METHYL-BRANCHED FATTY-ACIDS AND THEIR BIOSYNTHESIS IN THE HOUSEFLY, MUSCA-DOMESTICA L (DIPTERA, MUSCIDAE)

Eighteen methyl-branched fatty acids from the housefly, Musca domestica L. (Diptera:Muscidae) were identified by gas chromatography-mass spectrometry after reduction to the corresponding hydrocarbons. A deuterium was inserted on what was the carboxyl carbon and the straight chain components were removed by molecular sieve. The deuterium allowed the mass spectral determination of the methyl branch position with respect to the carboxyl end of the parent fatty acid. The methyl-branched fatty acids were characterized as n-2, n-3, n-4, n-5, n-6, n-7, n-8 and n-9 monomethyl fatty acids of 15-19 carbons and a n-3,7 dimethyl fatty acid of 18 total carbons. These methyl-branched fatty acids have similar branching patterns and are presumed to be the precursors to the methyl-branched hydrocarbons, some of which function as an arrestant in the female sex pheromone. With increasing concentrations of methylmalonyl-CoA, its incorporation into methyl-branched fatty acids increased in both day 1 and day 4 males and females using both microsomal and soluble fatty acid synthases (FAS). Methylmalonyl-CoA inhibited both the soluble and microsomal FAS in a competitive manner. The data on the incorporation of methylmalonyl-CoA into methyl-branched fatty acids by day 1 and day 4 males and females indicate that the regulation of methyl-branched hydrocarbon synthesis does not reside at the level of fatty acid synthesis, but must occur during the process of the fatty acyl-CoA elongation or reductive conversion of very long chain fatty acyl-CoAs to hydrocarbons.