Uptake of Glycyrrhizin by Isolated Rat Hepatocytes

抄録

The mechanism of uptake of glycyrrhizin (GLZ) by isolated rat liver cells was studied. The uptake rate was dependent on the unbound GLZ concentation. 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 K_m of 11.3 μM, V_max of 0.112 nmol/min/10⁶ cells, and a first-order rate constant (K_d) of 0.195 nmol/min/10⁶ cells/mM. GLZ adsorption on the cell membrane occurs at two types of binding sites with a linear adsorption coefficient=2.81 nmol/10⁶ cells/mM and a dissociation constant=18.3 μM and its adsorption capacity=0.12 nmol/10⁶ 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 Arrhenium plot of uptake of 10 μM GLZ presented a single straight line in the range of 4-37°C, with an activation energy of 15.9 kcal/mol. An energy requirement was also demonsrated, as all metabolic inhibitors studied (rotenone, antimycin A, 2, 4-dinitrophenol, and KCN) significantly reduced the uptake of 10 μM GLZ (p<0.01). The uptake was competitively inhibited by glycyrrhetinic acid (GLA), taurocholate (TCA), and probenecid (PBC) with inhibition constants, K_i, of 13.7, 48.5, and 115.9 μM, respectively, and it was noncompetitively inhibited by bromosulfophthalein (K_i 9.2 μM) and indocyanine green (K_i 13.5 μM) only at low GLZ concentrations (5 and 10 μM).It was concluded that a carrier-mediated transport system participates in the uptake of GLZ into isolated rat hepatocytes and the affinity site of the transport carrier commonly binds GLA, TCA, and PBC.