Manfredi J P, Holmes E W
Arch Biochem Biophys. 1984 Sep;233(2):515-29. doi: 10.1016/0003-9861(84)90475-2.
The enzymes of the purine nucleotide cycle-AMP deaminase, adenylosuccinate synthetase, and adenylosuccinate lyase-were examined as a functional unit in an in vitro system which simulates the purine nucleotide composition of sarcoplasm. Activity of each cycle enzyme in extracts of rat skeletal muscle was observed to increase as ATP/ADP, reflecting the energy state of the system, was lowered from approximately 50 to 1. The increase in AMP deaminase activity could be attributed to effects of energy state and factors such as AMP concentration, which are obligatorily coupled to energy state. The increases in synthetase and lyase activities were accounted for by increases in the concentration of IMP and adenylosuccinate, respectively. The inhibitory influence of IMP concentration on synthetase activity reported in other systems was not observed in this system; synthetase activity progressively increased as IMP concentration was raised to approximately 4 mM, and apparent saturation occurred at concentrations above 4 mM. Also, adenylosuccinate was found to be an activator of AMP deaminase. The results of this study document that the activities of the enzymes of the purine nucleotide cycle increase in parallel at low energy states, and the components of the cycle function as a coordinated unit with individual enzyme activities linked via concentrations of cycle intermediates.
嘌呤核苷酸循环的酶——AMP脱氨酶、腺苷酸琥珀酸合成酶和腺苷酸琥珀酸裂解酶——在一个模拟肌浆嘌呤核苷酸组成的体外系统中作为一个功能单元进行了研究。当反映系统能量状态的ATP/ADP从大约50降至1时，观察到大鼠骨骼肌提取物中每种循环酶的活性增加。AMP脱氨酶活性的增加可归因于能量状态的影响以及诸如AMP浓度等与能量状态必然相关的因素。合成酶和裂解酶活性的增加分别是由于IMP和腺苷酸琥珀酸浓度的增加。在该系统中未观察到其他系统中报道的IMP浓度对合成酶活性的抑制作用；随着IMP浓度升高至约4 mM，合成酶活性逐渐增加，且在浓度高于4 mM时出现明显饱和。此外，发现腺苷酸琥珀酸是AMP脱氨酶的激活剂。这项研究的结果表明，在低能量状态下，嘌呤核苷酸循环的酶活性平行增加，并且该循环的各组分作为一个协调单元发挥作用，各酶活性通过循环中间产物的浓度相互关联。
