Isoepoxydon, a new metabolite of the patulin pathway in Penicillium urticae

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Abstract
A patulin-negative mutant (J1) of P. urticae (N.R.R.L. 2159A) was known to accumulate about 100 mg/l quantities of the 5,6-epoxygentisyl quinone, (-)-phyllostine and another metabolite (UIII). Both were derived from acetate and hence were polyketides. Purified UIII (m.p. [melting point] 53.degree. C, .**GRAPHIC**. +206.degree., .**GRAPHIC**. 240 nm; .epsilon. 3806 l .cntdot. mol-1 .cntdot. cm-1) was characterized as a partially reduced derivative of (-)-phyllostine and was found to be a diastereoisomer of the known phytotoxin, (+)-epoxydon. Hence its designation as (+)-iso- or epi-epoxydon. From 1H NMR and CD [circular dichroism] data the stereochemistry of the epoxide ring in (+)-isoepoxydon was determined to be identical with that in (+)-epoxydon (i.e., R,R), but the configuration of the secondary alcohol at C-4 was S rather than R as in (+)-epoxydon. Isoepoxydon (compound UIII) is therefore (4S,5R,6R)-5,6-epoxy-4-hydroxy-2-hydroxymethylcyclohex-2-en-1-one. The boat conformation in which the C-4 hydroxy group is axial is preferred. In the range of 1 mM to 5 mM, the antibiotic activity of (+)-isoepoxydon against Bacillus subtilis was 56% of that obtained with patulin. Over a period of 1-3 h, [14C]isoepoxydon was efficiently converted into patulin by a shake culture of the parent strain of P. urticae. The precursor relationship of isoepoxydon to patulin was confirmed by feeding unlabeled isoepoxydon (1 mM) to a washed-cell suspension of a mutant (J2) in which, over a period of 3-5 h, a better than 60% conversion into patulin was attained. The enzymic relationship between isoepoxydon and phyllostine and their positions in the late portion of the patulin biosynthetic pathway are discussed.