Stereoisomeric inversion of (25R)- and (25S)-3 alpha,7 alpha,12 alpha-trihydroxy-5 beta-cholestanoic acids in rat liver peroxisome

Major biliary bile acids of the medaka (Oryzias latipes): 25R- and 25S-epimers of 3alpha,7alpha,12alpha-trihydroxy-5beta-cholestanoic acid

The biliary bile salts of the medaka, the Japanese rice fish (Oryzias latipes) were isolated and identified. Only bile acids were present, and all were N-acylamidated with taurine. Three bile acids, constituting 98% of total bile acids, were isolated by chromatography and their structure inferred from their properties compared to those of synthetic standards when analyzed by liquid chromatographytandem mass spectrometry. The dominant bile acid was the 25R-epimer (82%) of 3alpha,7alpha,12alpha-trihydroxy-5beta-cholestan-27-oic acid. The 25S-epimer was also present (11%), as was cholic acid (5%). Complete (1)H and (13)C NMR signal assignments of the C-25 epimers were made by using a combination of several 1D- and 2D-NMR techniques. The (1)H and (13)C NMR chemical shifts and spectral patterns of the hydrogen and carbon atoms, being close to the asymmetric centered at C-25, provided confirmatory evidence in that they distinguished the two epimeric diastereomers. The medaka is the first fish species identified as having C(27) biliary bile acids as dominant among its major bile salts.

A new, major C27 biliary bile acid in the red-winged tinamou (Rhynchotus rufescens):25R-1beta, 3alpha,7alpha-trihydroxy-5beta-cholestan-27-oic acid

The chemical structures of the three major bile acids present in the gallbladder bile of the Red-winged tinamou (Rhynchotus rufescens), an early evolving, ground-living bird related to ratites, were determined. Bile acids were isolated by preparative reversed-phase HPLC. Two of the compounds were identified as the taurine N-acylamidates of 25R-3alpha,7alpha-dihydroxy-5beta-cholestan-27-oic acid (constituting 22% of biliary bile acids) and 25R-3alpha,7alpha,12alpha-trihydroxy-5beta-cholestan-27-oic acid (constituting 51%). The remaining compound, constituting 21% of biliary bile acids, was an unknown C27 bile acid. Its structure was elucidated by LC/ESI-MS/MS and NMR and shown to be the taurine conjugate of 25R-1beta, 3alpha, 7alpha-trihydroxy-5beta-cholestan-27-oic acid, a C27 trihydroxy bile acid not previously reported. Although C27 bile acids with a 1beta-hydroxyl group have been identified as trace bile acids in the alligator, this is the first report of a major biliary C27 bile acid possessing a 1beta-hydroxyl group.

Characterization of cholyl-adenylate in rat liver microsomes by liquid chromatography/electrospray ionization-mass spectrometry

The cholyl-adenylate, covalently bound 3alpha, 7alpha, 12alpha-trihydroxy-5beta-cholanoic acid (cholic acid) with adenosine 5'-monophosphate having an acid anhydride linkage, has been characterized by means of liquid chromatography/mass spectrometry in an incubation mixture with a rat liver microsomal fraction. The authentic specimen of cholyl-adenylate was synthesized using the carbodiimide method and the structure was confirmed by MS and nuclear magnetic resonance spectroscopy. After incubation of cholic acid with a hepatic microsomal fraction in the presence of adenosine 5'-triphosphate, bile acids were extracted and purified by solid-phase extraction on a Sep-Pak C18 cartridge and then subjected to a LC/MS analysis, where cholyl-adenylate and a CoA thioester of cholic acid (cholyl-CoA) were monitored with characteristic negative ions of m/z 736 and 577, respectively. Cholyl-adenylate was definitely characterized and preferential biotransformation into the acyl-adenylate prior to formation of cholyl-CoA was noted. The nonenzymatic formation of taurine-conjugated cholic acid by incubation of cholyl-adenylate with taurine in a buffer solution was also demonstrated.

Substrate specificity of THCA-CoA oxidases from rat liver light mitochondrial fractions on dehydrogenation of 3 alpha,7 alpha,12 alpha-trihydroxy-5 beta-cholestanoic acid CoA thioester

The substrate specificity of rat liver peroxisomal 3 alpha,7 alpha,12 alpha-trihydroxy-5 beta-cholestanoyl-CoA (THCA-CoA) oxidases, which catalyze the dehydrogenation of 3 alpha,7 alpha,12 alpha-trihydroxy-5 beta-cholestanoic acid (THCA) CoA thioester, having an asymmetric center at C-25, to form (24E)-3 alpha,7 alpha,12 alpha-trihydroxy-5 beta-cholest-24-enoic acid (delta 24-THCA) CoA thioester, was studied. The stable isotope labeled substrates, [3,7,12-18O3]-(25R)- and (25S)-THCA CoA thioesters were synthesized by an exchange reaction of carbonyl oxygens on a steroid nucleus of 3,7,12-trioxo-5 beta-cholestanoic acid, followed by metal hydride reduction and condensation reaction with CoA. After incubation of a mixture of unlabeled (25R)- and 18O-labeled (25S)-THCA CoA thioester, or vice versa, with hepatic peroxisomal THCA-CoA oxidases, biotransformed delta 24-THCA was determined by liquid chromatography/atmospheric pressure chemical ionization mass spectrometry. The delta 24-THCA was derived only from (25S)-THCA CoA thioester, indicating that the 25S epimer of THCA is a preferential substrate on dehydrogenation by THCA-CoA oxidases.

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.

2-methylacyl racemase: a coupled assay based on the use of pristanoyl-CoA oxidase/peroxidase and reinvestigation of its subcellular distribution in rat and human liver

Because of the 2S-methyl-stereospecificity of the acyl-CoA oxidases acting on the CoA esters of 2-methyl-branched fatty carboxylates such as pristanic acid and the side chain of trihydroxycoprostanic acid (Van Veldhoven P.P., Croes K., Asselberghs S., Herdewijn P. and Mannaerts G.P. (1996) FEBS Lett. 388, 80-84), naturally occurring 2R-pristanic acid and 25R- (corresponding to 2R in the side chain) trihydroxycoprostanic acid, after activation to their CoA-esters, need to be racemized to the S-isomers before they can be degraded by peroxisomal beta-oxidation. A coupled assay to measure 2-methyl-acyl racemases was developed by using purified rat pristanoyl-CoA oxidase. Upon incubation of rat and human liver homogenates with 2R-methyl-pentadecanoyl-CoA, the formed 2S-methyl isomer was desaturated by an excess of added oxidase and the concomitant production of hydrogen peroxide was monitored by means of peroxidase in the presence of a suitable hydrogen donor. Application of this assay to subcellular fractions of rat liver revealed the presence of racemase activity not only in mitochondria, as described by Schmitz W., Albers C., Fingerhut R. and Conzelmann E. (Eur. J. Biochem. (1995) 231, 815-822), but also in peroxisomes and cytosol. A similar distribution was seen in human liver. In rat the highest activities were found in liver, followed by Harderian gland, kidney and intestinal mucosa.

Effect of the hydroxyl group on the oxidative cleavage (beta-oxidation) of steroidal side chain for bile acid biosynthesis in rat liver homogenate

Mono-, di-, tri-, and tetrahydroxy-5 beta-cholestan-26-oic acids were incubated with rat liver homogenate (800 x g supernatant and light mitochondrial fraction) to study substrate specificity in the side-chain cleavage reaction (beta-oxidation) of bile acid biosynthesis. The C27-intermediates (5 beta-cholest-24-en-26-oic acids and 24-hydroxy-5 beta-cholestan-26-oic acids) in beta-oxidation and the corresponding C24-bile acids were quantitatively determined by capillary gas chromatography. Monohydroxy-5 beta-cholestan-26-oic acid was not converted into C24-bile acid. Di- and trihydroxy-5 beta-cholestan-26-oic acids were effectively transformed into the C27-intermediates and C24-bile acids. Tetrahydroxy-5 beta-cholestan-26-oic acids were also converted into C27-intermediates and corresponding C24-bile acids. The intermediate 24-hydroxy-5 beta-cholestan-26-oic acids could not be detected in the products by incubation with the light mitochondrial fraction. The total specific activity of protein in the light mitochondrial fraction for the production of C27-intermediates and C24-bile acids was higher than that of 800 x g supernatant solution. The effects of the number and the position of hydroxyl groups on the side-chain degradation are discussed.

Analysis of stereoisomeric C27-bile acids by high performance liquid chromatography with fluorescence detection

A method for differentially measuring the 24-hydroxylated stereoisomeric intermediates (3 alpha,7 alpha,12 alpha,24-tetrahydroxy- and 3 alpha,7 alpha,24-trihydroxy-5 beta-cholestan-26-oic acids) and related C27-bile acids in beta-oxidation of bile acid biosynthesis has been developed by high performance liquid chromatography with fluorescence detection. The method involved the derivatization of the above intermediable C27-bile acids into fluorescent esters with 3-(4-bromomethylphenyl)-7-diethylaminocoumarin, a newly synthesized labeling reagent for carboxylic acids. The fluorescent derivatives were subjected to a short silica gel column to eliminate interfering products prior to analysis by high performance liquid chromatography. The separation of the 16 kinds of bile acids containing stereoisomers was carried out using a reversed-phase Inertsil C8-column by gradient elution and detected with a fluorometer (Ex. 400 nm, Em. 475 nm). The linearity of calibration curve for each bile acid was from 0.5 to 250 pmol (r = 0.999) and the detection limits were about 15 fmol at a signal-to-noise ratio of 3. The method was applied to the determination of intermediates in beta-oxidation of bile acid biosynthesis using rat liver homogenate. The results showed that two stereoisomers of 24-hydroxylated C27-bile acids were predominantly produced, indicating the formation of the isomers by the cis-hydration with water.

Stereochemistry of peroxisomal and mitochondrial beta-oxidation of alpha-methylacyl-CoAs

The stereochemistry of beta-oxidation of alpha-methyl-branched fatty acids was analyzed, in rat liver and in human cells, with (2R)- and (2S)-2-methyltetradecanoic acid as model substrates. In rat liver, formation of the alpha,beta-unsaturated compound was found to be concentrated in mitochondria while in human cells, this activity co-distributed mainly with peroxisomal marker enzymes. In both cases, the dehydrogenating enzymes were absolutely specific for the (2S)-enantiomer. In human liver, activation was some three times faster with the (2R)- than with the (2S)-isomer while in rat liver both were activated at about the same rate.

Peroxisomal beta-oxidation of 2-methyl-branched acyl-CoA esters: stereospecific recognition of the 2S-methyl compounds by trihydroxycoprostanoyl-CoA oxidase and pristanoyl-CoA oxidase

Trihydroxycoprostanoyl-CoA oxidase and pristanoyl-CoA oxidase, purified from rat liver, both catalyse the desaturation of 2-methyl-branched acyl-CoAs. Upon incubation with the pure isomers of 2-methylpentadecanoyl-CoA, both enzymes acted only on the S-isomer. The R-isomer inhibited trihydroxycoprostanoyl-CoA oxidase but did not affect pristanoyl-CoA oxidase. The activity of both enzymes was suppressed by 3-methylheptadecanoyl-CoA. Valproyl-CoA and 2-ethylhexanoyl-CoA, however, did not influence the oxidases. Although only one isomer of 25R,S-trihydroxycoprostanovl-CoA was desaturated by trihydroxycoprostanoyl-CoA oxidase, isolated peroxisomes were able to act on both isomers, suggesting the presence of a racemase in these organelles. Given the opposite stereoselectivity of the 26-cholesterol hydroxylase and of the oxidase, the racemase is essential for bile acid formation.