To clear the structural specificity of calmodulin (CaM) on the specific 125I-omega-CTX binding to crude membranes from whole chick brain, the following experiments were investigated in this study: (i) the attenuating effect of semisynthetic tetrahydroisoquinoline derivatives on the inhibitory effect of Ca2+/CaM, (ii) the effects of chimeras of yeast and chicken Ca2+/CaM, and (iii) the effects of Ca2+-binding proteins (such as troponin c, S 100 a and b, and annexin I, III-V). The inhibitory effect of Ca2+/CaM was attenuated by isoquinoline derivatives (PX 28, 34, 216, 224, and CPU57) and a CaM antagonist W-7. PX 34, a typical synthesized isoquinoline derivative, showed the attenuating effect in a dose-dependent manner. The ED50 value for the attenuating effect of PX 34 was about 20 microM, which is similar to that of W-7 reported previously. Some chimeric CaMs such as YC 51-53 (which are close to the properties of vertebrate CaM) showed a significant inhibitory effect on the specific 125I-omega-CTX binding, but YC 129 and 130 (which retain the properties of yeast CaM), troponin c, S100 a, b, and annexin I, III-V had no effect on the specific 125I-omega-CTX binding. These results suggest that the characteristic structure containing the EF-hand structure of CaM itself is needed to cause the inhibitory effect on the specific 125I-omega-CTX binding.
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