Newsholme E A, Crabtree B, Zammit V A
Ciba Found Symp. 1979(73):245-58. doi: 10.1002/9780470720561.ch14.
It can be shown theoretically and experimentally that the maximum activities in vitro of enzymes that catalyse near-equilibrium reactions in vivo must be considerably higher than the maximum flux through that pathway. Consequently, the activities of such enzymes cannot provide quantitative information on the maximum possible flux through a pathway. On the other hand, the maximum activity of an enzyme that catalyses a non-equilibrium reaction in vivo may provide quantitative information. Such possibilities must be tested experimentally. Thus the maximum flux through a given metabolic pathway is measured (or calculated) and compared with the maximum in vitro activities of enzymes that catalyse non-equilibrium reactions in that pathway. Catalytic activities similar to the flux suggest that such enzymes may be useful as flux indicators. For example, phosphorylase or phosphofructokinase activities provide a quantitative indication of maximum flux through glycolysis-from-glycogen (i.e. anaerobic glycolysis); hexokinase activities provide a quantitative indication of maximum flux through glycolysis-from-glucose; 2-oxoglutarate dehydrogenase activities provide a quantitative indication of maximum flux through the citric acid cycle. The advamtages of the use of enzyme activities in this manner include simplicity, general applicability to pathways, tissues and animals, and minimum intervention (particularly in larger animals including the human species). One disadvantage is that the properties of the enzyme must be known in detail before an assay that gives maximum activities can be developed, and the properties of enzymes that catalyse non-equilibrium reactions may be complex. These considerations emphasize the dangers of quantitative interpretation of the maximum flux through pathways from 'near-equilibrium' enzymes or from 'non-equilibrium' enzymes whose properties have been inadequately studied.
理论和实验均表明，体内催化近平衡反应的酶在体外的最大活性必定远高于该途径的最大通量。因此，这类酶的活性无法提供关于某一途径最大可能通量的定量信息。另一方面，体内催化非平衡反应的酶的最大活性可能提供定量信息。此类可能性必须通过实验来检验。因此，需测定（或计算）给定代谢途径的最大通量，并将其与该途径中催化非平衡反应的酶的体外最大活性进行比较。与通量相似的催化活性表明这类酶可能可用作通量指标。例如，磷酸化酶或磷酸果糖激酶的活性定量指示了糖原酵解（即无氧糖酵解）的最大通量；己糖激酶的活性定量指示了葡萄糖酵解的最大通量；2-氧代戊二酸脱氢酶的活性定量指示了柠檬酸循环的最大通量。以这种方式使用酶活性的优点包括操作简单、普遍适用于各种途径、组织和动物，以及干预最小（特别是在包括人类在内的大型动物中）。一个缺点是，在开发出能给出最大活性的测定方法之前，必须详细了解酶的性质，而且催化非平衡反应的酶的性质可能很复杂。这些考虑因素强调了对来自“近平衡”酶或性质研究不充分的“非平衡”酶的途径最大通量进行定量解释的危险性。
