Stereochemistry during aflatoxin biosynthesis: conversion of norsolorinic acid to averufin.
A reaction sequence, norsolorinic acid (NA)-->averantin (AVN)-->5'-hydroxyaverantin (HAVN)-->averufin (AVR), is the early part of a biosynthetic pathway for aflatoxins. In this study, we determined the stereochemical relationship among these metabolites by using chiral high-performance liquid chromatography. In cell-free experiments using the cytosol fraction of Aspergillus parasiticus NIAH-26, (1'S)-AVN was exclusively produced from NA in the presence of NADPH. Also, only (1'S)-AVN, and not (1'R)-AVN, served as a substrate for the reverse reaction from AVN to NA. When the microsome fraction of NIAH-26 was incubated with (1'S)-AVN in the presence of NADPH, two HAVN diastereomers and one AVR enantiomer were formed, whereas these substances were never produced from (1'R)-AVN. Moreover, (1'S,5'R)-AVR was exclusively formed from both HAVN diastereomers by the cytosol fraction in the presence of NAD. The feeding experiments using this mutant showed that aflatoxins were produced from (1'S,5'R)-AVR but not from (1'R,5'S)-AVR. These results indicate that the enzymes involved in this pathway show strict stereospecificity to their substrates and that the configuration of (1'S,5'R)-AVR leading to the formation of aflatoxins is due to the stereospecificity of NA dehydrogenase which catalyzes the reaction between (1'S)-AVN and NA.
Yabe
K
K
National Institute of Animal Health, Ibaraki, Japan.
Matsuyama
Y
Y
Ando
Y
Y
Nakajima
H
H
Hamasaki
T
T
D000348:Aflatoxins / Q000096:biosynthesis* / Q000737:chemistry; D000880:Anthraquinones / Q000737:chemistry / Q000378:metabolism; D001230:Aspergillus / Q000378:metabolism; D003600:Cytosol / Q000378:metabolism; D013237:Stereoisomerism
D000348:Aflatoxins; D000880:Anthraquinones; C029856:averufin; C002754:norsolorinic acid