Phenylbutazone peroxidatic metabolism and conjugation

Phenylbutazone, a nonsteroidal anti-inflammatory drug, elicits therapeutic as well as toxic effects by unknown pathways. Phenylbutazone was shown to form a conjugate with the heterocyclic amine bladder carcinogen 2-amino-4-(5-nitro-2-furyl)-thiazole (ANFT). To understand further the reactivity of these compounds, this study was conducted to identify the conjugate formed and determine the mechanism of conjugate formation. Both prostaglandin H synthase and horseradish peroxidase catalyzed conjugate formation. This conjugate was identified by 1H-NMR to be 4-[2-amino-4-(5-nitro-2-furyl)-5-thiazolyl]-4-butyl-1,2-diphenyl-3,5- pyrazolidinedione. Phenylbutazone-mediated oxygen uptake was inhibited by ANFT (0.1 mM) and the spin traps 5,5-dimethyl-1-pyrroline-N-oxide (200 mM) and tert-nitrosobutane (4 mM). By contrast, phenol (0.005 to 0.25 mM) and aminopyrine (0.4 mM) stimulated oxygen uptake. None of these agents mediated oxygen uptake in the absence of phenylbutazone. Conjugate formation was significantly increased by phenol (0.005-0.25 mM) and aminopyrine (0.4 mM), as well as in the absence of oxygen. Conjugate formation was inhibited by 5,5-dimethyl-1-pyrroline-N-oxide (200 mM), tert-nitrosobutane (4 mM), ascorbic acid (2 mM), and 95% oxygen. Horseradish peroxidase initiated conjugate formation at much lower concentrations than it metabolized ANFT. The stoichiometric relationship between phenylbutazone and ANFT, with respect to conjugate formation, was complex. With the concentration of ANFT fixed at 0.05 mM, phenylbutazone exhibited saturation kinetics with a Km of 0.2 mM. In contrast, saturation kinetics were not observed with ANFT.Km values for ANFT varied with the concentration of phenylbutazone used.(ABSTRACT TRUNCATED AT 250 WORDS)