Omar, Rabeea Fahmy (1990) Mechanism of ochratoxin a-stimulated lipid peroxidation. Masters thesis, Memorial University of Newfoundland.
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Ochratoxin A (OTA) is a mycotoxin and a nephrotoxic carcinogen. The mechanism by which it stimulates lipid peroxidation was investigated in a reconstituted system consisting of phospholipid vesicles (liposomes), flavoprotein NADPH-cytochrome P-450 reductase, EDTA, Fe³⁺ ions, and NADPH. Lipid peroxidation, measured either as malondialdehyde formed or by oxygen uptake, was greatly stimulated in the presence of OTA, Omission of EDTA lowered the extent of lipid peroxidation but did not eliminate it. Fluorometric and spectrophotometric studies demonstrated the formation of a 1:1 Fe³⁺-OTA complex. The rate of reduction of Fe³⁺ to Fe²⁺ was greatly enhanced in the presence of OTA and there was a further increase in the rate when EDTA was also included. Cytochrome P-450 (an enzyme normally present in microsomes) was found to effectively replace EDTA in the reconstituted system and its role in microsomal lipid peroxidation was also implicated suggesting that this hemoprotein could play an important role in OTA-stimulated lipid peroxidation in vivo. -- ESR studies showed that the Fe³⁺-OTA complex produced hydroxyl radicals in the presence of NADPH and NADPH-cytochrome P-450 reductase. The lack of any diminution of lipid peroxidation by catalase and several hydroxyl radical scavengers suggests that hydroxyl radical production by the Fe³⁺-OTA complex may not be a significant factor in the lipid peroxidation in vitro, but these results do not preclude hydroxyl radicals produced by the Fe³⁺-OTA complex from playing an important role in the toxicity of OTA. -- Structure-activity relationship studies indicated that the presence of a free carboxyl group and chlorine atom as well as the L-Phe moiety on OTA contributed significantly to the stimulatory effect on lipid peroxidation. Earlier studies had shown an absolute requirement for a free phenolic hydroxyl group on OTA. -- My results indicate that OTA stimulates lipid peroxidation by complexing with Fe³⁺ and facilitating its reduction. Subsequent to oxygen binding, an iron-oxygen complex of undetermined nature initiates lipid peroxidation. The extent of OTA-dependent lipid peroxidation in vivo and its role in the toxicity of OTA remain to be determined.
|Item Type:||Thesis (Masters)|
|Additional Information:||Bibliography: leaves 139-156.|
|Department(s):||Science, Faculty of > Biochemistry|
|Library of Congress Subject Heading:||Ochratoxin A--Toxicology; Mycotoxins; Lipids--Peroxidation|
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