Inhibition of Aflatoxin Production by Citrinin and Non-Enzymatic Formation of a Novel Citrinin-Kojic Acid Adduct.

Screening for microorganisms that inhibit aflatoxin production from environments showed that Penicillium citrinum inhibited aflatoxin production by Aspergillus parasiticus . The inhibitory substance in the culture medium of P. citrinum was confirmed to be citrinin (CTN). RT-PCR analyses showed that CTN did not inhibit expressions of aflatoxin biosynthetic genes ( aflR , pksL1 , and fas-1 ) of A. parasiticus , whereas feeding experiments using A. parasiticus showed that CTN inhibited the in vivo conversion of dihydrosterigmatocystin to AFB ₂ ·AFG ₂ . These results suggest that CTN inhibits a certain post-transcriptional step in aflatoxin biosynthesis. CTN in the culture medium of A. parasiticus was found to be decreased or lost with time, suggesting that a certain metabolite produced by A. parasiticus is the cause of the CTN decrease; we then purified, characterized, and then analyzed the substance. Physico-chemical analyses confirmed that the metabolite causing a decrease in CTN fluorescence was kojic acid (KA) and the resulting product was identified as a novel substance: (1 R ,3 S ,4 R )-3,4-dihydro-6,8-dihydroxy-1-(3-hydroxy-6-(hydroxymethyl)-4-oxo-4 H -pyran-2-yl)-3,4,5-trimethyl-1 H -isochromene-7-carboxylic acid, which was named "CTN-KA adduct". Our examination of the metabolites' toxicities revealed that unlike CTN, the CTN-KA adduct did not inhibit aflatoxin production by A. parasiticus . These results indicate that CTN's toxicity was alleviated with KA by converting CTN to the CTN-KA adduct.