SOYBEAN LIPOXYGENASE IS ACTIVE ON NONAQUEOUS MEDIA AT LOW MOISTURE: A CONSTRAINT TO XEROPHILIC FUNGI AND AFLATOXINS?

Authors: Gardner, Harold; Grove, Marilyn; KELLER, NANCY - TX A&M UNIV,COLLEGE STA,

Submitted to: Journal of the American Oil Chemists' Society
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/16/1998
Publication Date: N/A
Citation: N/A

Interpretive Summary: Aflatoxin, a toxic chemical produced by the fungus Aspergillus flavus, is a serious concern in our food supply. Lipoxygenase, an enzyme found in plants, was previously shown to form a product that inhibits aflatoxin formation in the fungus. Most enzymes, like lipoxygenase, are only active at high moisture levels, not in "dry" stored grains where A. flavus often grows. We have demonstrated that lipoxygenase is also active on "dry" media at a wide range of humidities, even far below those at which A. flavus thrives. We demonstrated that aflatoxin formation is inhibited in soy flour containing large amounts of lipoxygenase activity, even though the fungus is present. After we destroyed the enzyme activity in the soy flour, aflatoxin formation increased significantly suggesting lipoxygenase is responsible, in part, for inhibiting formation of the toxin. This work should be of particular interest to scientists involved in enzyme reactions and aflatoxin biosynthesis, and to breeders and genetic engineers looking for new mechanisms of reducing aflatoxin prevalence.

Technical Abstract: In model systems employing soybean lipoxygenase at relative humidities between 52% and 95%, oxidation of linoleate occurred at all relative humidities compared to much lower values of the heat-inactivated controls. Because it was determined that the model required from 7 to 24 h to achieve moisture equilibrium, experiments were then completed after an overnight equilibration at the designated relative humidity before mixing the cellulose/linoleate with cellulose/soybean extract. Under these conditions, there was a notable positive correlation in oxidation rate with increasing relative humidity. That the oxidation was principally enzymatic was shown by chiral analysis of the linoleate hydroperoxides formed. Because the dry reaction products were somewhat more racemic than a short-time aqueous incubation of soybean aqueous extract with linoleate and racemization increased with time of incubation, autoxidation appeared to be initiated by the lipoxygenase reaction in dry media. To assess the biological relevance of lipoxygenase, enzyme-active and enzyme-inactivated defatted soy flour amended with linoleic acid (3.5%) or without amendment was inoculated with Aspergillus parasiticus. The enzyme-active flour showed significantly less aflatoxin production with or without linoleic acid amendment, compared to heat-inactivated soy flour. Because hexane-defatted flour contained significant glyceride linoleic acid (1.7%), the results are consistent with a role for lipoxygenase in preventing aflatoxin production.