Lipoxygenase—Another Pathway for Glutathione Conjugation of Xenobiotics: A Study with Human Term Placental Lipoxygenase and Ethacrynic Acid

Abstract

In this study, we examined the ability of human term placental lipoxygenase (HTPLO) to catalyze glutathione (GSH) conjugate formation from ethacrynic acid (EA) in the presence of linoleic acid (LA) and GSH. HTPLO purified by affinity chromatography was used in all the experiments. The results indicate that the process of EA-SG is enzymatic in nature. The reaction shows dependence on pH, the enzyme, and the concentration of GSH, LA, and EA. The optimal assay conditions to observe a maximal rate of EA-SG formation required the presence of 0.3 mM LA, 0.2 mM EA, 2.0 mM GSH, and ∼300 μg HTPLO in the reaction medium buffered at pH 9.0. Under the experimental conditions employed, the reaction exhibited Km values of 1.1 mM, 200 μM, and 130 μM for GSH, LA, and EA, respectively. The estimated specific activity of HTPLO-catalyzed EA-GS formation was ∼4.4 ± 0.4 μmol/min/mg protein. This rate is more than twofold greater than the rate noted for the reaction mediated by the purified human term placental glutathione transferase. Under physiologically relevant conditions (20 μM LA, 2.0 mM GSH, at pH 7.4), HTPLO produced EA-SG at 56% of the maximal rate noted under optimal assay conditions. Nordihydroguaiaretic acid, the classical inhibitor of different lipoxygenases, significantly blocked the reaction. It is proposed that free radicals are involved in the process of EA-SG formation by HTPLO. The evidence gathered in this in vitro study suggests for the first time that lipoxygenase present in the human term placenta is capable of EA-SG formation.