Biosynthetic relationship among aflatoxins B1, B2, M1, and M2

Abstract

Aflatoxins are a family of toxic, acetate-derived decaketides that arise biosynthetically through polyhydroxyanthraquinone intermediates. Most studies have assumed that aflatoxin B1 is the biosynthetic precursor of the other aflatoxins. We used a strain of Aspergillusflavus which accumulates aflatoxin B2 to investigate the later stages of afiatoxin biosynthesis. This strain produced aflatoxins B2 and M2 but no detectable aflatoxin B1 when grown over 12 days in a low-salt, defined growth medium containing asparagine. Addition of dichlorvos to this growth medium inhibited aflatoxin production with concomitant accumulation of versiconal hemiacetal acetate. When mycelial pellets were grown for 24, 48, and 72 h in growth medium and then transferred to a replacement medium, only aflatoxins B2 and M2 were recovered after 96 h of incubation. Addition of sterigmatocystin to the replacement medium led to the recovery of higher levels of aflatoxins B2 and M2 than were detected in control cultures, as well as to the formation of aflatoxins B1 and M, and O-methylsterigmatocystip. These results support the hypothesis that aflatoxins B1 and B2 can arise independently via a branched pathway.

Preparation of 4,15-diacetoxyscirpenol from cultures of Fusarium sambucinum NRRL 13495

“…The filtrates of replacement culture from Fernbach flasks contained only 77 ± 8 mg of DAS per liter. This finding of less rather than more DAS in replacement cultures differs from previous work with other mycotoxins (7,12). Replacement culture filtrates from Erlenmeyer flasks, on the other hand, contained 458 ± 60 mg of DAS per liter, of which 96% was obtained as crystals.…”

Aflatoxin levels in dried figs, nuts and paprika for export from Turkey

“…Although some authors claimed that biosynthesis of aflatoxins B 2 , G 1 and G 2 occurs by interconversion of aflatoxin B 1 (Maggon & Venkitasubramanian, 1973;Lin et al, 1980), others did not find any evidence for conversion of B 1 to other toxins (Dutton et al, 1985;Henderberg et al, 1988). Moreover, Bhatnagar et al (1991) demonstrated separate pathways for aflatoxins B 1 and B 2 production.…”

Conversion of a new metabolite to aflatoxin B2 by Aspergillus parasiticus

“…In a recent study (11) no interconversion of AFB1, AFB2, AFG1, or AFG2 by intact cells of blocked mutants of A. parasiticus was observed, indicating that AFB2 or the AFG toxins might not arise from AFB1. In the proposed metabolic grid for the branched AFB1 and AFB2 biosynthetic pathway, the biosyntheses diverge, beginning at the metabolic precursor versiconal hemiacetal acetate (9,17). The chemical intermediates placed in the AFB1 pathway after the versiconal hemiacetal acetate branch have been demonstrated to be in the following sequence: versicolorin A (15) sterigmatocystin (ST) (13) --+O-methylsterigmatocystin (OMST) (5) --AFB1.…”

“…The sequence of biosynthetic steps resulting in AFB2 formation remains largely hypothetical. It has been Speculated, however, that versicolorin C (9) and 5-hydroxydihydrosterigmatocystin (10) may be AFB2 precursors.…”