Vol. 291, Issue 3, 1210-1219, December 1999

Uptake and Glutathione Conjugation of Ethacrynic Acid and Efflux of the Glutathione Adduct by Periportal and Perivenous Rat Hepatocytes¹

Rommel G. Tirona, Eugene Tan, Galina Meier and K. Sandy Pang

Department of Pharmaceutical Sciences, Faculty of Pharmacy (R.G.T., E.T., G.M., K.S.P.), and Department of Pharmacology, Faculty of Medicine (K.S.P.), University of Toronto, Toronto, Ontario, Canada

We assessed the impact of zonal factors on the hepatic reduced glutathione (GSH) conjugation of ethacrynic acid (EA). Uptake of EA by enriched periportal (PP) and perivenous (PV) rat hepatocytes was characterized by both saturable (V_max^uptake = 3.4 ± 1.7 and 3.2 ± 0.8 nmol/min/mg protein and K_m^uptake = 51 ± 13 and 44 ± 15 µM) and nonsaturable (12 ± 5 and 12 ± 3 µl/min/mg protein) components. Values for the overall GSH conjugation rates of EA (200 µM) were similar among the zonal hepatocytes and resembled those for the influx transport rates. In the absence of the hepatocyte membrane, GSH conjugation in PV and PP hepatocyte cytosol was similar, but a higher perivenous GSH conjugation activity toward EA (PV/PP of 2.4) that mirrored the higher PV/PP ratios of immunodetectable GSTs Ya (1.7) and Yb2 (2.5) was found in cell lysates obtained by the dual-digitonin-pulse perfusion technique. The GSH conjugation rates in the subcellular fragments were, however, much greater than those observed for intact hepatocytes. Efflux rates of the glutathione conjugate EA-SG from zonal hepatocytes were similar, as were levels of the immunodetectable multidrug-resistance protein 2/canalicular multispecific organic anion transporter (Mrp2/cMoat) in the 100,000g pellets. The composite results suggest that the GSTs responsible for EA metabolism are more abundant in the PV region, albeit that the gradient of enzymatic activities is shallow. Despite the existence of zonal metabolic activity, the overall GSH conjugation rate of EA is homogeneous among cells because the reaction is rate limited by uptake, which occurs evenly. Results on EA-SG efflux suggest the acinar homogeneity in Mrp2/cMoat function for canalicular transport.