Evidence for the Localization of Hydrogen Peroxide-Stimulated Cyclooxygenase Activity in Rat Brain Mitochondria: A Possible Coupling with Monoamine Oxidase

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Abstract
The distribution of basal and of H2O2-stimulated cyclooxygenase activity in the primary fractions of rat brain homogenates and in the subfractions of crude mitochondrial fraction was studied. The localization of H2O2-generating monoamine oxidase (MAO) as well as that of the mitochondrial marker succinate dehydrogenase (SDH) was also examined. H2O2 was generated by MAO using 5 .times. 10-4 M noradrenaline [norepinephrine (NA)] or 2 .times. 10-4 M 2-phenylethylamine (PEA) as substrates, or by 25 .mu.g glucose oxidase (GO)/ml in the presence of 1 mM glucose. For nonstimulated (basal) cyclooxygenase, the relative specific activity (RSA) was high in microsomes (1.79) and in the free mitochondria-containing subfraction of the crude mitochondrial fraction (1.94). Parallel distribution of MAO and H2O2-stimulated cyclooxygenase was observed in all fractions studied in the presence of NA. The highest RSA was found in the purified mitochondria for both enzymes (1.85 for MAO and 1.97 for H2O2-stimulated cyclooxygenase). The enrichment of SDH (RSA = 2.21) indicated a high concentration of mitochondria in this fraction. The same distribution of H2O2-stimulated cyclooxygenase was obtained when, instead of the MAO-NA system, H2O2 was generated by GO in the presence of glucose. H2O2 generated by deamination of NA or PEA by MAO, or during the enzymatic oxidation of glucose by GO, caused a 3-fold increase in in mitochondrial endoperoxide formation. Indomethacin (2 .times. 10-4 M), catalase (50 .mu./ml) and pargyline (2 .times. 10-4 M) eliminated the MAO-dependent mitochondrial synthesis of PG endoperoxides. The GO-dependent cyclooxygenase activity of this fraction was abolished by indomethacin or catalase, but not by pargyline. The results show the existence of a mitochondrial cyclooxygenase in brain tissue. The enzyme is sensitive to H2O2 and produces prostaglandin endoperoxides from an endogenous source of arachidonic acid. The identical localization of H2O2-producing MAO and H2O2-sensitive cyclooxygenase suggests a possible coupling between monoamine and arachidonic acid metabolism.