Biosynthesis of canescin, a metabolite of Aspergillus malignus: incorporation of methionine, acetate, succinate, and isocoumarin precursors, labelled with deuterium and carbon-13

Abstract

The biosynthesis of canescin (1), a metabolite of Aspergillus malignus, has been studied using [Me13C2H3] methionine, sodium [1,2-13C2]- and [1-13C, 2-2H3]acetate, and sodium [2,3-13C2]- and [2-13C, 2-2H2]-succinate as simple precursors, and deuterium labelled isocoumarins as potential advanced precursors. Analysis of the labelled products by both n.m.r. and mass spectrometry suggests that 6,8-dihydroxy-3,7-dimethylisocoumarin (13) is the first enzyme-free intermediate produced by the polyketide synthase, and the 7-formyl-6,8-dihydroxy-3-methylisocoumarin (16) is a later intermediate on the pathway. Incorporation into canescin of 13C together with one deuterium atom from [13C2H3]methionine proves that the formyl group of (16) is derived from this source and that it is not oxidised to the level of a carboxylic acid in subsequent steps. The 13C labels in the succinate units are incorporated into the γ-lactone carbons (C-11 to C-13) of canescin, but with complete loss of the neighbouring deuterium label.