Chemiluminescence studies on the generation of oxygen radicals from the interaction of NADPH-cytochrome P-450 reductase with iron

Peroxide-dependent and -independent lipid peroxidations catalyzed by chelated iron

Oxidation of linoleic acid (LA) in tetradecyltrimethylammonium bromide micelles was induced by ferrous- and ferric-chelates in the presence of linoleic acid hydroperoxide (LOOH). Ferrous-chelates also induced lipid peroxidation in the presence of H2O2, but ferric-chelates did not, thought they could generate OH-radicals in the presence of H2O2, resulting in deoxyribose degradation. Of the chelators tested, nitrilotriacetic acid (NTA) chelated with iron showed the highest activity for induction of H2O2- and LOOH-dependent lipid peroxidations and H2O2-dependent deoxyribose degradation. NTA with ferrous ion, but not with ferric ion, also initiated oxidation of LA after a short lag period in the absence of peroxides such as H2O2 and LOOH, but other chelators with ferrous ion did not. The peroxide-independent lipid peroxidation and associated oxidation of ferrous-NTA to ferric-NTA progressed in two steps: an induction step in a lag period and then a propagation step. Ferrous ion complexed with NTA was autoxidized pH-dependently and synchronously with oxygen uptake. The rates of both reactions increased with increase of pH, but were not related to the length of the lag period, which was also dependent on pH, and was shortest at pH 4.2. The EPR spectrum of the ferric-NTA complex prepared directly from ferric salt was different from that of the complex prepared from ferrous salt, confirming that some ferric-type active oxygen participated in induction of peroxide-independent lipid peroxidation. From these results, we propose a possible mechanism of lipid peroxidation induced by ferrous-NTA without peroxides. The finding that iron-NTA had the highest activity for induction of the oxidations of LA and deoxyribose is discussed in relation to the carcinogenic and nephrotoxic effects of this chelating agent.

Comparison of the ability of ferric complexes to catalyze microsomal chemiluminescence, lipid peroxidation, and hydroxyl radical generation

The interaction of microsomes with iron and NADPH to generate active oxygen radicals was determined by assaying for low level chemiluminescence. The ability of several ferric complexes to catalyze light emission was compared to their effect on microsomal lipid peroxidation or hydroxyl radical generation. In the absence of added iron, microsomal light emission was very low; chemiluminescence could be enhanced by several cycles of freeze-thawing of the microsomes. The addition of ferric ammonium sulfate, ferric-citrate, or ferric-ADP produced an increase in chemiluminescence, whereas ferric-EDTA or -diethylenetriaminepentaacetic acid (detapac) were inhibitory. The same response to these ferric complexes was found when assaying for malondialdehyde as an index of microsomal lipid peroxidation. In contrast, hydroxyl radical generation, assessed as oxidation of chemical scavengers, was significantly enhanced in the presence of ferric-EDTA and -detapac and only weakly elevated by the other ferric complexes. Ferric-desferrioxamine was essentially inert in catalyzing any of these reactions. Chemiluminescence and lipid peroxidation were not affected by superoxide dismutase, catalase, or competitive hydroxyl radical scavengers whereas hydroxyl radical production was decreased by the latter two but not by superoxide dismutase. Chemiluminescence was decreased by the antioxidants propylgallate or glutathione and by inhibiting NADPH-cytochrome P-450 reductase with copper, but was not inhibited by metyrapone or carbon monoxide. The similar pattern exhibited by ferric complexes on microsomal light emission and lipid peroxidation, and the same response of both processes to radical scavenging agents, suggests a close association between chemiluminescence and lipid peroxidation, whereas both processes can be readily dissociated from free hydroxyl radical generation by microsomes.

Oxidation of pyrazole to 4-hydroxypyrazole by intact rat hepatocytes

4-Hydroxypyrazole has been identified as a major metabolite found in the urine of rats and mice after in vivo administration of pyrazole, a potent inhibitor of alcohol dehydrogenase and of ethanol metabolism. The locus and the enzyme systems responsible for the oxidation of pyrazole have not been identified. In the current report, isolated hepatocytes from fed rats were shown to oxidize pyrazole to 4-hydroxypyrazole. An HPLC procedure employing UV and electrochemical detection was utilized to separate and quantify the 4-hydroxypyrazole. The apparent Km for pyrazole by intact hepatocytes was about 2 mM, whereas the apparent Vmax was about 0.06 nmol 4-hydroxypyrazole per min per mg liver cell protein. The production of 4-hydroxypyrazole was inhibited by carbon monoxide and metyrapone, as well as by competitive drug substrates such as aniline or aminopyrine. These results implicate a role for cytochrome P-450 in the oxidation of pyrazole by the hepatocytes. Ethanol was an effective inhibitor of pyrazole oxidation. Hepatocytes were also isolated from rats treated with acetone and 4-methylpyrazole, to attempt to evaluate whether pyrazole oxidation is induced. The rate of 4-hydroxypyrazole production by hepatocytes after acetone and 4-methylpyrazole treatment was actually lower than that of controls. Kinetic assays suggested the presence of an endogenous inhibitor (perhaps the inducer itself) in the induced hepatocytes. In contrast, hepatocytes isolated from rats fasted for 48 hr showed a 2-fold increase in the oxidation of pyrazole to 4-hydroxypyrazole. The Km for pyrazole was the same in hepatocytes from fasted and fed rats, whereas Vmax was increased after fasting. The locus and enzyme system responsible for the oxidation of pyrazole to 4-hydroxypyrazole, and the site of sensitivity to ethanol, appears to be the cytochrome P-450 system of the hepatocyte.