The transport of sulfate ions across the membrane of the ehrlich ascites tumor cell

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Abstract
The applicability of the membrane fixed charge hypothesis to anion transport in Ehrlich ascites tumor cells was studied by investigating the dependence of steady state sulfate transport on the extracellular pH, chloride and sulfate concentration. When the extracellular sulfate was maintained at 10 mM both cellular sulfate and sulfate transport increased with decreasing pH and chloride concentration. The dependence of sulfate transport on the cellular sulfate concentration suggests a saturation phenomenon. The relationship between sulfate transport and cellular sulfate was also studied as a function of extracellular sulfate, both in the presence and absence of chloride. In both cases, sulfate transport is a saturable function of the cellular sulfate. However, in the presence of chloride the maximal flux is twice that in its absence. The discrepancy between the maximal fluxes suggests that the transport system mediates chloride-sulfate exchange in addition to sulfate self exchange. Unidirectional sulfate effluxes into chloride and sulfate-free medium; into 50 mM sulfate medium or 50 mM chloride medium were: 0.38, 1.95 and 3.91 nmoles/107 cells min-1, respectively. These results indicate that in the absence of either sulfate or chloride the net efflux, of sulfate is low. However, chloride or sulfate on the trans side of the membrane is effective in accelerating unidirectional sulfate efflux. Taken together, the results of this investigation cannot be explained in terms of the membrane fixed charge hypothesis. Rather, they support the contention that sulfate transport across the tumor cell membrane is a carrier-mediated process.