METHYL-BRANCHED FATTY-ACID BIOSYNTHESIS IN THE GERMAN-COCKROACH, BLATTELLA-GERMANICA - KINETIC-STUDIES COMPARING A MICROSOMAL AND SOLUBLE FATTY-ACID SYNTHETASE

The fatty acid synthetase (FAS) activity of integument-enriched tissue from the German cockroach, Blattella germanica, was separated into soluble and microsomal forms. Both the soluble and microsomal FAS incorporated methylmalonyl-CoA into fatty acids, but their kinetic characteristics were quite different. The specific activity for soluble FAS was 144 +/- 7 nmol NADPH oxidized/min/mg protein and for microsomal FAS it was 20 +/- 1 nmol/min/mg. Both have similar Michaelis constants. There was a clear-cut substrate preference: the soluble FAS had almost no activity with methylmalonyl-CoA as the only elongating substrate whereas the microsomal FAS readily utilized methylmalonyl-CoA in the absence of malonyl-CoA. The specific activity with methylmalonyl-CoA for the microsomal FAS was 7 +/- 1 nmol/min/mg. Methylmalonyl-CoA was a competitive inhibitor against malonyl-CoA for both the soluble and microsomal FAS, which indicated that methylmalonyl-CoA can bind to both forms of the enzyme. Methylmalonyl-CoA was a non-competitive inhibitor against acetyl-CoA for both the soluble and microsomal FAS. In the presence of malonyl-CoA, the microsomal FAS had a much higher ability to incorporate methylmalonyl-CoA into fatty acid than did the soluble FAS. The data indicated that the microsomal FAS from integument tissue plays a key role in methyl-branched fatty acid synthesis by incorporating methylmalonyl-CoA into growing fatty acid chains.