NADPH-dependent oxidation of benzidine by rat liver

This study used liver microsomes from control and beta-naphthoflavone-treated rats to evaluate NADPH-dependent oxidation of benzidine, With microsomes from beta-naphthoflavone-treated rats, the rates of formation of aqueous soluble metabolite (HPLC analysis) and protein and DNA binding were 835 +/- 81, 14.5 +/- 1.8 and 0.71 +/- 0.14 pmol/mg/min respectively. beta-Naphthoflavone treatment elicited 12.3-, 1.8- and 14.2-fold increases in benzidine metabolism compared with controls as judged by HPLC and protein and DNA binding respectively, For microsomes from treated animals, K-m and V-max values were 47 +/- 6 mu M and 1.13 +/- 0.16 nmol/mg protein/min respectively, All of the metabolic parameters were inhibited to varying degrees by glutathione (1 or 10 mM), N-acetylmethionine (10 mM) and ascorbic acid (10 mM), Following glutathione addition, at least two new metabolite peaks were observed, representing similar to 6% of the total radioactivity recovered by HPLC, Neither metabolite was 3-(glutathion-S-yl)benzidine. Cytochrome P450 inhibitors (10 mu M) specific for different members of cytochrome gene families 1-3 indicated that benzidine was metabolized by cytochrome P450 1A1/1A2, Ellipticine and alpha-naphthoflavone, specific 1A1/1A2 inhibitors, elicited 50% inhibition at similar to 0.2 and 0.5 mu M respectively, Electron impact and negative ion chemical ionization mass spectrometry identified the aqueous soluble metabolite as 3-hydroxybenzidine, The lability of 3-hydroxybenzidine observed at pH > 7.0 was prevented by ascorbic acid, Thus, cytochrome P450 1A1/1A2 NADPH-dependent metabolism of benzidine to 3-hydroxybenzidine was demonstrated.