Ochratoxin A-induced tumor formation:: Is there a role of reactive ochratoxin A metabolites?

Ochratoxin A is a nephrotoxic and tumorigenic mycotoxin which contaminates a variety of food items, resulting in chronic human exposure. Biotransformation reactions have been implicated in the tumorigenicity of ochratoxin A. The biotransformation of ochratoxin A by cytochromes P450 and other mammalian enzymes was investigated to optimize conditions for bacterial mutagenicity testing. Metabolite formation was assessed by HPLC with UV and fluorescence detection and by LC/MS/MS. When ochratoxin A was incubated with liver microsomes from rats and mice, formation of 4R- and 4S-hydroxyochratoxin A was observed at very low rates. However, oxidation of ochratoxin A was not observed using kidney microsomes from rats and mice. Significantly higher rates of oxidation were seen in liver microsomes from rats pretreated with 3-methylcholanthrene and dexamethasone. Other reported or postulated that ochratoxin A-metabolites were not formed in detectable concentrations. Human cytochromes P450 (3A4, 1A2, and 2C9-1 Supersomes(R)) also showed very low activity with ochratoxin A (<60 fmole/min x pmol P450). Other enzyme systems used to study possible biotransformation of ochratoxin A were rat and human liver and kidney S-9 fortified with NADPH and glutathione, semipurified glutathione S-transferases, horseradish peroxidase, and soybean lipoxygenase; none of these resulted in detectable biotransformation of ochratoxin A. Using rat liver microsomes with high activity for ochratoxin A oxidation and the other enzyme systems to activate ochratoxin A for mutagenicity testing in the Ames test, mutagenicity was not observed in Salmonella typhimurium TA 100 and TA 2638. The obtained results suggest that oxidative biotransformation of ochratoxin A occurs at low rates, is catalyzed by cytochromes P450, and is unlikely to form reactive intermediates capable of binding to DNA.