PHYSIOLOGICAL-ROLE OF D-3-HYDROXYACYL-COA DEHYDRATASE D-3-HYDROXYACYL-COA DEHYDROGENASE BIFUNCTIONAL PROTEIN

The second and third reactions of the peroxisomal beta-oxidation spira l are thought to be catalyzed by enoyl-CoA hydratase/L-3-hydroxyacyl-C oA dehydrogenase bifunctional protein (L-bifunctional protein), Recent ly, we found the presence of D-3-hydroxyacyl-CoA dehydratase/D-3-hydro xyacyl-CoA dehydrogenase bifunctional protein (D-bifunctional protein) in mammalian peroxisomes. Therefore, we studied the physiological rol e of the D-bifunctional protein, The contents of the L- and D-bifuncti onal proteins were about 0.01 and 0.5 mu g/mg protein, respectively, i n cultured human skin fibroblasts. The activity of conversion of hexad ecenoyl-CoA to 3-ketopalmitoyl-CoA by the D-bifunctional protein was e stimated to be about 0.5 milliunit/mg of fibroblast protein, This valu e was about 100-fold that of the L-bifunctional protein in the fibrobl asts. From comparison of the activities of the bifunctional proteins w ith the rate of palmitate oxidation and the activities of acyl-CoA oxi dase and 3-ketoacyl-CoA thiolase, it is proposed that the D-bifunction al protein plays a major role in the peroxisomal oxidation of palmitat e in the fibroblasts. The contents of both the L- and D-bifunctional p roteins in liver were about 2.5 mu g/mg protein, Therefore, it is sugg ested that the D-bifunctional protein also plays a significant role in human liver peroxisomal fatty acid oxidation, Actions of the bifuncti onal proteins on enoyl forms of other acyl-CoA derivatives were examin ed, The D-bifunctional protein but not the L-bifunctional protein reac ted with 2-methylhexadecenoyl-CoA and 3 alpha,7 alpha,12 alpha-trihydr oxy-5 beta-cholest-24-enoyl-CoA. We propose that, among the reactions of the distinct group of carboxylates oxidized specifically in peroxis omes, oxidation of 2-methyl-branched fatty acids and side-chain shorte ning of cholesterol for bile acid formation are catalyzed by the D-bif unctional protein, but not the L-bifunctional protein.