Versiconal acetate

Yao and Hsieh (1974) observed that treatment of A. parasiticus with dichlorvos [0,0-dimethyl-0-(2,2-dichlorovinyl)phosphate] resulted in reduced aflatoxigenicity and in the formation of an orange-red pigment, called versiconal acetate (19). Structure elucidation using and NMR techniques on versiconal acetate (Cox et al., 1977 Steyn et al., 1979a) and... 

The origin in nature of sterigmatocystin and aflatoxin is not obvious from simple chemical analysis of their structures. The biosyntheses of only averufin (8), versiconal acetate (19), versicolorin A (21), sterigmatocystin (32), and aflatoxin B (1) have been studied in detail. Each of these compounds, however, represents a specific stage in the biosynthetic sequence of aflatoxin Bj. 

The biochemical events prior to the elaboration of the aflatoxins were clarified by the use of mutants, and enzyme inhibitors and by incorporation studies of potential precursors. Norsolorinic acid (14), averufin (8), versiconal acetate (19) (from biosynthetic inhibitors), and versicolorin A (21) comply with two of the postulated requirements for an intermediate i.e., they are overproduced by a mutant blocked in aflatoxin synthesis and furthermore are converted into aflatoxin by the wild-type culture of A. parasiticus. In view of the low incorporation of acetate into aflatoxin Bj [relative specific activity (RSA) = 0.24 vs. a maximum of 9.0], the high RSA values and the higher quantities of aflatoxins derived from the above-mentioned four precursors (14, 8, 19, and 21), as compared to acetate, provide strong evidence for their conversion into the aflatoxins without the intermediary formation of acetate (Singh and Hsieh, 1977). 

Dichlorvos possesses a strong inhibitory effect on aflatoxin biosynthesis in A.flavus and A. parasiticus, presumably by specific enzyme inhibition. Dichlorvos inhibited 90% of the aflatoxin production by A. parasiticus ATCC 15517 (Hsieh, 1973). An early step in the secondary metabolic pathway was inhibited, as only additions of dichlorvos prior to the initiation of toxin production, which occurs 40 hr after incubation effectively stopped the toxin synthesis. The reduction in aflatoxin synthesis was accompanied by the excretion of versiconal acetate (19). Yao and Hsieh (1974) reported the efficient conversion of versiconal acetate (13.7%) into aflatoxin Bj by a nitrogen-free resting cell medium oiA. parasiticus. Averufin is converted into versiconal acetate, and not into aflatoxin Bj, in the presence of dichlorvos (Yao and Hsieh, 1974). 

Singh and Hsieh (1977) likewise observed the inhibition of versiconal acetate conversion into aflatoxin by dichlorvos. However, the conversion of versicolorin A (21), and therefore also that of sterigmatocystin (32), into the aflatoxins, was not affected. These findings indicated that 21 and 32 lay beyond the step in the biosynthetic sequence inhibited by dichlorvos and that versicolorin A is most probably derived from versiconal acetate. 

Fig. 17. Proposed structural rearrangement of averufin into versiconal acetate. C(2) of acetate.

The incorporation pattern resulting fron incorporation of C-labelled acetates into versiconal acetate (83) by dichlorvos-treated cultures of A. parasiticus is consistent with its origin... 

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