Further studies on the biosynthesis of the avermectins

Summary The biosynthesis of avermectins was studied further inStreptomyces avermitilis MA5502 by feeding experiments with labeled precursors.¹³C-NMR analysis of the compounds biosynthesized from 2-¹³C]acetate, 1,2-¹³C₂]acetate, 3-¹³C]propionate and 2,3-¹³C₂]propionate confirmed that the aglycone of avermectins is made from seven intact acetate and five propionate units. Feeding experiments with 1-¹³C]2-methylbutyrate and 1-¹³C]isobutyrate have shown that 2-methylbutyrate and isobutyrate are immediate precursors of the starter units of the polyketide chains of avermectin a and b components, respectively. The³H/¹⁴C doublelabeling experiments suggest that the two oleandrose moieties are derived from glucose.