methimazole and benzidine

Metabolism of benzidine was assessed with rabbit renal inner medullary slices. 3-(Glutathion-S-yl)-benzidine was identified as a product of metabolism. This thioether conjugate was shown to be identical to synthetic conjugate by chromatographically assisted hydrodynamic voltammetric and enzymatic techniques. A good correlation between PGE2 synthesis and conjugate formation was observed with a variety of incubation conditions including tissue weight, arachidonic acid concentration and incubation time. With 0-0.01 mM idomethacin, an inhibitor of the fatty acid cyclo-oxygenase component of prostaglandin H synthase (PHS), a linear relationship between conjugate formation and prostaglandin E2 synthesis was observed. In contrast, the peroxidase cosubstrates propylthiouracil, phenidone, ascorbate and methimazole inhibited arachidonic acid stimulation of conjugate formation but not prostaglandin E2 synthesis. These cosubstrates may be functioning as competitive inhibitors of benzidine co-oxidation. The results are consistent with peroxidatic metabolism of benzidine in intact tissue by a PHS-mediated process. 3-(Glutathion-S-yl)-benzidine may be a useful marker for studying peroxidatic metabolism in intact tissue and in investigating selective inhibition of this process.

Topics: Animals; Arachidonic Acid; Arachidonic Acids; Ascorbic Acid; Benzidines; Dinoprostone; In Vitro Techniques; Indomethacin; Kidney Medulla; Kinetics; Methimazole; Peroxides; Propylthiouracil; Prostaglandin-Endoperoxide Synthases; Prostaglandins E; Pyrazoles; Rabbits

Carcinogens which cause cancers in tissues distal to their entry are thought to require metabolic activation before covalent binding to macromolecules. The hydroperoxidase component of prostaglandin H synthase (PHS) activates certain carcinogens and a model describing this process is presented. The procarcinogen benzidine was used to identify sites at which microsomal PHS-catalyzed binding might be inhibited by pharmacologic agents. Activation of benzidine was determined by assessing free radical cation formation and covalent binding to protein. Reduction of benzidine diimine to diamine was also assessed. This study provides the first demonstration of inhibition of PHS-activated benzidine binding by propylthiouracil, methimazole, MK447, vitamin C and phenidone. The agents tested identified the following sites at which PHS-catalyzed binding of benzidine can be prevented: 1) inhibition of generation of the peroxide cosubstrate for benzidine oxidation; 2) inhibition of prostaglandin hydroperoxidase; 3) reduction of oxidized intermediate(s) to the parent compound; and 4) conjugation of the activated intermediate(s). This study provides a basis for further investigations of the pharmacologic intervention of chemical carcinogenesis.

Topics: Animals; Arachidonic Acid; Arachidonic Acids; Ascorbic Acid; Benzidines; Binding Sites; Butylated Hydroxytoluene; Carcinogens; Dose-Response Relationship, Drug; Drug Interactions; Indomethacin; Male; Methimazole; Microsomes; Models, Chemical; Peroxidases; Propylthiouracil; Prostaglandin Endoperoxides; Prostaglandin-Endoperoxide Synthases; Prostaglandins H; Protein Binding; Pyrazoles; Rabbits; Stimulation, Chemical