RT Journal Article
SR Electronic
T1 Bimolecular Glutathione Conjugation Kinetics of Ethacrynic Acid in Rat Liver: In Vitro and Perfusion Studies
JF Journal of Pharmacology and Experimental Therapeutics
JO J Pharmacol Exp Ther
FD American Society for Pharmacology and Experimental Therapeutics
SP 1230
OP 1241
VO 290
IS 3
A1 Tirona, Rommel G.
A1 Pang, K. Sandy
YR 1999
UL http://jpet.aspetjournals.org/content/290/3/1230.abstract
AB The conjugation kinetics of glutathione (GSH) and ethacrynic acid (EA) were studied in rat liver perfusion studies, where efficient removal occurred (steady-state extraction ratio Ess, ∼0.8–0.4 at concentrations ranging from 10–200 μM) despite the appreciable plasma protein binding. The declining Ess paralleled the saturation in GSH conjugate (EA-SG) formation; EA-SG primarily appeared in bile as the unchanged glutathionyl adduct (90%) and minimally as cleavage products. The GSH conjugation of EA in perfused liver was described by the constants Kmoverall of 67 μM and Vmaxoverall of 0.23 μmol/min/g liver. These differed from those observed for the bimolecular nonenzymatic (constant of 126 μM−1 min−1) and enzymatic (Km for GSH and EA were 1.2 mM and 94 μM, respectively; Vmax of 533 nmol/min/mg liver cytosolic protein or 32 μmol/min/g liver) GSH conjugation of EA in vitro. But they were similar to those estimated for EA uptake in isolated rat hepatocytes by saturable (Kmuptake = 57 μM, and Vmaxuptake = 0.55 μmol/min/g liver) and nonsaturable (0.015 ml/min/mg) processes. At increasing EA concentrations (>25 μM), time-dependent changes were observed for Ess and EA-SG formation, which rapidly decreased with time after the attainment of steady state due to the rapid loss of cellular GSH. The composite data were described adequately by a physiological model that accounted for transport and the GSH-dependent conjugation of EA. The results suggest that the rate-limiting process for hepatic EA GSH conjugation is cellular uptake, but cosubstrate availability controls the rate of metabolism when GSH becomes depleted. The American Society for Pharmacology and Experimental Therapeutics