Versiconal

The conversion fix>m averufin (AVF) to versiconal hemiacetal acetate (VHA) is catalyzed by an averufin oxidase encoded by afll (oyfA) (41). VHA was converted to versiconal (VAL) by an esterase (29, 63, 64, 65). VAL is converted to Versicolorin B (VERB) by a VERB synthase (31). The gene, qflK (ybs), was cloned, characterized and expressed (66,67,68). It was demonstrated that the versicolorin B synthase catalyzes the side chain cyclodehydration of racemic VHA to VERB. This is a key step in the aflatoxin formation since it closed the bisfuran ring of aflatoxin for binding to DNA. VERB is converted to versicolorin A (VERA) by a desaturase encoded by qflL (verB) (31). 

Versicon Condutive Polymer. [Allied-Signal] Inherently conductive polynter. 

Figure 11.39. Electron micrograph of Incoblend containing PAni Versicon to a concentration of 0.1 wt.% PAni. TEM revealed the flocculation of dispersed PAni particles even at these low concentrations [79], The diameters of the particles are roughly only 200 nm.

Polyaniline used in these dispersions was Versicon [112]. The coating process was repeated at least five times (after complete drying of the previous coating step) and a maximum of 20 times to increase thickness. The results described below are taken from experiments with 15 coating layers on the metal surface. 

VERSICON, Allied Signal Inc., Technical data sheets. 

Several specific enzyme activities associated with precursor conversions in the aflatoxin pathway " " have been partially purified (Fig.l 1.1) whereas others such as methyltransferases " have been purified to homogeneity. Several other enzymes involved in aflatoxin biosynthesis, such as a reductase and a cyclase, " have also been purified from A. parasiticus. A desaturase that converts VERA to VERB has been found in cell-free fungal extracts. Matsushima et al. have purified and characterized two versiconal hemiacetal acetate reductases involved in toxin synthesis, whereas Kusumoto and Hsieh purified to homogeneity an esterase that converts VHA to versiconal. ... 

YABE, K., ANDO, Y., HAMASAKI, T., A metabolic grid among versiconal hemiacetal acetate, versiconol acetate, versiconol and versiconal during aflatoxin biosynthesis, y. Gen. Microbiol., 1991,137, 2469-2475. 

YABE, K., HAMASAKI, T., Stereochemistry during aflatoxin biosynthesis Cyclase reaction in the conversion of versiconal to versicolorin B and racemi2ation of versiconal hemiacetal acetate, Appl. Environ. Microbiol., 1993, 59, 2493-2500. 

LIN, B.K., ANDERSON, J.A., Purification and properties of versiconal cyclase from Aspergillus parasiticus, Arch. Biochem. Biophys., 1992, 293, 67-70. 

BiosyntheticaUy, the aflatoxins are all formed from the same precursor, versiconal hemiacetal acetate (12) (25). Compound 12 is formed from acetate, the units of which are converted into a polyketide. The polyketide is then metabolized to the xanthone 12 (see Scheme 2.1) (26). Intermediate 12 can then be transformed either into versicolorin A (13) or versicolorin B (14) in several steps. Versicolorin A (13) may be converted to sterigmatocystin (15), while 14 can lead to dihydro-sterigmatocystin (16). Sterigmatocystin (15) can be metabolized to aflatoxins Gi (3) or Bi (1) and the latter may then be transformed to aflatoxin Mi (5). Aflatoxins B2 (2) and G2 (4) are formed from dihydrosterigmatocystin (16) and aflatoxin M2 (6) is formed by conversion from B2 (2). Pathways also exist to convert aflatoxin Bi (1) to B2 (2), Ml (5) to M2 (6), and Gj (3) to G2 (4), and vice versa. Important biosynthesis steps are shown in Scheme 2.1. 

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