Smith T, Bell J E
Biochemistry. 1982 Feb 16;21(4):733-7. doi: 10.1021/bi00533a023.
Hysteresis in glutamate dehydrogenase is observed only in the reductive amination reaction and only with GTP present. The rate of reductive amination with NADH as coenzyme increases during the time course of the reaction. Premixing experiments, where glutamate dehydrogenase is preincubated with various combinations of substrates and GTP, suggest that the hysteresis phenomenon is not due to a time-dependent conformational change in the enzyme. Enzyme dilution experiments show (i) that the hysteresis is not due to enzyme association-dissociation effects and (ii) that the onset of the activation occurs after accumulation of about 25 microM NAD+. Addition of NAD+ to the initial reaction mixture prevents hysteresis from occurring. Although with NADPH as coenzyme hysteresis does not occur, addition of NADP+ to initial reaction mixtures containing NADH blocks hysteresis. A model based on reciprocating subunits is proposed whereby hysteresis results from product (NAD+) accumulation resulting in a half-of-the-sites activation of reductive amination.
谷氨酸脱氢酶的滞后现象仅在还原胺化反应中且只有在存在GTP时才会观察到。以NADH作为辅酶的还原胺化反应速率在反应进程中会增加。预混合实验中，将谷氨酸脱氢酶与各种底物和GTP组合进行预孵育，结果表明滞后现象并非由于酶的时间依赖性构象变化所致。酶稀释实验表明：（i）滞后现象并非由于酶的缔合 - 解离效应；（ii）激活的起始发生在约25μM NAD⁺积累之后。向初始反应混合物中添加NAD⁺可防止滞后现象的发生。虽然以NADPH作为辅酶时不会出现滞后现象，但向含有NADH的初始反应混合物中添加NADP⁺会阻止滞后现象。提出了一个基于往复亚基的模型，据此滞后现象是由产物（NAD⁺）积累导致还原胺化反应半位点激活所致。
