Analysis of the transport system for inorganic anions in normal and transformed hepatocytes

Abstract

The transport system for inorganic anions has been investigated in hepatocytes and hepatoma tissue culture cells. Sulfate transport in hepatocytes is temperature sensitive and occurs against an electrochemical gradient. Uptake was shown to occur by a sodium-dependent and a sodium-independent route with Km values of 2.3 and 33 mM and Vmax values of 2.1 and 10 nmol/mg of protein/min, respectively. An analysis of the sodium dependency indicates a Hill coefficient of 1.05 suggesting an equimolar stoichiometry for sodium and sulfate transport. The transport of sulfate was decreased by metabolic and sodium transport inhibitors. Bicarbonate was shown to effect the transport of sulfate, where uptake was accelerated by intracellular bicarbonate and competitively inhibited by extracellular bicarbonate. In addition, sulfate efflux was stimulated by extracellular bicarbonate. These results suggested that bicarbonate is a substrate for the sulfate transport system and can accelerate uptake and efflux by an anion exchange mechanism. Inhibition of bicarbonate uptake by extracellular sulfate and by the anion transport inhibitor 4,4'-diisothiocyano-2,2'-stilbene disulfonate demonstrates that bicarbonate does not enter the cell exclusively by CO2 diffusion but can be transported in part as an anionic species. These results are consistent with its role in the sulfate-bicarbonate exchange system. This inorganic anion transport system was shown to be inhibited by approximately 80% in hepatoma tissue culture cells where altered sodium dependency, Km, and Vmax values reflect possible alterations in the structure and/or membrane content of the carrier.