Uptake of glycyrrhizin by isolated rat hepatocytes

Abstract

The mechanism of uptake of glycyrrhizin (GLZ) by isolated rat liver cells was studied. The uptake rate was dependent on the unbound GLZ concentration. The initial uptake rate with respect to the unbound GLZ concentration reflected the operation of both saturable and nonsaturable processes, which followed Michaelis-Menten type kinetics; the process involves a Km of 11.3 microM, Vmax of 0.112 nmol/min/10(6) cells, and a first-order rate constant (Kd) of 0.195 nmol/min/10(6) cells/mM. GLZ adsorption on the cell membrane occurs at two types of binding sites with a linear adsorption coefficient = 2.81 nmol/10(6) cells/mM and a dissociation constant = 18.3 microM and its adsorption capacity = 0.12 nmol/10(6) cells describing specific adsorption. GLZ uptake did not require the presence of Na+ in the incubation medium and was not significantly inhibited by ouabain. The Arrhenius plot of uptake of 10 microM GLZ presented a single straight line in the range of 4-37 degrees C, with an activation energy of 15.9 kcal/mol. An energy requirement was also demonstrated, as all metabolic inhibitors studied (rotenone, antimycin A, 2,4-dinitrophenol, and KCN) significantly reduced the uptake of 10 microM GLZ (p < 0.01). The uptake was competitively inhibited by glycyrrhetinic acid (GLA), taurocholate (TCA), and probenecid (PBC) with inhibition constants, Ki, of 13.7, 48.5, and 115.9 microM, respectively, and it was noncompetitively inhibited by bromosulfophthalein (Ki 9.2 microM) and indocyanine green (Ki 13.5 microM) only at low GLZ concentrations (5 and 10 microM).(ABSTRACT TRUNCATED AT 250 WORDS)