, Volume 8, Issue 1, pp 45-62

Cation transport and electrogenesis byStreptococcus faecalis

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Summary

Glycolyzing cells ofStreptococcus faecalis 9790 accumulate large amounts of lipid-soluble cations such as dimethyldibenzylammonium (DDA+). We showed in the preceding paper that uptake of DDA+ occurs in response to an electrical potential, interior negative, which arises by extrusion of H+ and Na+. The experiments described here deal with the mechanism of electrogenesis. Evidence is presented to indicate that extrusion of protons is an electrogenic, energy-linked process which can proceed against the electrochemical gradient for H+. Proton extrusion is blocked by dicyclohexylcarbodiimide (DCCD), an inhibitor of the membrane-bound ATPase ofS. faecalis. However, in a mutant whose ATPase is resistant to DCCD, proton extrusion is also resistant to the inhibitor. We conclude from these results that the membrane-bound ATPase is involved in proton extrusion. Extrusion of sodium can also occur against the electrochemical gradient, but we find no evidence for the existence of an electrogenic sodium pump. It rather appears, from studies with ionophorous agents and inhibitors of the ATPase, that the cells extrude Na+ in exchange for H+; the H+ is then extruded by the proton pump. Evidence is presented for an influx of H+ coupled to the efflux of Na+.

Among the mutants known to be defective in K+ accumulation one class is deficient in proton extrusion and another lacks the Na+/H+ exchange. Thus, proton extrusion and Na+/H+ antiport are essential elements in K+ accumulation.