Biosynthesis of ochratoxins by Aspergillus ochraceus

Shaken liquid fermentation of an isolate of Aspergillus ochraceus showed gr owth-associated production of ochratoxins A and B, followed by production o f a related polyketide diaporthin. Later, between 150 and 250 It, mellein a ccumulated transitorily. In contrast, shaken solid substrate (shredded whea t) fermentation over 14 days produced mainly ochratoxins A and B (ratio ca. 5: 1) in very high yield (up to 10 mg/g). In these systems experiments wit h C-14 -labelled precursors and putative intermediates revealed temporal se paration of early and late stages of the ochratoxin biosynthetic pathway, b ut did not support an intermediary role for mellein. The pentaketide interm ediate ochratoxin beta was biotransformed very efficiently into both ochrat oxins A and B, 14 and 19%, respectively. The already chlorinated ochratoxin alpha was only biotransformed significantly (4.85%) into ochratoxin A, ind icating that chlorination is mainly a penultimate biosynthetic step in the biosynthesis of ochratoxin A. This was supported by poor (1.5%) conversion of radiolabelled ochratoxin B into ochratoxin A. Experiments implied that s ome ochratoxin B may arise by dechlorination of ochratoxin A. (C) 2001 Else vier Science Ltd. All rights reserved.