Conversion of dihydro-O-methylsterigmatocystin to aflatoxin B2 by Aspergillus parasiticus.

Dihydro-O-methylsterigmatocystin (DHOMST) was identified in cultures of Aspergillus parasiticus SRRC 2043, an aflatoxin (AF) non-producer, by comparison of its mass spectrum to that of authentic DHOMST. In addition to DHOMST, SRRC 2043 is known to produce two other chemically related compounds in culture: O-methylsterigmatocystin (OMST) which is an AFB1 precursor and a hydrated-vinyl ether analogue of OMST (HOMST) which is an AFB2 precursor. In the present study, DHOMST also was shown to be an AFB2 precursor by demonstrating that DHOMST is converted to AFB2 by mycelia of an A. parasiticus mutant (SRRC 163) blocked early in the AF biosynthetic pathway; this conversion occurred at an even higher rate than observed for HOMST to AFB2 conversion by the same fungal strain. SRRC 2043 mycelia, although incapable of producing AF's when supplied with OMST, HOMST or DHOMST, were capable of converting HOMST to DHOMST. The results indicated that DHOMST can be placed after HOMST in the biosynthetic sequence which produces AFB2.