Lipid biosynthesis in the sebaceous glands. Synthesis of multibranched fatty acids from methylmalonyl-coenzyme A in cell-free preparations from the uropygial gland of goose
Cell-free extracts from the uropygial gland of goose catalyzed the incorporation of malonyl-CoA and methylmalonyl-CoA into n- and multi-branched fatty acids, respectively, with NADPH as the preferred reductant. Methylmalonyl-CoA was shown to be incorporated almost exclusively into the acyl portion of wax esters by the cell-free extract while malonyl-CoA was incorporated into polar lipids and both the acyl and alcohol portions of the wax. The optimal pH for the synthesis of both n- and multibranched acids was 6.0. Apparent Km and Vmax for malonyl-CoA were 2 times 10- minus-4 M and 250 nmol per min per mg, respectively, while the Km and Vmax for methylmalonyl-CoA were 7.7 times 10- minus-4 M and 0.8 nmol per min per mg, respectively with 105,000g supernatant; but partial purification resulted in a tenfold decrease in Km values. The partially purified synthetase preparation catalyzed the formation of n-C16 acid (80%) and n-C18 acid (20%) from acetyl-CoA and malonyl-CoA. With the same synthetase preparation and the appropriate primer methylmalonyl-CoA was converted into 2,4,6,8-tetramethyldecanoic acid and 2,4,6,8-tetramethylundecanoic acid which were identified by radio gas-liquid chromatography and combined gas chromatography-mass spectrometry. Experiments with an equimolecular mixture of acetyl-CoA and propionyl-CoA showed that the synthetase preferred acetyl-CoA as a primer. Since malonyl-CoA is known to be rapidly decarboxylated in the gland, acetyl-CoA and methylmalonyl-CoA are expected to be the major primer and elongating agent, respectively, available in the gland and therefore 2,4,6,8-tetramethyldecanoic acid should be the major product. Combined gas-liquid chromatography and mass spectrometry demonstrated that this acid was in fact the major acid of the gland.