Prostaglandin H synthase metabolism of the urinary bladder carcinogens benzidine and ANFT

Abstract

Prostaglandin H synthase (PHS) and horseradish peroxidase catalyze the oxidation of benzidine to the same free radical species. No radical was observed if either benzidine, H2O2 or enzyme was omitted. The similarity of the fine structure of this radical to a computer-simulated model suggests the presence of a free cation radical of benzidine. Neither superoxide nor hydroxyl radicals appear to be involved in the co-oxidation of benzidine or 2-amino-4-(5-nitro-2-furyl)-thiazole (ANFT) by PHS. Production of the benzidine radical by PHS was inhibited by ANFT, acetaminophen, cyanide and ascorbate. ANFT was metabolized by PHS but not by horseradish peroxidase. ANFT had no effect on either radical production or 14C-metabolism of benzidine by horseradish peroxidase. These results indicate that different peroxidases may exhibit specificity with respect to the carcinogens they activate. The free radical cation of benzidine may be the electrophilic intermediate responsible for PHS-catalyzed binding of benzidine to protein and nucleic acids.

Peroxygenase Metabolism of N-Acetylbenzidine by Prostaglandin H Synthase

“…The peroxygenation mechanism for metabolism of ABZ is dramatically different from the electron transfer mechanism observed for PHS metabolism of benzidine, the diamine analogue of ABZ (12)(13)(14)(41)(42)(43)(44). Both chemical and enzymatic experiments indicate the oxidation of benzidine to one-electron (cation radical) and two-electron (benzidinediimine) oxidized products.…”

“…The effects of indomethacin are consistent with the distinct fatty acid cyclooxygenase (inhibition) and the peroxidase (absence of inhibition) activities of PHS (13). Cyanide, a peroxidase inhibitor, prevented ABZ metabolism and has been shown to inhibit PHS metabolism of benzidine (12). 2,4-Dichloro-6-phenylphenoxyethylamine and SKF-525A prevented ABZ NADPH-dependent metabolism to NЈHA by rat liver microsomes (18).…”

“…Exposure to these amines is common due to their occurrence in cooked foods, cigarette smoke, and pharmaceuticals and in the manufacture or use of dyes, chemicals, and antioxidants (11). Benzidine, an aromatic amine reducing cofactor, is converted to a radical cation and subsequently converted to benzidinediimine, a two-electron oxidation product (12)(13)(14). In contrast, monoacetylated benzidine, N-acetylbenzidine (ABZ), yields an oxygenated metabolite, 4Ј-nitro-4-acetylaminobiphenyl (15).…”

Oxidative activation of benzidine and its derivatives by peroxidases.

“…Similarly, TMB oxidation by RSV microsomal preparation could be initiated with H202. Wise et al (41) obtained an ESR spectrum of the benzidine cation radical using Tween 20-solubilized RSV preparation, and initiating oxidation using H202 at pH 4.2. However, the authors did not report arachidonic acid-dependent metabolism studies, except to note that "results were essentially the same ... but with much reduced levels of metabolism at pH 5.0."…”

“…However, the authors did not report arachidonic acid-dependent metabolism studies, except to note that "results were essentially the same ... but with much reduced levels of metabolism at pH 5.0." (41).…”