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Cooxidation of the Clinical Reagent 3,5,3'5'-Tetramethylbenzidine by Prostaglandin Synthase

Laboratories of Pulmonary Function and Toxicology [P. D. J., T. E.] and of Environmental Biophysics [R. P.M.], National Institute of Environmental Health Sciences, Research Triangle Park, North Carolina 27709

Prostaglandin synthase catalyzes the oxidation of arachidonic acid to prostaglandin H₂ via a hydroperoxide intermediate, prostaglandin G₂. The prostaglandin synthase system cooxidizes 3,5,3'5'-tetramethylbenzidine (TMB), a derivative of the human carcinogen benzidine, to colored products. This process is arachidonic acid dependent and indomethacin sensitive. The reaction is also supported by hydroperoxides, and, in this case, indomethacin has no effect. This suggests that the cooxidation is mediated by the hydroperoxidase activity of prostaglandin synthase. The products of TMB oxidation by this system are the same as those obtained with lactoperoxidase and H₂O₂ or with horseradish peroxidase and H₂O₂. The initial products of TMB oxidation are the TMB radical cation and a charge-transfer complex composed of TMB and its two-electron (diimine) oxidation product. The TMB radical cation was identified by electron spin resonance spectroscopy. Ascorbic acid reduces the products, regenerating the parent compound.

¹ Research fellow of the National Cancer Institute of Canada.

Received 11/17/81. Accepted 3/30/82.