Stereochemistry of the α-oxidation of 3-methyl-branched fatty acids in rat liver

Abstract

The stereochemistry of the α-oxidation of 3-methyl-branched fatty acids was studied in rat liver. R- and S-3-methylhexadecanoic acid were equally well α-oxidized in intact hepatocytes and homogenates. Subcellular fractionation studies showed that α-oxidation of both isomers is confined to peroxisomes. Dehydrogenation of 2-methylpentadecanal, the end-product of the peroxisomal α-oxidation of 3-methylhexadecanoic acid, to 2-methylpentadecanoic acid, followed by derivatization with R-1-phenylethylamine and subsequent separation of the stereoisomers by gas chromatography, revealed that the configuration of the methylbranch is preserved throughout the whole α-oxidation process. Metabolism and formation of the 2-hydroxy-3-methylhexadecanoyl-CoA intermediate were also investigated. Separation of the methyl esters of the four isomers of 2-hydroxy-3-methylhexadecanoic acid was achieved by gas chromatography after derivatization of the hydroxy group with R-2-methoxy-2-trifluoromethylphenylacetic acid chloride and the absolute configuration of the four isomers was determined. Although purified peroxisomes are capable of metabolizing all four isomers of 2-hydroxy-3-methylhexadecanoyl-CoA, they can only form the (2S,3R) and the (2R,3S) isomers. Our experiments exclude the racemization of the 3-methyl branch during the α-oxidation process. The configuration of the 3-methyl branch does not influence the rate of α-oxidation, but determines the side of the 2-hydroxylation, hence the configuration of the 2-hydroxy-3-methylacyl-CoA intermediates formed during the process.—Croes, K., M. Casteels, M. Dieuaide-Noubhani, G. P. Mannaerts, and P. P. Van Veldhoven. Stereochemistry of the α-oxidation of 3-methyl-branched fatty acids in rat liver. J. Lipid Res. 1999. 40: 601–609.

Keywords

• peroxisome
• phytanic acid
• Refsum's disease
• hydroxylation