Patulin biosynthesis: the metabolism of phyllostine and isoepoxydon by cell-free preparations from Pencillium urticae

Abstract

Cell-free extracts of Penicillium urticae (NRRL 2159A), and its Par⁻ mutants, J2, J1, and S11, were found to contain significant NADP-dependent isoepoxydon dehydrogenase activity. This reversible interconversion of the epoxides (−)-phyllostine and (+)-isoepoxydon occurred optimally at pH5.8 and was completely inhibited by 1 mM p-chloromercuribenzoate (PCMB). The cytosol enzyme possessed specificity for both substrate and cofactor since neither (+)-epoxydon, an epimer of (+)-isoepoxydon, nor NADH was utilized. Cell extracts of the parent and of mutant J2, which is blocked before the epoxides in the patulin pathway, were found to convert phyllostine and isoepoxydon to a number of unknown metabolites which appeared as yellow spots on thin-layer chromatograms after spraying with a chromogenic reagent. Extracts of mutant J1 were unable to carry out this conversion, while whole cells of mutant S11 accumulated what appeared to be these same 'yellow' compounds. Since PCMB-treated extracts of J2 converted phyllostine but not isoepoxydon to these new metabolites, phyllostine appeared to be their more immediate precursor. The relative positions of isoepoxydon and phyllostine in the patulin pathway are discussed.