Kincaid R L, Kempner E, Manganiello V C, Osborne J C, Vaughan M
J Biol Chem. 1981 Nov 10;256(21):11351-5.
The apparent target sizes of the basal and calmodulin-dependent activities of calmodulin-activated phosphodiesterase from bovine brain were estimated using target theory analysis of data from radiation inactivation experiments. Whether crude or highly purified samples were irradiated, the following results were obtained. Low doses of radiation caused a 10 to 15% increase in basal activity, which, with further irradiation, decayed with an apparent target size of approximately 60,000 daltons. Calmodulin-dependent activity decayed with an apparent target size of approximately 105,000 daltons. The percentage stimulation of enzyme activity by calmodulin decreased markedly as a function of radiation dosage. These observations are consistent with results predicted by computer-assisted modeling based on the assumptions that: 1) the calmodulin-activated phosphodiesterase exists as a mixture of monomers which are fully active in the absence of calmodulin and dimers which are inactive in the absence of calmodulin; 2) in the presence of calmodulin, a dimer exhibits activity equal to that of two monomers; 3) on radiations destruction of a dimer, an active monomer is generated. This monomer-dimer hypothesis provides a plausible explanation for and definition of basal and calmodulin-dependent phosphodiesterase activity.
利用辐射失活实验数据的靶理论分析，估算了牛脑钙调蛋白激活的磷酸二酯酶的基础活性和钙调蛋白依赖性活性的表观靶标大小。无论照射的是粗样品还是高度纯化的样品，都得到了以下结果。低剂量辐射使基础活性增加10%至15%，随着进一步照射，其以约60,000道尔顿的表观靶标大小衰减。钙调蛋白依赖性活性以约105,000道尔顿的表观靶标大小衰减。钙调蛋白对酶活性的刺激百分比随辐射剂量的增加而显著降低。这些观察结果与基于以下假设的计算机辅助建模预测结果一致：1）钙调蛋白激活的磷酸二酯酶以单体混合物的形式存在，单体在没有钙调蛋白时完全有活性，二聚体在没有钙调蛋白时无活性；2）在有钙调蛋白存在时，二聚体的活性等于两个单体的活性；3）辐射破坏二聚体时，会产生一个有活性的单体。这种单体-二聚体假说是对基础和钙调蛋白依赖性磷酸二酯酶活性的合理解释和定义。
