Cox J A, Malnoë A, Stein E A
J Biol Chem. 1981 Apr 10;256(7):3218-22.
Comparison of the parameters of Ca and Sr binding to bovine brain calmodulin with the activation of bovine brain phosphodiesterase by Ca2+ and Sr2+ at different calmodulin concentrations allows a quantitative description of the mechanism of activation of the enzyme. Equilibrium dialysis studies show that calmodulin possesses three high affinity (K'diss = 6 micoM) and one low affinity (K'diss = 200 microM) sites for Ca2+. All four sites display the same affinity for Sr2+ with K'diss = 180 microM. In the presence of calmodulin, soluble bovine brain phosphodiesterase is activated by Sr2+ to the same extent as by Ca2+. The activation of the enzyme shows the same Ca2+/Sr2+ selectivity ratio of 30 as the binding of the metal ions to calmodulin. Based on the findings that the Ca2+ or Sr2+ concentration at half-maximal activation of the enzyme depends on the concentration of calmodulin present, a quantitative analysis of activation was carried out as a function of the four calmodulin-metal complex species (CaM . Men). The data show that the activating species are CaM . Ca3, CaM . Ca4 or CaM . Sr3, CaM . Sr4. The interaction of these activating species with phosphodiesterase follows the Hill equation with a dissociation constant of 10(-9) M and a Hill coefficient of 2, irrespective of the binding characteristics of Ca2+ or Sr2+. The latter value agrees well with the fact that phosphodiesterase possesses two binding sites for calmodulin.
通过比较钙（Ca）和锶（Sr）与牛脑钙调蛋白结合的参数，以及在不同钙调蛋白浓度下Ca2+和Sr2+对牛脑磷酸二酯酶的激活作用，可以对该酶的激活机制进行定量描述。平衡透析研究表明，钙调蛋白对Ca2+有三个高亲和力位点（解离常数K'diss = 6 μM）和一个低亲和力位点（K'diss = 200 μM）。所有这四个位点对Sr2+的亲和力相同，K'diss = 180 μM。在钙调蛋白存在的情况下，可溶性牛脑磷酸二酯酶被Sr2+激活的程度与被Ca2+激活的程度相同。该酶的激活显示出与金属离子与钙调蛋白结合相同的Ca2+/Sr2+选择性比率30。基于酶半最大激活时的Ca2+或Sr2+浓度取决于所存在的钙调蛋白浓度这一发现，对激活作用进行了定量分析，作为四种钙调蛋白 - 金属络合物（CaM·Men）的函数。数据表明，激活物种是CaM·Ca3、CaM·Ca4或CaM·Sr3、CaM·Sr4。这些激活物种与磷酸二酯酶的相互作用遵循希尔方程，解离常数为10^(-9) M，希尔系数为2，与Ca2+或Sr2+的结合特性无关。后一个值与磷酸二酯酶具有两个钙调蛋白结合位点这一事实非常吻合。
