In vitro metabolism of cyanohydroxybutene: Formation of a glutathione-S- transferase catalyzed product

The pancreatotoxin cyanohydroxybutene (CHB) causes a significant and prolonged elevation in glutathione (GSH) in liver and pancreas (Wallig and Jeffery, 1990). Here we report that urinary thiols also increase. This suggests that CHB may react with GSH, either directly or following phase I oxidation, to form an adduct, which is further metabolized to the corresponding mercapturic acid for urinary excretion. Metabolism of CHB by hepatic mixed function oxidase and cytosolic alcohol dehydrogenase enzymes was evaluated by monitoring microsomal NADPH consumption and alcohol dehydrogenase-dependent NADH generation, respectively. There was no apparent increase in the rate of microsomal NADPH consumption or alcohol dehydrogenase-dependent NADH generation in the presence of CHB. To evaluate in vitro formation of a glutathione-S-transferase (GST) catalyzed adduct, [³H-glycyl]-GSH and [¹⁴C-cyano]-CHB were incubated at 37°C for 1 h, with or without GST. Dinitrophenol derivatization and high performance liquid chromatographic (HPLC) analysis (Farris and Reed, 1987) revealed no double- labeled peaks, suggesting that no stable conjugate was formed. However a tritiated product, not present in control samples, and with an identical retention time to cysteinyl-glycine (cys-gly) was formed. In addition, the product has a fast atom bombardment mass-spectrum consistent with cys-gly. These results suggest that while CHB may not undergo phase I oxidation, in the presence of CHB, GSH may break down to form cys-gly. A mechanism for CHB- dependent breakdown of GSH to cys-gly is proposed, and the pharmacological implications of this finding are discussed.