Stereochemistry of the alpha-oxidation of 3-methyl-branched fatty acids inrat liver

The stereochemistry of the alpha-oxidation of 3-methyl-branched fatty acids was studied in rat liver. R- and S-3-methylhexadecanoic acid were equally well alpha-oxidized in intact hepatocytes and homogenates, Subcellular frac tionation studies showed that alpha-oxidation of both isomers is confined t o peroxisomes, Dehydrogenation of 2-methylpentadecanal, the end-product of the peroxisomal alpha-oxidation of 3-methylhexadecanoic acid, to 2-methylpe ntadecanoic acid, followed by derivatization with R-1-phenylethylamine and subsequent separation of the stereoisomers by gas chromatography, revealed that the configuration of the methyl-branch is preserved throughout the who le alpha-oxidation process. Metabolism and formation of the 2-hydroxy-3-met hylhexadecanoyl-CoA intermediate were also investigated. Separation of the methyl esters of the four isomers of 2-hydroxy-3-methylhexadecanoic acid wa s achieved by gas chromatography after derivatization of the hydroxy group with R-2-methoxy-2-trifluoromethylphenylacetic acid chloride and the absolu te configuration of the four isomers was determined. Although purified pero xisomes are capable of metabolizing all four isomers of 2-hydroxy-3-methylh exadecanoyl-CoA, they can only form the (2S,3R) and the (2R,3S) isomers. Ou r experiments exclude the racemization of the 3-methyl branch during the al pha-oxidation process. The configuration of the 3-methyl branch does not in fluence the rate of alpha-oxidation, but determines the side of the 2-hydro xylation, hence the configuration of the 2-hydroxy-3-methylacyl-CoA interme diates formed during the process.