Cleavage of the lactone ring of aflatoxin B1 results in a nonfluorescent compound that has greatly reduced biological activity. Mutagenicity, as measured by the Ames test, is reduced 450-fold compared to that of B1, and toxicity, as measured by the chick embryo test, is reduced 18-fold.
Two activities involved in terminal pathway conversion of sterigmatocystin to aflatoxin B1 were isolated from an aflatoxin-nonproducing mutant of Aspergillus parasiticus (avn-1), and the time course of appearance of the activities in culture was determined. Subcellular fractionation of fungal mycelia resolved the two activities into a postmicrosomal activity which catalyzed conversion of sterigmatocystin to O-methylsterigmatocystin and a microsomal activity which converted O-methylsterigmatocystin to aflatoxin B1. The two activities were absent in 24-hold cells, increased to optimum levels during the stationary phase, and then declined.
A new blocked mutant of Aspergillus parasiticus produces no detectable aflatoxin B1, but accumulates several polyhydroxyanthraquinones. One of these pigments was identified as averantin. This is the first report of its formation by A. parasiticus. Radiotracer studies with [14C]averantin showed that 15.3% of label from averantin was incorporated into aflatoxin B1. This incorporation was blocked by dichlorvos. With radiotracers and other mutants, averantin was placed after norsolorinic acid and before averufin in the biosynthetic pathway in which the general steps are norsolorinic acid-. averantin-. averufin-. versiconal hemiacetal acetate-versicolorin A-sterigmatocystin-aflatoxin B1. Aflatoxins are a family of toxic, carcinogenic secondary metabolites produced by certain strains of Aspergillus flavus and A. parasiticus. Aflatoxin biosynthesis proceeds through a polyketide pathway in which the general steps are acetate-anthraquinones-+ xanthones-. aflatoxin B1. There are several recent reviews of aflatoxin biosynthesis (la, 10, 19, 20).
A mutant strain of Aspergillus parasiticus blocked in aflatoxin biosynthesis accumulates versicolorin A and versicolorin C. The effect of trace elements on the growth and versicolorin production by this strain was studied in a defined medium. The omission of manganese was slightly stimulatory to versicolorin production; when zinc was omitted from the medium, no detectable versicolorins were produced. Experiments on nitrogen sources in a high-sucrose medium indicated that fourfold to fivefold increases in versicolorin yields could be obtained by substituting 3 ml/l corn steep liquor or 0.1 M NH4NO3 for the 0.023 M (NH4)2SO3 used previously as the nitrogen source in studies on versicolorin production by this strain. These improved yields will facilitate attempts to accumulate enough versicolorin A and versicolorin C for toxicity and carcinogenicity testing. Chromatographic profiles of mycelial extracts of cultures grown in a defined medium with 0.1 M NH4NO3 as the nitrogen source revealed 2 previously unrecognized compounds. The accumulation of these new metabolites in a mutant blocked in aflatoxin production may indicate that they are biosynthetically related to aflatoxin.
Adjusting dilute aqueous solutions of aflatoxins B1 and G1 to pH 1, 2 and 3, and heating over a range of 40–100 C resulted in the conversion of B1 to B2a and G1 to G2a as major products. Both B2a and G2a were identified by co‐thin layer chromatography with authentic B2a and G2a and M1 on silica gel plates developed in two different solvents. The rate of disappearance of B1 or G1 at given temperature and at constant pH was found to be first order with respect to each aflatoxin. At given temperature the conversion is strongly pH dependent, a 10‐fold increase in H+ ion (1 pH unit) producing about a 9‐fold increase in the reaction rate, indicating first order dependent of the rate on H+ ion concentration.
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