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.
Zepnik
H
H
Institut für Toxikologie, Universität Würzburg, Versbacher Str. 9, 97078 Würzburg, Germany.
Pähler
A
A
Schauer
U
U
Dekant
W
W
D000818:Animals; D001711:Biotransformation; D002273:Carcinogens / Q000493:pharmacokinetics* / Q000633:toxicity; D002458:Cell Fractionation; D002851:Chromatography, High Pressure Liquid; D003577:Cytochrome P-450 Enzyme System / Q000378:metabolism; D003907:Dexamethasone / Q000494:pharmacology; D005260:Female; D006801:Humans; D007527:Isoenzymes; D007668:Kidney / Q000187:drug effects; D008297:Male; D013058:Mass Spectrometry; D008748:Methylcholanthrene / Q000494:pharmacology; D008862:Microsomes, Liver / Q000187:drug effects / Q000201:enzymology; D009183:Mycotoxins / Q000493:pharmacokinetics* / Q000633:toxicity; D009793:Ochratoxins / Q000493:pharmacokinetics* / Q000633:toxicity; D010084:Oxidation-Reduction; D051381:Rats; D017208:Rats, Wistar; D012486:Salmonella typhimurium / Q000187:drug effects / Q000235:genetics
D002273:Carcinogens; D007527:Isoenzymes; D009183:Mycotoxins; D009793:Ochratoxins; C025589:ochratoxin A; D008748:Methylcholanthrene; D003907:Dexamethasone; D003577:Cytochrome P-450 Enzyme System