Ochratoxin production by Aspergillus ochraceus as affected by methionine and structurally related compounds

Abstract

When 10-2 M of L- or D-Met was added to a synthetic medium containing xylose as the sole C source, ochratoxin production by A. ochraceus was strongly inhibited. At that concentration methionine derivatives, .alpha.-methyl-DL-methionine, DL-methionine sulfoxide and L-methionine sulfone, did not inhibit ochratoxin production; DL-methionine S-methyl sulfonium chloride (MMSC) inhibited ochratoxin production to a large extent. L-Met and MMSC also completely inhibited sclerotia formation, while D-Met and DL-methionine sulfoxide caused only a partial inhibition. At lower concentrations (10-3 and 10-4 M), none of the compounds exhibited inhibitory effects. In cases where strong ochratoxin inhibition was detected, fungal radial growth or mycelial dry weight was inhibited by only 10-25%, while the initial pH of the medium dropped from .apprx. 6.5 to .apprx. 4.4-5.0. Adjustment of the initial pH of media supplemented with 10-2 M L-Met, D-Me or MMSC to a pH of .apprx. 7.8 did not change the inhibitory effects on ochratoxin production in media containing L-Met. Sclerotia formation was restored in all 3 treatments.