Verkman A S, Skorecki K L, Ausiello D A
Am J Physiol. 1986 Jan;250(1 Pt 1):C103-14. doi: 10.1152/ajpcell.1986.250.1.C103.
Radiation inactivation has been applied extensively to determine the molecular weight of soluble enzyme and receptor systems from the slope of a linear ln (activity) vs. dose curve. Complex nonlinear inactivation curves are predicted for multimeric enzyme systems, composed of distinct subunits in equilibrium with multimeric complexes. For the system A1 + A2----A1A2, with an active A1A2 complex (associative model), the ln (activity) vs. dose curve is linear for high dissociation constant, K. If a monomer, A1, has all the enzyme activity (dissociative model), the ln (activity) vs. dose curve has an activation hump at low radiation dose if the inactive subunit, A2, has a higher molecular weight than A1 and has upward concavity when A2 is smaller than A1. In general, a radiation inactivation model for a multistep mechanism for enzyme activation fulfills the characteristics of an associative or dissociative model if the reaction step forming active enzyme is an associative or dissociative reaction. Target theory gives the molecular weight of the active enzyme subunit or complex from the limiting slope of the ln (activity) vs. dose curve at high radiation dose. If energy transfer occurs among subunits in the multimer, the ln (activity) vs. dose curve is linear for a single active component and is concave upward for two or more active components. The use of radiation inactivation as a method to determine enzyme size and multimeric subunit assembly is discussed with specific application to the hormone-sensitive adenylate cyclase system. It is shown that the complex inactivation curves presented in the accompanying paper can be used select the best mechanism out of a series of seven proposed mechanisms for the activation of adenylate cyclase by hormone.
辐射失活已被广泛应用于根据线性ln（活性）与剂量曲线的斜率来确定可溶性酶和受体系统的分子量。对于由与多聚体复合物处于平衡状态的不同亚基组成的多聚体酶系统，预计会出现复杂的非线性失活曲线。对于系统A1 + A2----A1A2，具有活性A1A2复合物（缔合模型），对于高解离常数K，ln（活性）与剂量曲线是线性的。如果单体A1具有所有酶活性（解离模型），当无活性亚基A2的分子量高于A1时，ln（活性）与剂量曲线在低辐射剂量处有一个活化峰，而当A2小于A1时则具有向上的凹度。一般来说，如果形成活性酶的反应步骤是缔合或解离反应，那么酶激活多步机制的辐射失活模型符合缔合或解离模型的特征。靶理论从高辐射剂量下ln（活性）与剂量曲线的极限斜率得出活性酶亚基或复合物的分子量。如果多聚体中亚基之间发生能量转移，对于单个活性成分，ln（活性）与剂量曲线是线性的，而对于两个或更多活性成分则是向上凹的。本文讨论了将辐射失活作为一种确定酶大小和多聚体亚基组装的方法，并特别应用于激素敏感腺苷酸环化酶系统。结果表明，随附论文中呈现的复杂失活曲线可用于从为激素激活腺苷酸环化酶提出的七种机制中选择最佳机制。
