Wood J M
J Biol Chem. 1975 Jun 25;250(12):4477-85.
The multiple active transport systems mediating L-leucine accumulation in Escherichia coli strain 7 (K12) and ML 308-225 have been examined. In addition to the previously characterized osmotic shock-sensitive LS (L-leucine-specific) and LIV-I (L-leucine; L-isoleucine-, and L-valine-specific) activities, a third system (designated LIV-II) has been detected, confirming a report by Rahmanian et al. (RAHMANIAN, M., CLAUS, D.R., and OXENDER, D. L. (1973) J. Bacteriol. 116, 1258-1266). This third system transports L-leucine, L-isoleucine, and L-valine with a relatively low affinity (apparent transport Km equals 4 muM for L-leucine) and it is resistant to repression by cell growth on L-leucine. Exploitation of these properties and of the differential sensitivity of the three transport activities to inhibition by L-leucine analogues permits estimation of the contribution by each system to the total transport activity under varying conditions. Such experiments show that, unlike systems LS and LIV-I, system LIV-II is resistant to osmotic shock. The L-leucine, L-iosleucine, and L-valine transport activity in membrane vesicles from strain ML 308-225 has the properties of system LIV-II. Although the L-leucine transport activities in strains 7 and ML 308-225 are in all other respects similar, membrane vesicles from strain 7 do not transport L-leucine, L-isoleucine, or L-valine. L-leucine transport under various conditions of energy supply has been measured in strain ML 308-225 and the corresponding Mg-2+-ATP-ASE-DEFICIENT STRAIN, DL-54. These measurements support the view that the osmotic shock-sensitive LS and LIV-I activities depend on the synthesis of ATP, while the osmotic shock-resistant LIV-II activity depends on the energized membrane state generated by electron flow but not on ATP synthesis, per se. This conclusion is not supported by the inhibitory effects of the energy poisons arsenate and 2,4-dinitrophenol, but these compounds may have secondary chemical effects on the transport systems.
对介导大肠杆菌7（K12）和ML 308 - 225菌株中L - 亮氨酸积累的多种主动转运系统进行了研究。除了先前已表征的对渗透休克敏感的LS（L - 亮氨酸特异性）和LIV - I（L - 亮氨酸；L - 异亮氨酸和L - 缬氨酸特异性）活性外，还检测到了第三种系统（命名为LIV - II），这证实了拉赫曼尼亚等人的报告（拉赫曼尼亚，M.，克劳斯，D.R.，和奥克森德，D.L.（1973年）《细菌学杂志》116，1258 - 1266）。这个第三种系统以相对较低的亲和力转运L - 亮氨酸、L - 异亮氨酸和L - 缬氨酸（L - 亮氨酸的表观转运Km等于4μM），并且它对细胞在L - 亮氨酸上生长时的阻遏具有抗性。利用这些特性以及三种转运活性对L - 亮氨酸类似物抑制的不同敏感性，可以估计每个系统在不同条件下对总转运活性的贡献。此类实验表明，与LS和LIV - I系统不同，LIV - II系统对渗透休克具有抗性。来自ML 308 - 225菌株的膜囊泡中的L - 亮氨酸、L - 异亮氨酸和L - 缬氨酸转运活性具有LIV - II系统的特性。尽管7菌株和ML 308 - 225菌株中的L - 亮氨酸转运活性在所有其他方面都相似，但来自7菌株的膜囊泡不转运L - 亮氨酸、L - 异亮氨酸或L - 缬氨酸。在ML 308 - 225菌株和相应的Mg - 2 + - ATP酶缺陷菌株DL - 54中测量了在各种能量供应条件下的L - 亮氨酸转运。这些测量结果支持这样一种观点，即对渗透休克敏感的LS和LIV - I活性依赖于ATP的合成，而对渗透休克抗性的LIV - II活性依赖于电子流产生的带电膜状态，而不是ATP本身的合成。能量毒物砷酸盐和2,4 - 二硝基苯酚的抑制作用不支持这一结论，但这些化合物可能对转运系统有次生化学作用。
