TIMING OF APPEARANCE OF VERSICONAL HEMIACETAL ACETATE ESTERASE AND VERSICONAL CYCLASE ACTIVITY IN CULTURES OF ASPERGILLUS-PARASITICUS

The sequence of steps versiconal hemiacetal acetate (VHA) to versiconal (VL) catalyzed by an esterase and VL to versicolorin B (VB) catalyzed by VL cyclase has been previously demonstrated in cell-free systems from Aspergillus parasiticus. VHA esterase and VL cyclase activities were estimated by determining the amounts of VL and VB after incubation of VHA in cell-free extracts from mycelia that were either synthesizing or not synthesizing aflatoxins. VHA esterase activity but not VL cyclase activity was present in extracts from cells grown in a nonaflatoxin-producing medium. VHA esterase activity was present in extracts from mycelia grown in aflatoxin-producing medium harvested after one to six days of incubation. VL cyclase activity was absent at one day, low at two days, maximal at three to five days, and lower at six days. VL cyclase activity appears in the later part of the growth period which is also the period of aflatoxin biosynthesis. This supports a role for VL cyclase in the aflatoxin biosynthetic pathway.