Biochemical analysis of oxidative stress in the production of aflatoxin and its precursor intermediates

The relevance of oxidative stress in the production of aflatoxin and its precursors was examined in different mutants of Aspergillus parasiticus, which produce aflatoxin or its precursor intermediates, and compared with results obtained from a non-toxigenic strain. In comparison to the non-toxigenic strain (SRRC 255), an aflatoxin producing strain (NRRL 2999) or mutants that accumulate aflatoxin precursors such as norsolorinic acid (by SRRC 162) or versicolorin (by NRRL 6196) or O-methyl sterigmatocystin (by SRRC 2043) had greater oxygen requirements and higher contents of reactive oxygen species. These changes were in the graded order of NRRL 2999 > SRRC 2043 > NRRL 6196 > SRRC 162 > SRRC 255, indicating incremental accumulation of reactive oxygen species, being least in the non-toxigenic strain and increasing progressively during the ternary steps of aflatoxin formation. Oxidative stress in these strains was evident by increased activities of xanthine oxidase and free radical scavenging enzymes (superoxide dismutase and glutathione peroxidase) as compared to the non-toxigenic strain (SRRC 255). Culturing the toxigenic strain in presence of 0.1-10 mu M H2O2 in the medium resulted in enhanced aflatoxin production, which could be related to dose-dependent increase in [C-14]-acetate incorporation into aflatoxin B-1 and increased acetyl CoA carboxylase activity. The combined results suggest that formation of secondary metabolites such as aflatoxin and its precursors by A. parasiticus may occur as a compensatory response to reactive oxygen species accumulation.