Biocatalytic synthesis of butein and sulfuretin by Aspergillus alliaceus

Aspergillus alliaceus UI315 was examined for its potential to catalyze biotransformation reactions of chalcones that mimic plant biosynthetic processes. 3-(4' '-Hydroxyphenyl)-1-(2',4'-dihydroxyphenyl)propenone (4,2',4'-trihydroxychalcone, isoliquiritigein) (1) was efficiently transformed to two major metabolites that were isolated chromatographically and identified by spectroscopic methods as 3-(3' ',4' '-dihydroxyphenyl)-1-(2',4'-dihydroxyphenyl)propenone (butein) (7) and 2-[(3,4-dihydroxyphenyl)methylene]-6-hydroxy-3(2H)benzofuranone (7,3',4'-trihydroxyaurone, sulfuretin) (10). Inhibition experiments suggested that initial C-3 hydroxylation of 1 to 7 was catalyzed by a cytochrome P450 enzyme system. A second A. alliaceus enzyme, partially purified and identified as a catechol oxidase, catalyzed the oxidation of the catechol butein (7) likely through an ortho-quinone (8) that cyclized to the aurone product 10. This work showed that A. alliaceus UI315 contains oxidative enzyme systems capable of converting phenolic chalcones such as 1 into aurones such as 10 in a process that mimics plant biosynthetic pathways.