Martin D D, Ciulla R A, Robinson P M, Roberts M F
Merkert Chemistry Center, Boston College, 2609 Beacon Street, Chestnut Hill, MA 02167, USA.
Biochim Biophys Acta. 2001 Nov 15;1524(1):1-10. doi: 10.1016/s0304-4165(00)00131-8.
Methanococcus thermolithotrophicus, a thermophilic methanogenic archaeon, produces and accumulates beta-glutamate and L-alpha-glutamate as osmolytes when grown in media with <1 M NaCl. When the organism is adapted to grow in >1 M NaCl, a new zwitterionic solute, N(epsilon)-acetyl-beta-lysine, is synthesized and becomes the dominant osmolyte. Several techniques, including in vivo and in vitro NMR spectroscopy, HPLC analyses of ethanol extracts, and potassium atomic absorption, have been used to monitor the immediate response of M. thermolithotrophicus to osmotic stress. There is a temporal hierarchy in the response of intracellular osmolytes. Changes in intracellular K(+) occur within the first few minutes of altering the external NaCl. Upon hypoosmotic shock, K(+) is released from the cell; relatively small changes occur in the organic osmolyte pool on a longer time scale. Upon hyperosmotic shock, M. thermolithotrophicus immediately internalizes K(+), far more than would be needed stoichiometrically to balance the new salt concentration. This is followed by a decrease to a new K(+) concentration (over 10-15 min), at which point synthesis and accumulation of primarily L-alpha-glutamate occur. Once growth of the M. thermolithotrophicus culture begins, typically 30-100 min after the hyperosmotic shock, the intracellular levels of organic anions decrease and the zwitterion (N(epsilon)-acetyl-beta-lysine) begins to represent a larger fraction of the intracellular pool. The observation that N(epsilon)-acetyl-beta-lysine accumulation occurs in osmoadapted cells but not immediately after osmotic shock is consistent with the hypothesis that lysine 2,3-aminomutase, an enzyme involved in N(epsilon)-acetyl-beta-lysine synthesis, is either not present at high levels or has low activity in cells grown and adapted to lower NaCl. That lysine aminomutase specific activity is 8-fold lower in protein extracts from cells adapted to low NaCl compared to those adapted to 1.4 M NaCl supports this hypothesis.
嗜热自养产甲烷球菌是一种嗜热产甲烷古菌，当在氯化钠浓度低于1M的培养基中生长时，它会产生并积累β - 谷氨酸和L - α - 谷氨酸作为渗透溶质。当该生物体适应在高于1M的氯化钠中生长时，会合成一种新的两性离子溶质N(ε)-乙酰-β - 赖氨酸，并成为主要的渗透溶质。包括体内和体外核磁共振光谱、乙醇提取物的高效液相色谱分析以及钾原子吸收在内的多种技术已被用于监测嗜热自养产甲烷球菌对渗透胁迫的即时反应。细胞内渗透溶质的反应存在时间上的层级关系。细胞内钾离子（K⁺）的变化在改变外部氯化钠浓度后的最初几分钟内发生。在低渗休克时，K⁺从细胞中释放出来；有机渗透溶质池在较长时间尺度上发生相对较小的变化。在高渗休克时，嗜热自养产甲烷球菌会立即摄取K⁺，其摄取量远远超过化学计量所需以平衡新的盐浓度。随后K⁺浓度会下降到一个新水平（在10 - 15分钟内），此时主要是L - α - 谷氨酸的合成和积累。一旦嗜热自养产甲烷球菌培养物开始生长，通常在高渗休克后30 - 100分钟，细胞内有机阴离子水平下降，两性离子（N(ε)-乙酰-β - 赖氨酸）开始在细胞内池中占更大比例。N(ε)-乙酰-β - 赖氨酸在渗透适应细胞中积累但不在渗透休克后立即积累这一观察结果与以下假设一致：赖氨酸2,3 - 氨基变位酶（一种参与N(ε)-乙酰-β - 赖氨酸合成的酶）在生长并适应较低氯化钠浓度的细胞中要么不存在高水平表达，要么具有低活性。与适应1.4M氯化钠的细胞相比，适应低氯化钠浓度的细胞蛋白质提取物中赖氨酸氨基变位酶的比活性低8倍，这支持了这一假设。
