Characterization of a lipid-rich fraction synthesized by Streptomyces avermitilis

Isolation of the macrocyclic lactone parasiticide avermectin and other closely related natural products produced by Streptomyces avermitilis also yields a lipid-rich fraction. The latter has been characterized by techniques based on gas-liquid chromatography (GLC) and mass spectrometry (MS). Initial examination of the lipid-rich fraction by direct probe electron-impact (EI) MS and packed-column GLC showed that it consists primarily of a mixture of triglycerides possessing C14-C17 acyl groups. Further examination of this fraction by capillary column GLC-MS demonstrated that it contains low levels of C15-C17 free fatty acids, squalene and diglycerides and, as the major components, at least ten mixed acyl triglycerides (total number of acyl carbon atoms ranging from 43 to 50). Prominent among the triglycerides were a C15-C15-C16 species, a C15-C16-C16 species and a C15-C16-C17 species. Capillary-column GLC and GLC-MS of the fatty acid methyl esters resulting from transesterification demonstrated that the major triglyceride acyl groups are anteiso-C15 (12-methyltetradecanoyl), iso-C16 (14-methylpentadecanoyl), n-C16 (hexa-decanoyl) and anteiso-C17 (14-methylhexadecanoyl). Lower levels of the methyl esters of the following fatty acids were observed: iso-C14 (12-methyltridecanoic), n-C14 (tetradecanoic), iso-C15 (13-methyltetradecanoic), n-C15 (pentadecanoic), iso-C17 (15-methylhexadecanoic) and n-C17 (heptadecanoic). Little evidence was seen for either unsaturated acyl groups or acyl groups of less than 13 or more than 18 carbon atoms. Desorption chemical ionization MS (ammonia reagent gas) analysis confirmed the nature of the lipid-rich fraction, and is an attractive one-step approach for determining the molecular weights and distribution of triglycerides in a mixture.