The in vivo disposition of 2-bromo-[¹⁴C]hydroquinone and the effect of γ-glutamyl transpeptidase inhibition

We have previously shown that the renal necrosis observed after 2-bromohydroquinone (2-BrHQ) administration to rats is probably caused by the formation of 2-Br-(diglutathion-S-yl)HQ (2-Br-[diGSyl]HQ), since injection of this conjugate caused severe proximal tubular necrosis. In the present study we report the in vivo metabolism and covalent binding of 2-[¹⁴C]-BrHQ in male Sprague-Dawley rats. The major urinary and biliary metabolite was a glucuronide conjugate. In addition, 2-Br-(di-GSyl)HQ, 2-Br-3-(GSyl)HQ, 2-Br-5-(GSyl)HQ, and 2-Br-6-(GSyl)HQ were all detected as urinary and biliary metabolites of 2-BrHQ. The in vivo covalent binding of 2-[¹⁴C]BrHQ to kidney, pancreas, seminal vesicles, intestine, bone marrow, and liver was 21.8, 1.5, 1.2, 4.4, 1.8, and 2.6 nmol/mg protein, respectively. γ-Glutamyl transpeptidase (γ-GT) activity measured in these tissues was 947, 159, 55, 31, and 5.5 U/mg. Liver γ-GT activity was negligible (0.07 U/mg). Thus, maximum covalent binding and γ-GT activity occurred in the kidney. Renal covalent binding and γ-GT activity were positively correlated with nephrotoxicity. Pretreatment of rats with l(αS,5S)-α-amino-3-chloro-4,5-dihydro-5-isoxazoleacetic acid (AT-125) inhibited renal γ-GT, after 24 hr, by 76%, renal covalent binding by 73%, and 2-BrHQ-mediated nephrotoxicity, as assessed by elevations in blood urea nitrogen (BUN), by 70%. These alterations were accompanied by an increase in the urinary excretion of each of the GSH conjugates, an increase in the fecal excretion of total radioactivity, and a decrease in plasma radioactivity at 24 hr. The present data provide evidence that 2-BrHQ is metabolized in vivo to nephrotoxic GSH conjugates. In addition, AT-125 probably inhibits nephrotoxicity by decreasing the γ-GT-mediated renal proximal tubule accumulation of the toxic metabolites, thereby facilitating their excretion into urine. Although AT-125 inhibited extrarenal γ-GT activity by 34-77%, it had variable effects on extrarenal covalent binding. Whereas covalent binding to renal tissue is probably mediated by reactive metabolites of the isomeric 2-Br-(GSyl)HQ conjugates, binding to extrarenal tissue may be mediated by both the conjugates and by 2-bromohydroquinone per se.