Griffiths M W, Sundaram T K
J Bacteriol. 1973 Dec;116(3):1160-9. doi: 10.1128/jb.116.3.1160-1169.1973.
The isocitrate lyase from a thermophilic Bacillus is activated about threefold by a variety of salts. Such strong stimulation of activity is not seen with isocitrate lyase from the mesophiles, Bacillus licheniformis, Bacillus megaterium, Escherichia coli, and Aspergillus nidulans. The salt activation is markedly pH-dependent. At pH values above 8.6, salt (KCl) indeed inhibits the enzyme activity. Potassium chloride also causes a significant shift of the pH optimum of the enzyme towards the acid side. As the temperature of the enzyme reaction is raised, activation becomes progressively weaker. Potassium chloride also affords considerable protection against enzyme denaturation at 55 C. The activation and the stabilization, however, appear to be independent effects. Of six other enzymes in the thermophile that were examined, isocitrate dehydrogenase was equally strongly activated by KCl and malate synthase was less strongly, but significantly, activated; citrate synthase, malate dehydrogenase, glutamate dehydrogenase, and lactate dehydrogenase were unaffected or slightly inhibited by KCl. The property of being strongly activated by salt appears to be a peculiar characteristic of the thermophile isocitrate lyase and possibly evolved concomitantly with its thermostability.
嗜热芽孢杆菌中的异柠檬酸裂解酶可被多种盐激活约三倍。而嗜温菌地衣芽孢杆菌、巨大芽孢杆菌、大肠杆菌和构巢曲霉中的异柠檬酸裂解酶则未观察到这种对活性的强烈刺激。盐激活明显依赖于pH值。在pH值高于8.6时，盐（氯化钾）确实会抑制酶的活性。氯化钾还会使该酶的最适pH值显著向酸性方向偏移。随着酶反应温度的升高，激活作用逐渐减弱。氯化钾在55℃时也能为酶变性提供相当大的保护作用。然而，激活和稳定作用似乎是相互独立的效应。在所检测的嗜热菌中的其他六种酶中，异柠檬酸脱氢酶同样被氯化钾强烈激活，苹果酸合酶被激活的程度稍弱，但也很显著；柠檬酸合酶、苹果酸脱氢酶、谷氨酸脱氢酶和乳酸脱氢酶不受氯化钾影响或略有抑制。被盐强烈激活的特性似乎是嗜热菌异柠檬酸裂解酶的一个独特特征，并且可能与其热稳定性同时进化而来。
