Biosynthesis of the avermectins by Streptomyces avermitilis. Incorporation of labeled precursors.

The biosynthesis of the avermectins, a group of 16 membered macrolides with potent anthelmintic and insecticidal activity produced by Streptomyces avermitilis, was studied by supplying cultures with 14C and 13C precursors. [1-14C] and [2-14C]acetate and propionate were poor precursors of the avermectins and were instead rapidly oxidized to 14CO2. The S-methyl of methionine in contrast was incorporated extensively and equally into the three methoxyl groups of the avermectins. The carbon backbone of methionine was not a precursor of the avermectins. Feeding of [1-13C]glucose yielded avermectins labeled specifically in the C1' and C1" of the oleandrose moiety and in the aglycone moiety in carbons known to be derived from the methyl of acetate. Feeding [U-13C]glucose showed that the entire avermectin molecule is derived from glucose carbons.