A 13C NMR study of methyl-branched hydrocarbon biosynthesis in the housefly

Abstract

Carbon 13 NMR, radiotracer and mass spectrometry studies were performed to confirm that propionate and methylmalonate are incorporated into long chain branched hydrocarbons as the methyl branch unit, to determine whether the branching methyl group was added initially or toward the end of the elongation process, to determine the precursor of methylmalonate, and to examine the metabolism of propionate in the housefly, Musca domestica. The labeled carbons from [3-13C]propionate and [methyl-13C]methylmalonate were incorporated into the branching methyl carbon of the methylalkanes, and into the even numbered carbons in alkanes and alkenes. The labeled carbons from [2-13C]propionate and [1-13C]propionate labeled the tertiary carbon and the carbon adjacent to the teritary carbon, respectively, in the methylalkanes. Mass spectral analysis of the methylalkanes after enrichment from [1-13C]propionate showed that propionate was incorporated during the initial stages of chain synthesis of the terminally branched alkanes. Sodium [2-13C]propionate labeled the odd numbered carbons of both the alkanes and alkenes. These data suggest that propionate is converted to an acetyl derivative, with carbon 3 of propionate converted to the carboxyl carbon of acetate, and carbon 2 of propionate converted to the methyl carbon of acetate. This pathway of propionate metabolism has hitherto only been reported in plants. The labeled carbons from DL-[3,4,5-13C3]valine were incorporated intact (as determined by 13C--13C coupling) into the branching methyl carbon, tertiary carbon, and carbon adjacent to the tertiary carbon, respectively, demonstrating that valine serves as a precursor to the methylmalonate used in branched alkane biosynthesis.