Kalabokis V N, Santoro M M, Hardwicke P M
Department of Medical Biochemistry, Southern Illinois University, Carbondale 62901-4409.
Biochemistry. 1993 Apr 27;32(16):4389-96. doi: 10.1021/bi00067a031.
In membranous scallop sarcoplasmic reticulum, the alkali metal cations Na+ and K+ and nucleotide together promote dimer formation by the Ca(2+)-free Ca-ATPase and stabilize the enzyme activity [Kalabokis, V. N., Bozzola, J. J., Castellani, L., & Hardwicke, P. M. D. (1991) J. Biol. Chem. 266, 22044-22050]. The dependence of stabilization of the Ca(2+)-free membranous scallop Ca-ATPase on Na+ concentration does not show saturation and may involve several superimposed effects. In order to assess the contribution of dimer toward stabilization, i.e., determine the relative importance of intra- and intermolecular effects on stabilization, the influence of varying Na+ concentration and nucleotide on the decay of enzyme activity of the Ca(2+)-free detergent-solubilized Ca-ATPase was studied. Loss of enzyme activity on removal of Ca2+ with EGTA was associated with loss of capacity for phosphorylation by ATP, a Ca(2+)-dependent function. Stabilization of the soluble Ca(2+)-free enzyme by Na+ showed major differences from that seen with the membranous enzyme. The extent of stabilization of the Ca(2+)-free soluble enzyme by Na+ showed clear saturation with increasing Na+ concentration. In contrast to the Ca(2+)-free membranous enzyme, which is inactivated at pH 7.0 with biphasic first-order kinetics, loss of enzymatic function by the solubilized Ca-ATPase at pH 6.92, 0 degrees C, followed monophasic first-order kinetics.(ABSTRACT TRUNCATED AT 250 WORDS)
在膜状扇贝肌浆网中，碱金属阳离子Na⁺和K⁺以及核苷酸共同促进无Ca²⁺的Ca - ATP酶形成二聚体，并稳定酶活性[卡拉博基斯，V. N.，博佐拉，J. J.，卡斯特拉尼，L.，& 哈德威克，P. M. D.（1991年）《生物化学杂志》266卷，22044 - 22050页]。无Ca²⁺的膜状扇贝Ca - ATP酶稳定性对Na⁺浓度的依赖性未表现出饱和现象，可能涉及多种叠加效应。为了评估二聚体对稳定性的贡献，即确定分子内和分子间效应在稳定性方面的相对重要性，研究了不同Na⁺浓度和核苷酸对无Ca²⁺去污剂增溶的Ca - ATP酶活性衰减的影响。用乙二醇双（2 - 氨基乙基醚）四乙酸（EGTA）去除Ca²⁺时酶活性的丧失与ATP磷酸化能力的丧失相关，这是一种Ca²⁺依赖性功能。Na⁺对可溶性无Ca²⁺酶的稳定作用与膜状酶有很大差异。Na⁺对无Ca²⁺可溶性酶的稳定程度随Na⁺浓度增加呈现明显的饱和现象。与在pH 7.0时以双相一级动力学失活的无Ca²⁺膜状酶不同，在pH 6.92、0℃时，增溶的Ca - ATP酶的酶功能丧失遵循单相一级动力学。（摘要截短于250字）
