Conjugation of Acrylonitrile and 2-Cyanoethylene Oxide with Hepatic Glutathione

Abstract

The glutathione (GSH) conjugation of the rat carcinogen acrylonitrile (ACN) and its epoxide metabolite 2-cyanoethylene oxide (CEO) by rat, mouse, and human liver enzymes was characterized in vitro since GSH conjugation is the major disposition pathway for these chemicals in vivo. Mass spectral analyses indicated that S-(2-cyanoethyl)GSH was the product from reaction of GSH and ACN and that S-(cyanohydroxyethyl)GSH reaction products were formed from CEO. Because of the rapid nonenzymic reactions of ACN and CEO with GSH at pH 7.3, the steady-state kinetics of hepatic GSH conjugation were determined at pH 6.5 by HPLC analysis of the products. Hyperbolic kinetics were observed with respect to GSH for the reactions catalyzed by mouse or rat hepatic cytosols at pH 6.5, whereas sigmoidal kinetics were observed with respect to ACN or CEO. This kinetic pattern is consistent with the random sequential kinetic mechanism that has been described for GSH S-transferases. Estimates of the maximal velocities of the reaction at pH 6.5 showed that mouse enzymes had a 4- to 6-fold greater capacity for GSH conjugation of ACN and CEO than rat enzymes. ACN appeared to be conjugated with GSH more efficiently than CEO under these conditions. At physiological pH (7.3), rapid nonenzymic conjugation of GSH (10 mM) with ACN or CEO (5 mM) was observed (∼25 and 15 nmol product/min, respectively). Addition of hepatic cytosols or microsomes from rats or mice increased the velocity of GSH conjugation ∼1.6-fold. A similar velocity enhancement was observed with human liver cytosols for the GSH conjugation of ACN, but not for CEO. Human liver microsomes did not enhance the velocity of GSH conjugation of either substrate. These results suggest that ACN is a better substrate for human liver GSH S-transferases than CEO. Estimation of the initial velocities of the GSH conjugation reactions in intact rodent liver from the in vitro data at pH 7.3 suggests that the enzyme-mediated GSH conjugation of ACN and CEO will be ~4-fold greater than the velocity of the direct chemical reaction with GSH.