Heefner D L, Harold F M
J Biol Chem. 1980 Dec 10;255(23):11396-402.
Streptococcus faecalis, like other bacteria, expels Na+ and accumulates K+. Sodium movements in several bacterial species have been attributed to secondary antiport of Na+ for H+, energized by the proton-motive force. We find a more complex pattern: a circulation of Na+ across the plasma membrane. One limb is the diffusion of Na+, into the cells or out, by a low affinity pathway in response to gradients of concentration and of electrical potential; Na+ movements are enhanced in metabolizing cells. The other limb is the vectorial extrusion of Na+ by a transport system that requires "ATP" (either ATP itself or a related metabolite), even when Na+ is moving downhill. Cells glycolyzing at alkaline pH in buffer containing excess K+ can expel Na+ against a concentration gradient of 100-fold, even in the presence of reagents that block or short circuit the proton circulation (pH gradient and membrane potential both zero). Evidently, under these conditions "ATP" can serve as the energy donor for a primary sodium pump. However, at acid pH, or in presence of low levels of K+, sodium extrusion requires both "ATP" and the proton-motive force. A mutant is described that retains the leak pathway but lacks the "ATP"-linked transport system.
粪链球菌与其他细菌一样，会排出Na⁺并积累K⁺。几种细菌中的钠转运被认为是由质子动力驱动的Na⁺与H⁺的继发性反向转运。我们发现了一种更复杂的模式：Na⁺在质膜上循环。一个分支是Na⁺通过低亲和力途径根据浓度梯度和电势梯度扩散进入或流出细胞；在代谢活跃的细胞中，Na⁺的转运增强。另一个分支是由一个需要“ATP”（ATP本身或相关代谢物）的转运系统进行的Na⁺的矢量性外排，即使Na⁺是顺浓度梯度移动。在含有过量K⁺的缓冲液中在碱性pH下进行糖酵解的细胞，即使在存在阻断或短路质子循环（pH梯度和膜电位均为零）的试剂的情况下，也能逆着100倍的浓度梯度排出Na⁺。显然，在这些条件下，“ATP”可以作为初级钠泵的能量供体。然而，在酸性pH下或在低钾水平存在时，钠的外排既需要“ATP”也需要质子动力。描述了一种突变体，它保留了泄漏途径但缺乏与“ATP”相关联的转运系统。
