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 pa thway are detrimental either to the development and growth of Aspergillus s pecies or to aflatoxin production by these organisms. Since Aspergillus oft en thrives on "dry" stored grains, depending on the level of the relative h umidity,we sought to determine if lipoxygenase could catalyze the oxidation of linoleic acid on these "dry" substrates equilibrated at various relativ e humidities. A desiccated model system, previously adjusted to pH 7.5, was composed of soybean extract, linoleic acid, and cellulose carrier. The mod el system was incubated for up to 24 h at four relative humidities ranging between 52 and 95% to determine the extent oxidation catalyzed by lipoxygen ase, compared with heat-inactivated controls. Oxidation in the active sampl es was much greater than in the controls at all relative humidities, and ox idation was principally enzymatic as demonstrated by chiral analysis of the linoleate hydroperoxides formed. The main product was 13S-hydroperoxy-9Z,1 1E-octadecadienoic acid, accompanied by a significant percentage of 9S-hydr operoxy-10E,12Z-octadecadienoic acid. Since the products became more racemi c with time of incubation, autoxidation appeared to be initiated by the lip oxygenase reaction in dry media. Additionally, the biological relevance of lipoxygenase activity was tested under these xerophilic conditions. Thus, e nzyme-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 signifi cantly less aflatoxin in the enzyme-active samples, compared to inactivated flour. Addition of linoleic acid had little effect, possibly because the d efatted soy flour was found to contain 1.7% residual linoleic acid as glyce ride lipid.