McLaggan D, Naprstek J, Buurman E T, Epstein W
Department of Molecular Genetics and Cell Biology, University of Chicago, Illinois 60637.
J Biol Chem. 1994 Jan 21;269(3):1911-7.
Escherichia coli responds to an increase in medium osmolarity by accumulating K+ and glutamate. At low osmolarity a large fraction of cytoplasmic K+ serves to balance charge on macromolecular anions. That fraction of K+ is here referred to as "bound," as distinguished from "free" K+ that serves to balance charge of small anions. At higher osmolarity where cytoplasmic K+ increases markedly, the bound fraction decreases but the absolute amount of bound K+ expressed per unit of dry weight increases. The increase in bound K+ can be explained largely by the reduction of cytoplasmic putrescine at high osmolarity. At high osmolarity, glutamate is the major cytoplasmic anion, equal to at least 70% of free cytoplasmic K+. A sudden increase in the osmolarity of the medium stimulates glutamate synthesis with a lag of only about a minute; glutamate synthesis is almost totally dependent on K+ uptake. The high rate of flow of nitrogen through the glutamate pool under control conditions of growth at low osmolarity indicates that glutamate accumulation immediately after shift to high osmolarity must be due to inhibition of utilization of glutamate in the synthesis of other nitrogen-containing compounds rather than stimulation of glutamate synthesis. In agreement with this reasoning we find the kinetics of glutamate accumulation to be independent of the specific path of synthesis, whether by glutamate dehydrogenase or by glutamate synthase. Synthesis of glutamate appears to be required to attain normal values of the electrical membrane potential after shift to high osmolarity.
大肠杆菌通过积累钾离子（K⁺）和谷氨酸来应对培养基渗透压的升高。在低渗透压下，大部分细胞质中的K⁺用于平衡大分子阴离子上的电荷。这部分K⁺在此被称为“结合态”，以区别于用于平衡小阴离子电荷的“游离态”K⁺。在较高渗透压下，细胞质中的K⁺显著增加，结合态部分减少，但每单位干重所表达的结合态K⁺的绝对量增加。结合态K⁺的增加在很大程度上可以通过高渗透压下细胞质中腐胺的减少来解释。在高渗透压下，谷氨酸是主要的细胞质阴离子，至少等于游离细胞质K⁺的70%。培养基渗透压的突然升高会刺激谷氨酸的合成，滞后时间仅约一分钟；谷氨酸的合成几乎完全依赖于K⁺的摄取。在低渗透压生长的对照条件下，氮通过谷氨酸池的高流速表明，转移到高渗透压后谷氨酸的立即积累一定是由于在其他含氮化合物合成中谷氨酸利用的抑制，而不是谷氨酸合成的刺激。与这一推理一致，我们发现谷氨酸积累的动力学与合成的具体途径无关，无论是通过谷氨酸脱氢酶还是通过谷氨酸合酶。转移到高渗透压后，似乎需要合成谷氨酸才能达到正常的膜电位值。
