycholate synthesis was variably increased (24% to 93%) during an infusion of 304 mumol taurocholate/kg per hour. The increase in bile acid synthesis and biliary cholesterol output was predominantly due to the utilization of preformed (unlabeled) cholesterol. The addition of 52 mumol/kg per hour of deoxycholate to 258 mumol/kg per hour of taurocholate had a comparable effect. In the late period (30 to 54 hours), the taurocholate infusion had little impact on total muricholate and chenodeoxycholate synthesis but caused by a significant increase of the proportion from performed cholesterol. Both total cholate production and its synthesis from de novo (labeled) cholesterol was inhibited by 30% (P < .05) and 64% (P < .01), respectively. The secretion rate of total and de novo biliary cholesterol was higher (65% and 72%; P < .01) compared with controls. In comparison, the combined bile acid infusion led to a further increase of total muricholate synthesis (P < .05), which was again due to an enhanced synthesis from performed cholesterol (P < .001). Similar changes were observed in chenodeoxycholate. The more pronounced suppression of total cholate synthesis by 81% (P < .05) was due to a diminished cholate synthesis from both de novo cholesterol by 72% (P < .001) and preformed cholesterol by 91% (P > .05). We conclude that the modulation of the synthesis of the various primary bile acids in the rat differs and feedback regulation of cholate synthesis by taurocholate and deoxycholate is mediated by different mechanisms of control, including inhibition of cholesterol 7 alpha-hydroxylase, HMG-CoA reductase, and uptake of lipoprotein cholesterol.

Bile acid synthesis in HepG2 cells: effect of cyclosporin

The hypothesis that cyclosporin specifically affects the pathway of bile acid synthesis that begins with 27-hydroxylation of cholesterol was evaluated in HepG2 cells, which synthesize chenodeoxycholic acid and cholic acid from endogenous 7 alpha-hydroxycholesterol. At a concentration in the medium of 8.3 microM cyclosporin, the proportion of cholic acid increased from 29 +/- 7% to 44 +/- 6% (P < 0.001) with no major change in total bile acid production. Chenodeoxycholic acid synthesis was enhanced by the addition of either 7 alpha-hydroxycholesterol or 5 beta-cholestane-3 alpha,7 alpha-diol to the medium and cholic acid synthesis was enhanced by the addition of 5 beta-cholestane-3 alpha,7 alpha,12 alpha-triol to the medium. Cyclosporin significantly inhibited only enhanced chenodeoxycholic acid synthesis, indicating a selective interference in mitochondrial side chain oxidation of less polar intermediates in bile acid synthesis derived from either initial 7 alpha- or initial 27-hydroxylation of cholesterol. The increase in the proportion of cholic acid that occurs in the presence of cyclosporin mimics that occurring in genetically determined sterol 27-hydroxylase deficiency (cerebrotendinous xanthomatosis). Cyclosporin is useful for dissecting the subcellular pathways of bile acid synthesis.

Non-stereoselective formation of 3 alpha,7 alpha,12 alpha,24-tetrahydroxy-5 beta-cholestan-26-oic acid during cholic acid biosynthesis

Incubation of (25RS)-, (25R)- and (25S)-3 alpha,7 alpha,12 alpha-trihydroxy-5 beta-cholestan-26-oic acid (THCA, 6, 6a, 6b) and (24E)-3 alpha,7 alpha,12 alpha-trihydroxy-5 beta-cholest-24-en-26-oic acid (7) with rat liver mitochondria gave all four stereoisomers (9a,9b,9c,9d) of 3 alpha,7 alpha,12 alpha,24-Tetrahydroxy-5 beta-cholestan-26-oic acid (TeHCA). The corresponding 27-nor analogs (10,11) were also converted non-stereoselectively to a 1:1 mixture of the epimeric 24-hydroxy compounds (12).

Bile acid and very low density lipoprotein production by cultured hepatocytes from hypo- or hyperresponsive rabbits fed cholesterol

Two groups of rabbits, either hyperresponsive or hyporesponsive to dietary cholesterol, were selected after ten weeks of cholesterol feeding (0.2 g cholesterol/kg body weight per day). Bile acids and very low density lipoprotein (VLDL) production were determined in primary hepatocyte cultures from control, hyper- and hyporesponsive rabbits. Free cholesterol and cholesteryl ester contents in hepatocytes of the hyperresponsive rabbits was significantly increased. In contrast, lipid composition in hepatocytes of the hyporesponders was similar to that of control cells. Cholic acid was the predominant bile acid in the culture medium of hepatocytes together with small amounts of chenodeoxycholic and deoxycholic acids. The rate of cholic acid production by hepatocytes in the hyporesponsive group was two times higher than that in the hyperresponsive group. Bile acid production by control hepatocytes was slightly higher than in the hyperresponsive group. In contrast, secretion of VLDL cholesteryl ester was significantly increased by hepatocytes of the hyperresponsive rabbits. Similar differences in bile acid production were found between hypo- and hyperresponsive rabbits selected after five days of cholesterol feeding and subsequent maintenance on a low cholesterol diet for a period of one month. The results suggest that the increased rate of bile acid production could contribute to the apparent resistance of hyporesponders to the atherogenic diet.

Effect of dietary n-3 versus n-6 polyunsaturated fatty acids on hepatic excretion of cholesterol in the hamster

Dietary polyunsaturated fatty acids of the n-6 and the n-3 class show differing effects on serum lipids and hepatic lipoprotein metabolism, which could be induced by alterations in hepatocellular cholesterol balance. As both fatty acid classes exert parallel effects on lipoprotein uptake and synthesis of cholesterol in the liver, we studied whether they have differing effects on the excretory pathways for cholesterol. Male Syrian hamsters were fed for 3 weeks low-cholesterol diets supplemented (9% w/w) with either saturated (coconut fat), n-6 unsaturated (safflower oil) or n-3 unsaturated fatty acids (fish oil), which shifted the serum lipid levels. N-6 unsaturated fatty acids increased both the synthesis of cholic acid (+57%; P = 0.05) and, in fistula bile, the secretion of cholesterol (+37%; P < 0.05 vs. saturated fatty acids). By contrast, n-3 unsaturated fatty acids did not enhance synthesis of cholic acid or biliary secretion of cholesterol (-30%, NS). The fatty acid pattern of biliary phospholipids was modified according to the major unsaturated fatty acids in the diet. The alterations both in phospholipid fatty acid composition and in secretory ratio of cholesterol to phospholipids and bile acids persisted during controlled secretion of taurocholic acid at increasing rates. In conclusion, hepatic excretion of cholesterol is increased on dietary n-6 unsaturated fatty acids, and low on n-3 unsaturated fatty acids. These two dietary fatty acid classes change differently the fatty acid composition of biliary phospholipids and the secretory ratio of cholesterol to phospholipids and bile acids in bile.

Bile acid synthesis from newly synthesized vs. preformed cholesterol precursor pools in the rat

The present study defines the origin of cholesterol subserving bile acid synthesis in male rats with an extracorporal bile duct by labeling newly formed cholesterol with tritiated water. Within 6 hr after interruption of the enterohepatic circulation, the bile acid pool was depleted. At this early time point the proportion from de novo cholesterol was 8% and 12% for biliary cholesterol and cholate, but 18% and 19% for muricholate and chenodeoxycholate, respectively. This proportion gradually rose to 40%, 34%, 51% and 51%, respectively, at 15 to 30 hr. At 78 hr after bile diversion, 64% of cholate was labeled, compared with 84% to 88% of the other biliary lipids and 71% of plasma cholesterol. Total and labeled bile acid secretion exhibited the same diurnal rhythm. To allow differentiation between direct hepatocytic de novo synthesis of bile acids from acetate and recycling of labeled plasma cholesterol, an inhibitor of 3-hydroxy-3-methylglutaryl coenzyme A reductase (pravastatin) was infused from 54 to 78 hr. It suppressed total synthesis of primary bile acids by 60% to 80% but decreased the tritium label of bile acids only from a range of 74% to 92% (54 hr) to a range of 54% to 63% (78 hr), which was in the range of plasma cholesterol (58%). We conclude that bile acids and biliary cholesterol are synthesized mostly from preformed (i.e., plasma) cholesterol, both immediately after depletion of the pool in enterohepatic circulation and after derepression. Moreover, the hepatic cholesterol pools subserving the synthesis of different bile acids and biliary cholesterol secretion are not identical.

Formation of cholic acid from 3 alpha, 7 alpha, 12 alpha-trihydroxy-5 beta-cholestanoic acid in human skin fibroblasts

Whether 3 alpha, 7 alpha, 12 alpha-trihydroxy-5 beta-cholestanoic acid (THCA) was converted into cholic acid in human skin fibroblasts was examined. THCA was incubated with subcellular fractions of cultured skin fibroblasts in the presence of NAD+, ATP, CoA, and Mg2+. The reaction products were analyzed by thin-layer chromatography and high-performance liquid chromatography after p-bromophenacyl ester derivatization. The highest specific activity was found in the light mitochondrial fraction (2.71 nmol/mg protein/h). The specific activity was about 9-fold higher than that in heavy mitochondrial fraction. The peroxisomal fraction prepared from the light mitochondrial fraction by sucrose gradient centrifugation was also able to catalyze the conversion of THCA into cholic acid. The specific activity in this fraction was a further 2.2-fold higher than that in the light mitochondrial fraction. These results suggest that cultured human skin fibroblasts are able to convert THCA into cholic acid, and that the activity exists in peroxisomes.

Enterohepatic circulation in hamsters with an extracorporeal bile duct

The present study describes a novel technique for investigations of the enterohepatic circulation in the hamster with an extracorporeal bile duct that allows long-term bile collection in the free-moving animal. The animals recovered for 7 days after the operation before the external loop was cut and bile was collected over a period of 78 h. Under these optimal conditions, initial bile flow (651 +/- 89 microliters per 100 g.h-1) and the secretion rates of biliary lipids were several-fold higher than reported in an earlier study using the acute fistula hamster. Biliary cholesterol secretion amounted to 369 +/- 32 nmol per 100 g.h-1, phospholipid secretion was 2.6 +/- 0.3 mumol per 100 g.h-1, and total bile acid secretion was 31.9 +/- 2.2 mumol per 100 g.h-1. A clearcut diurnal rhythm was demonstrated for bile flow and all biliary constituents. After 9 h the depletion of the bile acid pool was complete and cholic acid synthesis derepressed 1.4-fold from a basal rate of 818 nmol per 100 g.h-1, whereas the derepression of chenodeoxycholic acid synthesis was even less pronounced. Biliary cholesterol output increased 2.2-fold, but the phospholipid secretion was constant during the full experiment. It may be concluded that the technique of an extracorporeal bile duct in the free-moving animal allows studies of bile secretion under optimal conditions. Most likely the bile secretion rates given above approach the physiological rates in the hamster.

Effect of simvastatin, ursodeoxycholic acid and simvastatin plus ursodeoxycholic acid on biliary lipid secretion and cholic acid kinetics in nonfamilial hypercholesterolemia

It has been recently shown that the newest hypocholesterolemic agent, simvastatin, lowers the biliary cholesterol saturation index and that its association with ursodeoxycholic acid renders it more effective. To determine the mechanism by which simvastatin decreases the biliary cholesterol saturation index, we evaluated hepatic secretion rates of cholesterol, bile acids and phospholipids, and cholic acid pool size, turnover and synthesis in eight hyperlipidemic patients (five women and three men, age range = 38 to 65 yr). These assessments were conducted before treatment, after 4 wk of simvastatin (40 mg/day), after 4 wk of ursodeoxycholic acid (600 mg/day) and after a further 4 wk of a combination therapy of simvastatin (40 mg/day) plus ursodeoxycholic acid (600 mg/day). The cholesterol saturation index was significantly reduced with simvastatin (from 1.51 +/- 0.10 to 0.94 +/- 0.05, mean +/- S.E.; p less than 0.02), with ursodeoxycholic acid (from 1.51 +/- 0.10 to 0.86 +/- 0.03, mean +/- S.E.; p less than 0.02) and with the combination of simvastatin plus ursodeoxycholic acid (from 1.51 +/- 0.01 to 0.70 +/- 0.05, p less than 0.02). The cholesterol saturation index during combination therapy was significantly lower (p less than 0.02) than that reached during the use of simvastatin and ursodeoxycholic acid. Both simvastatin and ursodeoxycholic acid significantly reduced the hepatic secretion rate of cholesterol (from 130 +/- 14 mumols/hr to 81 +/- 12 mumols/hr, p less than 0.01, and 70 +/- 9 mumols/hr, p less than 0.01) without affecting bile acid and phospholipid outputs.(ABSTRACT TRUNCATED AT 250 WORDS)

Alteration of bile acid metabolism in two-thirds hepatectomized rat

Bile acid metabolism after two-thirds partial hepatectomy (PH) in rat was studied. Bile acid kinetics (i.e. synthesis rate and pool size) were determined by wash out method combined with gas liquid chromatography, and serum bile acids by gas liquid chromatography-mass spectrometry. Serum bile acid concentration was highest on the third day after PH, as the liver regeneration progressed but it gradually decreased with increasing cholic acid (CA)/chenodeoxycholic acid (CDCA), reflecting impaired hepatic uptake of bile acids and/or cholestasis during the early post-hepatectomy period. Predominance of CA in bile acid synthesis, pool, and biliary secretion was also found. On the third day after PH, liver weight recovered to 66% of the control value, but enhancement of bile acid synthesis was not observed. Consequently, pool size remained at 50% of control. On the seventh day, synthesis of bile acid, especially of CA, was enhanced and pool size and liver weight returned to 68 and 72% of the respective control values. Bile acid synthesis was returned to the control value on the fourteenth day with concomitant restoration of liver weight and bile acid pool size. These changes in bile acid kinetics parallel the events during hepatic regeneration after PH.

Biosynthesis of cholic acid accelerated by diabetes: its mechanism and effect of vanadate administration

[7 beta-3H]3 alpha,7 alpha-Dihydroxy-5 beta-cholestane was infused into the femoral vein of bile-fistula rats. The main metabolites in bile were cholic, chenodeoxycholic, and alpha-muricholic acids. Cholic acid accounted for only 7.5% of the total biliary [3H]bile acids in the normal rats, but 51.8% in the diabetic rats. This increased proportion of cholic acid was partially cancelled by treatment of the diabetic rats with insulin (29.8%) or vanadate (28.8%). These results clearly confirm that an alternative biosynthetic pathway of cholic acid via 3 alpha,7 alpha-dihydroxy-5 beta-cholestane is accelerated by diabetes, and indicate that vanadate has an insulin-like effect on the formation of cholic acid in an insulin-deficient state.

Potential bile acid precursors in plasma--possible indicators of biosynthetic pathways to cholic and chenodeoxycholic acids in man

The plasma concentrations of 3 beta-hydroxy-5-cholestenoic acid, 3 beta,7 alpha-dihydroxy-5-cholestenoic acid and 7 alpha-hydroxy-3-oxo-4-cholestenoic acid have been compared with that of 7 alpha-hydroxy-4-cholesten-3-one in healthy subjects and in patients with an expected decrease or increase of the bile acid production. In controls and patients with liver disease, the level of 7 alpha-hydroxy-3-oxo-4-cholestenoic acid was positively correlated to that of 3 beta,7 alpha-dihydroxy-5-cholestenoic acid and not to that of 7 alpha-hydroxy-4-cholesten-3-one. In patients with stimulated bile acid formation the levels of the acids were not correlated to each other but there was a significant positive correlation between the levels of 7 alpha-hydroxy-3-oxo-4-cholestenoic acid and 7 alpha-hydroxy-4-cholesten-3-one. These findings indicate that the precursor of 7 alpha-hydroxy-3-oxo-4-cholestenoic acid differs depending on the activity of cholesterol 7 alpha-hydroxylase. Since the activity of this enzyme is reflected by the level of 7 alpha-hydroxy-4-cholesten-3-one in plasma the findings are compatible with a formation of 7 alpha-hydroxy-3-oxo-4-cholestenoic acid from 3 beta,7 alpha-dihydroxy-5-cholestenoic acid when the rate of bile acid formation is normal or reduced and from 7 alpha-hydroxy-4-cholesten-3-one under conditions of increased bile acid synthesis. In support of this interpretation, 7 alpha,26-dihydroxy-4-cholesten-3-one was identified at elevated levels in plasma from patients with ileal resection or treated with cholestyramine. The levels of 7 alpha,12 alpha-dihydroxy-4-cholesten-3-one were also higher than normal in these patients. Based on these findings and previous knowledge, a model is proposed for the biosynthesis of bile acids in man. Under normal conditions, two major pathways, one "neutral" and one "acidic" or "26-oxygenated", lead to the formation of cholic acid and chenodeoxycholic acid, respectively. These pathways are separately regulated. When the activity of cholesterol 7 alpha-hydroxylase is high, the "neutral" pathway is most important whereas the reverse is true when cholesterol 7 alpha-hydroxylase activity is low. In cases with enhanced activity of cholesterol 7 alpha-hydroxylase, the "neutral" pathway is connected to the "acidic" pathway via 7 alpha,26-dihydroxy-4-cholesten-3-one, whereas a flow from the acidic pathway to cholic acid appears to be of minor importance.

Oxidation of 5 beta-cholestane-3 alpha,7 alpha, 12 alpha-triol into 3 alpha,7 alpha,12 alpha-trihydroxy-5 beta-cholestanoic acid by cytochrome P-450(26) from rabbit liver mitochondria

The mitochondrial cytochrome P-450(26), previously shown to catalyze 26-hydroxylation of 5 beta-cholestane-3 alpha, 7 alpha, 12 alpha-triol, was found to convert this substrate also into 3 alpha,7 alpha,12 alpha-trihydroxy-5 beta-cholestanoic acid. The formation of 3 alpha,7 alpha,12 alpha-trihydroxy-5 beta-cholestanoic acid increased with increasing incubation time and enzyme concentration. Addition of NAD+ to the incubation mixture did not increase the formation of the acid. Incubation with 5 beta-cholestane-3 alpha,7 alpha,12 alpha,26-tetrol, cytochrome P-450(26), ferredoxin, ferredoxin reductase and NADPH resulted in one major product, 3 alpha,7 alpha, 12 alpha-trihydroxy-5 beta-cholestanoic acid. The cytochrome P-450 required both ferredoxin, ferredoxin reductase and NADPH for activity. NADPH could not be replaced by NAD+ or NADP+.

[Cholate-forming function of the liver in patients with chronic alcoholism]

The level of bile acids was studied in the bile and feces in 119 patients with chronic alcoholism using thin-layer chromatography. The cholate-forming function of the liver was found to increase in patients with short alcohol anamnesis in chronic alcoholism. With advance of the alcohol anamnesis the number of patients with reduced cholate-forming function of the liver increased.

Effects of cholecystectomy on the kinetics of primary and secondary bile acids

Removal of the gallbladder is thought to increase formation and pool size of secondary bile acids, mainly deoxycholic acid (DCA), by increased exposure of primary bile acids (cholic acid [CA], chenodeoxycholic acid [CDCA]) to bacterial dehydroxylation in the intestine. We have tested this hypothesis by simultaneous determination of pool size and turnover of DCA, CA, and CDCA in nine women before and at various intervals after removal of a functioning gallbladder. An isotope dilution technique using marker bile acids labeled with stable isotopes (2H4-DCA, 13C-CA, 13C-CDCA) was used. After cholecystectomy, concentration and output of bile acids relative to bilirubin increased (P less than 0.02) in fasting duodenal bile and cholesterol saturation decreased by 27% (P less than 0.05) consistent with enhanced enterohepatic cycling of bile acids. Three months after removal of the gallbladder bile acid kinetics were in a new steady state: pool size and turnover of CDCA were unchanged. Synthesis of CA, the precursor of DCA, was diminished by 37% (P = 0.05), probably resulting from feedback inhibition by continuous transhepatic flux of bile acids. The fraction of CA transferred after 7 alpha-dehydroxylation to the DCA pool increased from 46 +/- 16 to 66 +/- 32% (P less than 0.05). However, this enhanced transfer did not lead to increased input or size of the DCA pool, because synthesis of the precursor CA had decreased.

Cholic acid synthesis from 26-hydroxycholesterol and 3-hydroxy-5-cholestenoic acid in the rabbit

Intravenous administration of 26-hydroxycholesterol to the rabbit with a bile fistula yielded cholic acid in proportions (84 and 86%) not significantly different from that derived from cholesterol. By contrast, the naturally occurring C27 bile acid 3 beta-hydroxy-5-cholestenoic acid yielded not more than 8% cholic acid. Thus initial 26-hydroxylation of cholesterol followed by 7-alpha-hydroxylation can provide sufficient amounts of cholic acid to be considered a quantitatively significant pathway for bile acid synthesis, and in addition it is the only pathway that can be the source of the circulating levels of C24 and C27 monohydroxy bile acids.

Bile acid synthesis and secretion by rabbit hepatocytes in primary monolayer culture: comparison with rat hepatocytes

Rabbit hepatocytes isolated after liver perfusion with collagenase were maintained in primary monolayer culture for periods up to 96 h. Bile acid synthesis and secretion was measured by capillary gas-liquid chromatography and by a rapid enzymatic-bioluminescence assay. As expected from the bile acid profile of rabbit gallbladder bile, cholic acid was the only bile acid synthesized in detectable amounts and was produced at a linear rate of 170 pmol/h per mg cell protein from 24 to 96 h in culture. Ketoconazole (20 microM) inhibited cholic acid synthesis and secretion by 78%, whereas the bile acids chenodeoxycholic acid (100 microM), deoxycholic acid (100 microM) or lithocholic acid (2 microM) had no effect. When rat hepatocytes were cultured under identical conditions, the rate of bile acid synthesis was found to be only 12 pmol/h per mg cell protein, a value in agreement with previous work. The large difference in rates of bile acid synthesis between rabbit and rat hepatocytes may be due to rapid loss of cytochrome P-450 from rat hepatocytes when placed in monolayer culture. Although reportedly active in cholesterol 7 alpha-hydroxylation, form 4 cytochrome P-450 levels in rabbit hepatocytes did not correlate with rates of bile acid synthesis.

Increased rate of cholic acid formation from 3 alpha,7 alpha-dihydroxy-5 beta-cholestane in perfused livers from diabetic rats

The formation of bile acids from 3 alpha,7 alpha-dihydroxy-5 beta-cholestane was comparatively investigated in the perfused livers from normal, streptozotocin-diabetic, and insulin-treated diabetic rats. During the infusion of the sterol substrate, cholic acid accounted for only 11% of the total biliary bile acids in the normal rats, while it increased drastically to 57% in the diabetic rats. This increased proportion of cholic acid tended to be normalized by treatment of the diabetic animals with insulin. These results suggest the possible acceleration of cholic acid formation, involving 12 alpha-hydroxylation of 3 alpha,7 alpha-dihydroxy-5 beta-cholestane or one of its metabolites in an insulin-deficient state in rats.

Bile acid metabolism in hereditary forms of hypertriglyceridemia: evidence for an increased synthesis rate in monogenic familial hypertriglyceridemia

This study was undertaken to characterize bile acid metabolism in hereditary forms of hypertriglyceridemia. Ten hypertriglyceridemic patients (type IV phenotype) with familial combined hyperlipidemia and 7 patients with monogenic familial hypertriglyceridemia (FHTG) were compared with 18 healthy controls; all subjects were males. Pool size, synthesis rate, and fractional catabolic rate of cholic and chenodeoxycholic acids were determined with an isotope dilution technique. Patients with FHTG had synthesis rates of cholic acid, chenodeoxycholic acid, and total bile acids above those seen in normal controls (P less than 0.001); also the fractional catabolic rates of both bile acids were increased (P less than 0.001). In contrast, bile acid kinetic parameters were--with one exception--within normal limits in patients with familial combined hyperlipidemia. The abnormality of bile acid metabolism could also be identified in a normolipidemic individual presumed to carry the gene for FHTG. The postprandial rise of serum bile acids was blunted in FHTG, indicating that the intestinal uptake of bile acids may be deficient in this condition. We conclude that FHTG, but not familial combined hyperlipidemia, is frequently associated with a defective regulation of bile acid synthesis, resulting in abnormally high production rate of bile acids. It is hypothesized that this abnormality is important for the subsequent development of hypertriglyceridemia.

Isocholic acid formation from 7 alpha,12 alpha-dihydroxy-3-keto-5 beta-cholanoic acid with human liver enzyme

The formation of isocholic acid from 7 alpha, 12 alpha-dihydroxy-3-keto-5 beta-cholanoic acid by human liver preparations was examined in vitro. Liver preparations were incubated with 7 alpha, 12 alpha-dihydroxy-3-keto-5 beta-cholanoic acid at pH 7.4 in a phosphate buffer containing NADPH or NADH. The products formed were analyzed by gas chromatography and gas chromatography/mass spectrometry. Results showed that 7 alpha,12 alpha-dihydroxy-3-keto-5 beta-cholanoic acid was reduced mainly to isocholic acid and to cholic acid in a smaller amount in the presence of NADPH, while it was reduced only to cholic acid in the presence of NADH. The reducing enzyme participating in the formation of isocholic acid was localized largely in the cytosol and had more specificity to the unconjugated form as substrate than to the conjugated forms. 3-Keto bile acid analogues, 3-keto-5 beta-cholanoic and 7 alpha-hydroxy-3-keto-5 beta-cholanoic acids were not reduced to the corresponding iso-bile acids by the cytosol in the same conditions used in the isocholic acid formation and the activity of the enzyme catalyzing the reduction of 7 alpha,12 alpha-dihydroxy-3-keto-5 beta-cholanoic acid to isocholic acid was not inhibited by the addition of 3-keto-5 beta-cholanoic acid or 7 alpha-hydroxy-3-keto-5 beta-cholanoic acid to the reaction mixture. Furthermore, on column chromatography of Affi-Gel Blue, the peak of the enzyme catalyzing the reduction of 7 alpha,12 alpha-dihydroxy-3-keto-5 beta-cholanoic acid to isocholic acid was clearly distinguished from that of the enzyme catalyzing the reduction of 3-keto-5 beta-cholanoic acid to isolithocholic acid and that of alcohol dehydrogenase. These results indicate that this enzyme catalyzing the reduction of 7 alpha,12 alpha-dihydroxy-3-keto-5 beta-cholanoic acid to isocholic acid is different from the enzyme(s) catalyzing the reduction 3-keto-5 beta-cholanoic and 7 alpha-hydroxy-3-keto-5 beta-cholanoic acids to the corresponding iso-bile acids and from alcohol dehydrogenase, and has a stereospecific character for 7 alpha,12 alpha-dihydroxy-3-keto-5 beta-cholanoic acid.

Gas chromatography-mass spectrometry of isobutyl ester trimethylsilyl ether derivatives of bile acids and application to the study of bile sterol and bile acid biosynthesis in rat liver epithelial cell lines

The derivatization of bile acids into trimethylsilyl ether isobutyl ester (IBTMS) and of neutral sterols into trimethylsilyl ether (TMS) allowed the separation on an OV-1 capillary gas chromatography column of 15 bile steroids as follows: cholesterol, 7 alpha-hydroxycholesterol, 6 beta-hydroxycholesterol, 6 alpha-hydroxycholesterol, 7 beta-hydroxycholesterol, lithocholate, deoxycholate, 25-hydroxycholesterol, chenodeoxycholate, cholate, murocholate, hyodeoxycholate, ursodeoxycholate, hyocholate, and beta-muricholate. Fragmentation data of the coupled gas chromatographic-mass spectrometric (GC-MS) analysis of these nine bile acids as IBTMS derivatives under electron impact and chemical ionizations (methane, isobutane, and ammonia) are given. The ammonia chemical ionization appears to be the best mode for compound identification and quantitation due to fragmentations into high mass ions. The comparison of methylene units of the five sterols as TMS derivatives and of each type of methyl, TMS, or isobutyl ester of the nine bile acids as TMS ethers showed that isobutyl esterification increased dramatically the retention time of the bile acids, allowing their separation after the neutral sterols. Different methods of GC-MS analysis were applied to the study of bile steroid secretion in long-term rat liver epithelial cell lines, either serum-supplemented cell lines or serum-free cell lines, growing in serum-free medium since the primary explanation or after adaptation of serum-supplemented lines to this medium. It is demonstrated for the first time that liver epithelial cell lines maintain the metabolic pathway leading from synthesized cholesterol to dioxygenated sterols and the two normal main primary bile acids of the liver, chenodeoxycholic acid and cholic acid, up to 32-47% of the in vivo daily rate, and in addition the production of alpha-muricholic acid, the bile acid marker of murine liver.

In vitro formation of bile acids from di- and trihydroxy-5 beta-cholestanoic acid in human liver peroxisomes

The conversion of 3 alpha,7 alpha,12 alpha-trihydroxy-5 beta-[3H]cholestanoic acid into cholic acid and 3 alpha,7 alpha-dihydroxy-5 beta-[3H]cholestanoic acid into chenodeoxycholic acid has been studied in subcellular fractions of human liver. The products were separated from the substrates by high-pressure liquid chromatography and identified by combined gas chromatography-mass spectrometry. The highest rates of conversion were found in the light mitochondrial fraction. This fraction also contained the highest amount of the marker enzymes for peroxisomes. The maximal rates of cholic acid and chenodeoxycholic acid formation were 1.3 and 1.8 nmol/mg protein per h, respectively. The presence of KCN in the incubation medium stimulated the formation of bile acids. Peroxisomes were prepared from the light mitochondrial fraction by sucrose-gradient centrifugation. By use of different marker enzymes, it was confirmed that the major part of the activity for cholic acid formation in the light mitochondrial fraction was located in the peroxisomes. It is concluded that liver peroxisomes are important for the oxidative cleavage of the C27 steroid side chain in bile acid formation in man.

Formation of chenodeoxycholic acid from 3 alpha, 7 alpha-dihydroxy-5 beta-cholestanoic acid by rat liver peroxisomes

The oxidation of the side chain of 3 alpha, 7 alpha-dihydroxy-5 beta-cholestanoic acid (DHCA) into chenodeoxycholic acid has been studied in subcellular fractions of rat liver. The product was separated from the substrate by high pressure liquid chromatography and identified by gas-liquid chromatography-mass spectrometry. The highest specific rate of conversion was found in the heavy (M) and the light (L) mitochondrial fractions with the highest enrichment in the L fraction. Washing the M fraction reduced the side chain cleavage activity by 90%. The peroxisomal marker enzyme urate oxidase was reduced to the same extent. The activity found in the M fraction may thus be due to peroxisomal contamination. After centrifugation of the L fraction on a Nycodenz density gradient, the highest specific activity for side chain cleavage of DHCA (31 nmol X mg-1 X h-1) was found in the fraction with the highest peroxisomal marker enzyme activity. This fraction also catalyzed conversion of 3 alpha,7 alpha,12 alpha-5 beta-cholestanoic acid (THCA) into cholic acid at the highest rate (32 nmol X mg-1 X h-1). The peroxisomal oxidation of DHCA into chenodeoxycholic acid required the presence of ATP, CoA, Mg2+, and NAD in the incubation medium. The reaction was not inhibited by KCN. It is concluded that rat liver peroxisomes contain enzymes able to catalyze the cleavage of the side chain of both DHCA and THCA. The enzymes involved are similar to, but not necessarily identical to, those involved in the peroxisomal beta-oxidation of fatty acids.

Ursodeoxycholic acid treatment in humans: effects on plasma and biliary lipid metabolism with special reference to very low density lipoprotein triglyceride and bile acid kinetics

Ursodeoxycholic acid reduces biliary saturation with cholesterol and may induce dissolution of cholesterol gallstones in man. In order to characterize the effects of this potentially useful bile acid on plasma lipid metabolism, we determined lipoprotein levels and very low density lipoprotein (VLDL) triglyceride kinetics in six hypertriglyceridaemic and three normolipidaemic subjects before and after 4-6 weeks of ursodeoxycholic acid treatment at a daily dose of 15 mg kg-1 body weight. The plasma levels of low density lipoprotein (LDL), high density lipoprotein (HDL) and total cholesterol were not significantly affected by therapy. Nor were the plasma level and apparent formation of VLDL triglycerides changed. In five subjects, the effects of a low dose (7.5 mg kg-1 body weight day-1 for 4-6 weeks) of ursodeoxycholic acid on biliary lipid composition and kinetics of cholic acid and chenodeoxycholic acid were determined. The relative concentration of cholesterol in bile was reduced to the same level as during treatment with a high dose of ursodeoxycholic acid. The synthesis rates of bile acids were not suppressed with ursodeoxycholic acid. It is concluded that, unlike chenodeoxycholic acid, ursodeoxycholic acid does not suppress endogenous bile acid production. The efficiency at lower doses, and the lack of effects on plasma lipid metabolism, may make ursodeoxycholic acid a more attractive alternative for clinical attempts of gallstone dissolution.

Bile acids and plasma high density lipoproteins: biliary lipid metabolism in fish eye disease

Cholesterol in plasma high density lipoproteins (HDL) has been proposed to serve as preferential precursor for bile acid biosynthesis in the liver. Furthermore, a negative relationship between plasma levels of HDL cholesterol and biliary saturation with cholesterol has been reported in healthy females. We have performed metabolic studies on a female patient with fish eye disease, a familial condition where plasma HDL levels are reduced by 90% and the concentration of plasma triglycerides is moderately increased. Both the total production of bile acids and the net steroid 'balance' (reflecting total body cholesterol synthesis) were within the range seen in normolipidaemic as well as in hypertriglyceridaemic females. Also the biliary lipid composition and cholesterol saturation of bile were normal. A qualitative abnormality in the bile acid pattern was observed, however, in that the ratio between the synthesis of cholic acid and chenodeoxycholic acid was reduced. It is concluded that a low HDL cholesterol level is not necessarily associated with quantitative abnormalities of biliary lipid metabolism. The abnormal bile acid synthesis ratio may reflect changes in the hepatic precursor pools of cholesterol as the consequence of HDL deficiency, however.

Hepatocyte-hepatoma cell hybrids. Characterization and demonstration of bile acid synthesis

Hybrids were created by fusion of primary rat hepatocytes with well-differentiated Reuber H35 rat hepatoma cells. Seventeen hybrids were screened for bile acid synthesis using [26-14C]cholesterol. As [26-14C]cholesterol was converted to bile acid, 14CO2 was released. Using this assay, four hybrids (8B, 12C, 13C, and 13D) were identified which synthesized bile acid. These four hybrids also incorporated [14C]taurine into bile acid. Bile acids were identified by capillary gas chromatography/mass spectrometry, and their rates of synthesis were quantitated by isotope dilution. Reuber H35 cells synthesized little or no bile acid. However, hybrids 8B, 12C, 13C, and 13D synthesized chenodeoxycholic acid, alpha-muricholic acid, and cholic acid and secreted them into the media. The rates of synthesis of individual bile acids varied among these hybrids. For example, the relative percentage of cholic acid ranged from 11.1% (hybrid 8B) to 50.4% (hybrid 13C). The bile acids synthesized and secreted by the most active hybrid, 12C, were greater than 93% conjugated. In summary, hybrids were created that retain the capacity to synthesize, conjugate, and secrete three major rat bile acid species. Such hybrids are unique model systems that will allow the study of the biochemical and genetic regulation of bile acid synthesis.

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.