Conversion of 11-hydroxy-O-methylsterigmatocystin to aflatoxin G1 in Aspergillus parasiticus

In aflatoxin biosynthesis, aflatoxins G 1 (AFG 1 ) and B 1 (AFB 1 ) are independently produced from a common precursor, O -methylsterigmatocystin (OMST). Recently, 11-hydroxy -O -methylsterigmatocystin (HOMST) was suggested to be a later precursor involved in the conversion of OMST to AFB 1 , and conversion of HOMST to AFB 1 was catalyzed by OrdA enzyme. However, the involvement of HOMST in AFG 1 formation has not been determined. In this work, HOMST was prepared by incubating OrdA-expressing yeast with OMST. Feeding Aspergillus parasiticus with HOMST allowed production of AFG 1 as well as AFB 1 . In cell-free systems, HOMST was converted to AFG 1 when the microsomal fraction, the cytosolic fraction from A. parasiticus , and yeast expressing A. parasiticus OrdA were added. These results demonstrated (1) HOMST is produced from OMST by OrdA, (2) HOMST is a precursor of AFG 1 as well as AFB 1 , and (3) three enzymes, OrdA, CypA, and NadA, and possibly other unknown enzymes are involved in conversion of HOMST to AFG 1 .