Soybean lipoxygenase is active on nonaqueous media at low moisture: A constraint to xerophilic fungi and aflatoxins?

Previous workers have reported that certain products of the lipoxygenase pathway are detrimental either to the development and growth of Aspergillus species or to aflatoxin production by these organisms. Since Aspergillus often thrives on "dry" stored grains, depending on the level of the relative humidity,we sought to determine if lipoxygenase could catalyze the oxidation of linoleic acid on these "dry" substrates equilibrated at various relative humidities. A desiccated model system, previously adjusted to pH 7.5, was composed of soybean extract, linoleic acid, and cellulose carrier. The model system was incubated for up to 24 h at four relative humidities ranging between 52 and 95% to determine the extent oxidation catalyzed by lipoxygenase, compared with heat-inactivated controls. Oxidation in the active samples was much greater than in the controls at all relative humidities, and oxidation was principally enzymatic as demonstrated by chiral analysis of the linoleate hydroperoxides formed. The main product was 13S-hydroperoxy-9Z,11E-octadecadienoic acid, accompanied by a significant percentage of 9S-hydroperoxy-10E,12Z-octadecadienoic acid. Since the products became more racemic with time of incubation, autoxidation appeared to be initiated by the lipoxygenase reaction in dry media. Additionally, the biological relevance of lipoxygenase activity was tested under these xerophilic conditions. Thus, enzyme-active and heat-inactivated defatted soy flour amended either with or without 3.5% by weight linoleic acid was inoculated with fungal spores and incubated at 95% relative humidity. Although fungal growth occurred on all treatments, samples inoculated with Aspergillus parasiticus showed significantly less aflatoxin in the enzyme-active samples, compared to inactivated flour. Addition of linoleic acid had little effect, possibly because the defatted soy flour was found to contain 1.7% residual linoleic acid as glyceride lipid.