Ranieri-Raggi M, Raggi A
Biochim Biophys Acta. 1983 Feb 15;742(3):623-9. doi: 10.1016/0167-4838(83)90281-9.
Rabbit skeletal muscle AMP deaminase (AMP aminohydrolase, EC 3.5.4.6) at low temperature and pH value above 7 undergoes inactivation, with a half-time of the order of several minutes. The loss of activity becomes more extensive at lower enzyme concentrations and higher pH values. It is reversible, since cold-inactivated AMP deaminase can be reactivated by raising the temperature, but not by lowering the pH, of the incubation mixture. The residual activity at the end of the inactivation process at various temperatures, reflecting the equilibrium between active and inactive forms of the enzyme, has been studied as a function of pH to determine the apparent pK and heat of ionization of the process. A general mechanism of reversible inactivation of AMP deaminase is postulated which assumes that deprotonation of the enzyme is followed by isomerization to a form which at low temperature slowly dissociates into the less-active subunits. Cold-inactivated AMP deaminase no longer shows the pH-dependent sigmoidal behaviour of the native enzyme, but regains this property along with the reactivation process. This suggests that allosteric kinetics at basic pH are probably produced by the same isomerization process which is involved in the mechanism for cold lability of the enzyme.
兔骨骼肌AMP脱氨酶（AMP氨基水解酶，EC 3.5.4.6）在低温及pH值高于7时会发生失活，半衰期为几分钟左右。在较低的酶浓度和较高的pH值条件下，活性丧失更为显著。这种失活是可逆的，因为冷失活的AMP脱氨酶可通过提高孵育混合物的温度重新激活，但不能通过降低pH值来激活。研究了在不同温度下失活过程结束时的残余活性，该活性反映了酶的活性形式与非活性形式之间的平衡，作为pH的函数来确定该过程的表观pK值和电离热。提出了AMP脱氨酶可逆失活的一般机制，该机制假定酶的去质子化之后会异构化为一种形式，在低温下这种形式会缓慢解离为活性较低的亚基。冷失活的AMP脱氨酶不再表现出天然酶的pH依赖性S形行为，但在重新激活过程中会恢复此特性。这表明碱性pH下的别构动力学可能是由与酶的冷不稳定性机制相关的相同异构化过程产生的。
