Biosynthesis of chlorflavonin in Aspergillus candidus. 13C- and 14C-labelling evidence for a new route to the flavonoid structure

Authors:
  • MBMichael K. Burns
    Michael K. Burns
  • JCJane M. Coffin
    Jane M. Coffin
  • IKItsuo Kurobane
    Itsuo Kurobane
  • LVLeo C. Vining
    Leo C. Vining
  • AMA. Gavin McInnes
    A. Gavin McInnes
  • DSDonald G. Smith
    Donald G. Smith
  • JWJohn A. Walter
    John A. Walter
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Abstract
Aspergillus candidus growing in a glucose–leucine–salts medium synthesized chlorflavonin (1) from isotopically labelled phenylalanine, cinnamate, benzoate, or acetate. Radioactivity from [β-14C]cinnamic acid and [α-14C]-benzoic acid was incorporated less efficiently than that from L-[β-14C]phenylalanine but decarboxylation of 3-chlorosalicylic acid (3) formed by alkaline degradation of the chlorflavonin samples located all of the radioactivity at C-2 in the flavonoid. 4,5-Dimethoxyresorcinol (2), obtained by alkaline degradation of chlorflavonin labelled from sodium [1-14C]acetate and [α-14C]cinnamic acid, accounted for only part of the radioactivity. The label from both precursors was distributed between ring A and one or more of the C-3, C-4, and C-3 methoxy-carbon atoms. 13C N.m.r. spectrometry of chlorflavonin labelled from sodium [1-13C]acetate showed 13C incorporation into C-4, C-5, C-7, and C-8a. Sodium [1-13C0;1;1,2-13C1;0;1]acetate was incorporated intact into (C-3,C-4) and all adjacent pairs of ring-A carbon atoms. The results indicate a pathway of flavonoid biosynthesis differing from that of higher plants in that a C6-C1 precursor unit is condensed with four C2 units. In the route proposed, the heterocyclic ring is formed before ring A is substituted at C-8 and while it is free to rotate at the enzyme surface.