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

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 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.

Synthesis of diastereomers of 3 alpha,7 alpha,12 alpha, 24-tetrahydroxy- and 3 alpha,7 alpha,24-trihydroxy-5 beta-cholestan- 26-oic acids and their structures

Four stereoisomers of 3 alpha,7 alpha,12 alpha,24-tetrahydroxy-5 beta-cholestan-26-oic acids were synthesized as possible intermediates of the side-chain degradation step of bile acid biosynthesis. 3 alpha,7 alpha,12 alpha-Trihydroxy-5 beta-cholest-25-en-24-one prepared by thermolysis of beta-ketosulfoxide was reduced to the (24R)- and (24S)-allylic alcohols by reduction with sodium borohydride. Each isomeric alcohol was subjected to hydroboration and oxidation to give (25R)- and (25S)-3 alpha,7 alpha,12 alpha,24,26-pentahydroxy-5 beta-cholestanes. The separated four stereoisomers were converted into the corresponding 26-carboxylic acids. The stereoisomers of 3 alpha,7 alpha,24-trihydroxy-5 beta-cholestan-26-oic acids were synthesized in the same manner. To establish the stereochemistry of these carboxylic acids, the chemical transformation of methyl 3 alpha,7 alpha,12 alpha-trihydroxy- and 3 alpha,7 alpha-dihydroxy-5 beta-cholest-24-en-26-oates into the above stereoisomers and the reductive dehydroxylation of the 24-hydroxyl group into known 3 alpha,7 alpha,12 alpha,26-tetrahydroxy- and 3 alpha,7 alpha,26-trihydroxy-5 beta-cholestanes are described. The applications of spectroscopic methods (circular dichroism and 1H nuclear magnetic resonance) to elucidation of the stereochemistry are also discussed.