Possible role of an iron-oxygen complex in 4(S)-4-hydroxyochratoxin a formation by rat liver microsomes.
Rat liver microsomes were examined for their ability to oxidize the mycotoxin ochratoxin A (OTA) to 4(R)-4-hydroxyochratoxin A [(R)-4-OH-OTA] and 4(S)-4-hydroxyochratoxin A [(S)-4-OH-OTA] and to induce OTA-dependent lipid peroxidation. Microsomes isolated from rats pretreated with pregnenolone-16 alpha-carbonitrile greatly induced both (R)-4-OH-OTA and (S)-4-OH-OTA formation whereas isoniazid pretreatment primarily induced (S)-4-OH-OTA. (R)-4-OH-OTA and (S)-4-OH-OTA formation showed significant differences with respect to pH optima, effect of antioxidants, and iron chelators. (R)-4-OH-OTA showed a pH optimum of 6.5 and was not inhibited by the antioxidants butylated hydroxyanisole or N,N-diphenyl-1,4-phenylenediamine or the iron chelators. Desferal or bathophenanthrolinedisulfonic acid. In contrast, both (S)-4-OH-OTA and lipid peroxidation showed a pH optimum of 7.0 and both activities were sensitive to inhibition by the above antioxidants and iron chelators. Lipid peroxidation was not involved in (S)-4-OH-OTA formation since addition of linoleic acid hydroperoxide to microsomes did not give rise to (S)-4-OH-OTA. Cytochrome P450 appeared to be essential since other hemoproteins like horseradish peroxidase and hemoglobin were ineffective in metabolizing OTA in the presence of hydroperoxides. The results suggest that (R)-4-OH-OTA is formed by normal mixed-function oxidation but that (S)-4-OH-OTA formation may involve free iron. It is likely that an active Fe2(+)-oxygen complex, formed via NADPH-cytochrome P450 reductase and cytochrome P450-dependent reduction of free Fe3+ followed by oxygen binding, serves as the species inducing lipid peroxidation and at least part of (S)-4-OH-OTA formation.
Omar
R F
RF
Department of Biochemistry, Memorial University, St. John's, Newfoundland, Canada.
Rahimtula
A D
AD
D000818:Animals; D000975:Antioxidants / Q000494:pharmacology; D002083:Butylated Hydroxyanisole; D003577:Cytochrome P-450 Enzyme System / Q000378:metabolism; D016166:Free Radical Scavengers; D006863:Hydrogen-Ion Concentration; D007502:Iron Chelating Agents / Q000494:pharmacology; D015227:Lipid Peroxidation / Q000187:drug effects; D008297:Male; D008315:Malondialdehyde / Q000032:analysis; D008862:Microsomes, Liver / Q000378:metabolism*; D006899:Mixed Function Oxygenases / Q000378:metabolism*; D009793:Ochratoxins / Q000378:metabolism*; D010084:Oxidation-Reduction; D011285:Pregnenolone Carbonitrile; D051381:Rats; D017207:Rats, Sprague-Dawley; D013237:Stereoisomerism
D000975:Antioxidants; D016166:Free Radical Scavengers; D007502:Iron Chelating Agents; D009793:Ochratoxins; D011285:Pregnenolone Carbonitrile; C025589:ochratoxin A; D002083:Butylated Hydroxyanisole; D008315:Malondialdehyde; D003577:Cytochrome P-450 Enzyme System; D006899:Mixed Function Oxygenases; C025650:4-hydroxyochratoxin A