Role of versicolorin A and its derivatives in aflatoxin biosynthesis

Versicolorin A hemiacetal, hydroxydihydrosterigmatocystin, and aflatoxin G2 alpha reductase activity in extracts from Aspergillus parasiticus

Versicolorin A hemiacetal was converted to versicolorin C in cell-free systems from Aspergillus parasiticus. The rate of reaction catalyzed by the 35-70% ammonium sulfate fraction was 0.43 nmol min-1 mg-1 with NADPH as cosubstrate and 0.17 nmol min-1 mg-1 with NADH at 25 degrees C at pH 7.4. The product from incubation of 17-hydroxy-16,17-dihydrosterigmatocystin with the 35-70% ammonium sulfate fraction and NADPH was a polar compound which was converted to dihydrosterigmatocystin by 0.4 M HCl. The polar compound is proposed to be the 14,17-hydrated open-chain derivative of dihydrosterigmatocystin. Aflatoxin G2 alpha was also reduced in this system to a polar product tentatively identified as the 13,16-hydrated open-chain derivative of AFG2. The reductase activity may be involved in the formation of reduced intermediates and aflatoxins in cultures of A. parasiticus.

Enzymes in aflatoxin B1 biosynthesis: strategies for identifying pertinent genes

Recent work on the aflatoxin biosynthetic pathway is reviewed, with special emphasis on the enzymes of the late stages of the pathway involving conversion of sterigmatocystin (ST) to aflatoxin B1 (AFB1) through an O-methylsterigmatocystin intermediate. Two enzyme activities were discovered in subcellular fractions of cell-free extracts of a mutant strain of Aspergillus parasiticus (SRRC 163): 1) A post-microsomal methyltransferase (MT) catalyzed conversion of ST to OMST, and 2) a microsomal-associated activity (oxido-reductase) converted OMST to AFB1. The 168 KDa, anionic MT was purified to homogeneity and characterized (two subunits, 110 KDa and 58 KDa). Preliminary evidence indicated the presence of a cationic isozyme of the MT in mycelial extracts. The oxido-reductase has been partially purified and characterized. Polyclonal antibodies were prepared to the anionic MT and the enzyme's amino acid composition determined. A cDNA library has been constructed from mRNA isolated from Aspergillus parasiticus mycelia during the onset of AFB1 biosynthesis for the purpose of identifying the genes responsible for aflatoxin biosynthesis.

Transformation of Aspergillus flavus to study aflatoxin biosynthesis

Aflatoxin contamination of agricultural commodities continues to be a serious problem in the United States. Breeding for resistant genotypes has been unsuccessful and detoxification of food sources is not economically feasible. New strategies for control may become apparent once more is known about the biosynthesis and regulation of aflatoxin. Although the biosynthetic pathway of aflatoxin has been extensively studied, little is known about the regulation of the individual steps in the pathway. We have developed a genetic transformation system for Aspergillus flavus that provides a new and expedient approach to studying the biosynthesis of aflatoxin and its regulation. Through the use of this genetic transformation system, genes for aflatoxin biosynthesis can be identified and isolated by the complementation of aflatoxin negative mutants. In this paper we discuss molecular strategies for studying the regulation and biosynthesis of aflatoxin.

Purification and characterization of a methyltransferase from Aspergillus parasiticus SRRC 163 involved in aflatoxin biosynthetic pathway

A five step scheme has been developed for the purification of a methyltransferase (MT) from mycelia of 3-day old Aspergillus parasiticus (SRRC 163), which catalyzes one step in the aflatoxin biosynthetic pathway. The S-adenosylmethionine (SAM) requiring MT activity is essential for the conversion of sterigmatocystin (ST) to O-methylsterigmatocystin (OMST) prior to being converted to aflatoxin B1. The purification of the MT was carried out from cell-free extracts by CDR (Cell Debris Remover, a cellulosic weak anion exchanger, Whatman) treatment, QMA ACELL, Hydroxylapatite-Ultrogel, PBE 94 chromatofocusing and FractoGel TSK HW-50F filtration chromatography. The purified enzyme was only about 0.1% of the total extractable proteins. The pI of the protein was about 5.0 as judged by chromatofocusing. Results of gel filtration chromatography indicated the approximate molecular mass of the native protein to be 160-KDa. SDS-polyacrylamide gel electrophoresis revealed two protein subunit bands of molecular masses approximately 110-KDa and 58-KDa. The molar extinction coefficient of the enzyme at 280 nm was estimated to be 7.87 X 10(4) M-1 cm-1 in 50 mM potassium phosphate buffer (pH 7.5). The reaction catalyzed by the MT was optimum at pH 7.5 and between 25-35 degrees C. The Km of the enzyme for ST and SAM was determined to be 1.8 microM and 42 microM, respectively with an estimated turnover number of the enzyme for ST of 2.2 X 10(-2) per sec.

Fate of the methyl group during the conversion of sterigmatocystin into O-methylsterigmatocystin and aflatoxin B1 by cell-free preparations of Aspergillus parasiticus

Cell-free extracts of fungal mycelia of two aflatoxin non-producing isolates of Aspergillus parasiticus (SRRC 163 and SRRC 2043) were utilized for the study of enzyme activities involved in the latter stages of aflatoxin biosynthesis. The post-microsomal fractions (105,000 x g supernatant) of both SRRC 163 and SRRC 2043 were able to convert sterigmatocystin (ST) into O-methylsterigmatocystin (OMST); whereas the microsomal (105,000 x g pellet) preparation of only SRRC 163 was able to convert OMST into aflatoxin B1 (AFB1). S-Adenosylmethionine (SAM) was the primary substrate for the ST to OMST (methyltransferase) enzymatic conversion; [3H]OMST of specific activity 0.93 Ci/mmol was obtained in a reaction containing the [3H]SAM substrate (specific activity 1 Ci/mmol). After the terminal enzymatic conversion of OMST into AFB1, none of the radiolabel of the methyl group from OMST was found in AFB1. It is postulated that the methylation of ST may be required for subsequent enzymatic oxidation of OMST to aflatoxin B1.

Appearance of enzyme activities catalyzing conversion of sterigmatocystin to aflatoxin B1 in late-growth-phase Aspergillus parasiticus cultures

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-h-old cells, increased to optimum levels during the stationary phase, and then declined.

Identification of O-methylsterigmatocystin as an aflatoxin B1 and G1 precursor in Aspergillus parasiticus

An isolate of Aspergillus parasiticus CP461 (SRRC 2043) produced no detectable aflatoxins, but accumulated O-methylsterigmatocystin (OMST). When sterigmatocystin (ST) was fed to this isolate in a low-sugar medium, there was an increase in the accumulation of OMST, without aflatoxin synthesis. When radiolabeled [14C]OMST was fed to resting mycelia of a non-aflatoxin-, non-ST-, and non-OMST-producing mutant of A. parasiticus AVN-1 (SRRC 163), 14C-labeled aflatoxins B1 and G1 were produced; 10 nmol of OMST produced 7.8 nmol of B1 and 1.0 nmol of G1, while 10 nmol of ST produced 6.4 nmol of B1 and 0.6 nmol of G1. A time course study of aflatoxin synthesis in ST feeding experiments with AVN-1 revealed that OMST is synthesized by the mold during the onset of aflatoxin synthesis. The total amount of aflatoxins recovered from OMST feeding experiments was higher than from experiments in which ST was fed to the resting mycelia. These results suggest that OMST is a true metabolite in the aflatoxin biosynthetic pathway between sterigmatocystin and aflatoxins B1 and G1 and is not a shunt metabolite, as thought previously.

The conversion of sterigmatocystin to O-methylsterigmatocystin and aflatoxin B1 by a cell-free preparation

A cell-free system derived from a versicolorin A-accumulating mutant of Aspergillus parasiticus was found to convert sterigmatocystin to both O-methylsterigmatocystin and aflatoxin B1. It is suggested that the similarity in the chromatographic properties of these two metabolites has caused erroneous conclusions to be made with regards to the biosynthesis of aflatoxin B1.