Involvement of monooxygenases and amine oxidases in hydroxyl radical generation in vivo.

The levels of hydroxyl radicals in vivo were measured in rat blood plasma by determining the formation of 2,3-dihydroxybenzoate (2,3-DHB) resulting from the attack of hydroxyl radicals on injected salicylate. This approach was used to study the effects of alterations in the activities of cytochrome P-540 (CYP)-dependent hydroxylases (monooxygenases) and monoamine oxidase (MAO), two groups of enzymes that can produce reactive oxygen species in the reactions they catalyse. Pretreatment with inducers of the cytochrome P540 (CYP) hydroxylases, such as phenobarbital and dexamethasone, resulted in substantial increases in the plasma oxygen radical formation, suggesting that the action of these enzymes on endogenous substrates, or on exogenous substrates such as salicylate, may contribute to oxygen radical formation in vivo. In contrast, pretreatment with the monoamine oxidase (MAO) inhibitors clorgyline and deprenyl did not have any significant effect on the plasma 2,3-DHB levels, perhaps reflecting the different intracellular locations of MAO and the CYP-dependent hydroxylases. Injection of pentylamine, a substrate of MAO-B and semicarbazide-sensitive amine oxidase (SSAO) was also without significant effect.