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J. Biochem, 1982, Vol. 91, No. 4 1313-1320
© 1982 Japanese Biochemical Society

research-article

Ca2+-Dependent Binding of [3H]Calmodulin to the Microsomal Fraction of Brain1

Kenji SOBUE, Kouichi MORIMOTO, Keiko KANDA and Shiro KAKIUCHI

Department of Neurochemistry and Neuropharmacology, Institute of Higher Nervous Activity, Osaka University Medical School Nakanoshima, Kita-ku, Osaka, Osaka 530

The binding of calmodulin to a brain microsomal fraction rich in synaptic membranes and vesicles was studied using 3H-labeled calmodulin. The binding was Ca2+-dependent and highly specific to calmodulin since it was competitively displaced only by unlabeled calmodulin and not by 200—4,000-fold excesses of other proteins that included troponin-C and S-100 protein. Within the physiological pH range, the specific binding, defined as the amount of bound [3H]calmodulin which is displacable by the addition of an excess of unlabeled calmodulin, agreed well with the Ca2+-dependent binding defined as the difference between the total binding in the presence of Ca2+ and the binding obtained with EGTA in place of Ca2+. Both binding activities appeared to be greatest at about pH 7.0. The binding, either specific or Ca2+-dependent, is a calmodulin concentration-dependent saturable process. The dose-dependent curve obtained for increasing concentrations of [3H]-calmodulin in binding studies. Scatchard plot analysis of the curve gave two different Kd values for calmodulin, 8.2 x 10–8 and 5.3 x10–7M. The corresponding maximum binding capacities were 1.0 x 1014 and 1.6 x 1014 calmodulin molecules per mg of microsomal protein, respectively. The binding ability of the microsomal fraction was completely abolished by prior treatment with proteolytic enzymes.

1This investigation was supported in part by research grants from the Ministry of Education, Science and Culture, and the Research Committee of Therapeutic Agents for Spino-Cerebellar Degenerations, the Ministry of Health and Welfare, Japan.


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