Polycyclic aromatic hydrocarbon (PAH) quinones are toxic environmental pollutants arising from the incomplete combustion of organic substances. In some instances they are formed in vivo as a consequence of the metabolic activation of PAHs (via oxidation of PAH dihydrodiols). The oxidation-reduction (redox) cycling of PAH quinones is catalyzed by one- and two-electron quinone reductases. This generates H2O2 and free radicals, including semiquinone and other reactive oxygen species, e.g., the superoxide anion (02) and the hydroxy radical (OH), which cause damage to cells. Superoxide dismutase (SOD) inhibits the redox cycling catalyzed by several two-electron quinone reductases. This suggests that the combined effect of the two-electron reductase and SOD provides a defense mechanism to protect cells against this type of quinone toxicity. Whether a cell is damaged by a particular quinone or protected from damage will depend on the biochemical makeup of that cell (its one-and two-electron quinone reductases, SOD and other defense mechanisms) and the ability of the quinone to produce free radicals or form adducts with cellular constituents. The long term objective of this proposal is to learn how the biochemical makeup of human cells influences the potential toxicity of PAH quinones.
The specific aims of this proposal are: 1) to determine the role of one= and two- electron reductases, of SOD, and of individual PAH quinones in the redox cycling of PAH quinones in subcellular fractions of human placenta and liver, 2) to assess the relative contributions of specific two-electron PAH quinone reductases in human placenta and liver, 3) to determine whether human placenta or liver has a dihydrodiol dehydrogenase (DDH) that oxidizes non-K-region trans-dihydrodiols and 4) to test the hypothesis that either the human placental/liver carbonyl reductase or the placental 17Beta-hydroxysteroid dehydrogenase (17Beta-HSD) could be responsible for DDH activity in human tissues. The methods used to reach the first and second goals will involve measurement of PAH quinone-mediated NAD(P)H oxidation and O2 production in subcellular fractions of human placenta and liver. The addition of specific enzyme inhibitors and of SOD will be used to evaluate the role of individual reductases and of SOD in PAH quinone- mediated redox cycling. Various quinones will be tested as substrates to determine the effect of quinone structure on this process. Gel filtration and ion exchange chromatography of the cytosol of human placenta and liver will be used to reach the second and third goals. Chromatography fractions will be assayed for PAH quinone reductase and for PAH dihydrodiol dehydrogenase activities. For the final goal, highly purified carbonyl reductase and 17B-HSD from human placenta will be used to find out whether either of these has DDH activity.
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