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犬心肌肌浆网钙转运ATP酶。与兔快肌骨骼肌肌浆网钙转运ATP酶的比较。

Calcium transport ATPase of canine cardiac sarcoplasmic reticulum. A comparison with that of rabbit fast skeletal muscle sarcoplasmic reticulum.

作者信息

Shigekawa M, Finegan J A, Katz A M

出版信息

J Biol Chem. 1976 Nov 25;251(22):6894-900.

PMID:11210
Abstract

To define the mechanism responsible for the slow rate of calcium transport by cardiac sarcoplasmic reticulum, the kinetic properties of the Ca2+-dependent ATPase of canine cardiac microsomes were characterized and compared with those of a comparable preparation from rabbit fast skeletal muscle. A phosphoprotein intermediate (E approximately P), which has the stability characteristics of an acyl phosphate, is formed during ATP hydrolysis by cardiac microsomes. Ca2+ is required for the E approximately P formation, and Mg2+ accelerates its decomposition. The Ca2+ concentration required for half-maximal activation of the ATPase is 4.7 +/- 0.2 muM for cardiac microsomes and 1.3 +/- 0.1 muM for skeletal microsomes at pH 6.8 and 0 degrees. The ATPase activities at saturating concentrations of ionized Ca2+ and pH 6.8, expressed as ATP hydrolysis per mg of protein, are 3 to 6 times lower for cardiac microsomes than for skeletal microsomes under a variety of conditions tested. The apparent Km value for MgATP at high concentrations in the presence of saturating concentrations of ionized Ca2+ is 0.18 +/- 0.03 ms at pH 6.8 and 25 degrees. The maximum velocity of ATPase activity under these conditions is 0.45 +/- 0.05 mumol per mg per min for cardiac microsomes and 1.60 +/- 0.05 mumol per mg per min for skeletal microsomes. The maximum steady state level of E approximately P for cardiac microsomes, 1.3 +/- 0.1 nmol per mg, is significantly less than the value of 4.9 +/- 0.2 nmol per mg for skeletal microsomes, so that the turnover number of the Ca2+-dependent ATPase of cardiac microsomes, calculated as the ratio of ATPase activity to the E approximately P level is similar to that of the skeletal ATPase. These findings indicate that the relatively slow rate of calcium transport by cardiac microsomes, whem compared to that of skeletal microsomes, reflects a lower density of calcium pumping sites and lower Ca2+ affinity for these sites, rather than a lower turnover rate.

摘要

为了确定负责心脏肌浆网钙转运速率缓慢的机制,对犬心脏微粒体中Ca2+依赖性ATP酶的动力学特性进行了表征,并与来自兔快肌的类似制剂进行了比较。在心脏微粒体ATP水解过程中形成了一种磷蛋白中间体(E~P),它具有酰基磷酸的稳定性特征。E~P的形成需要Ca2+,Mg2+加速其分解。在pH 6.8和0℃时,心脏微粒体ATP酶半最大激活所需的Ca2+浓度为4.7±0.2μM,骨骼肌微粒体为1.3±0.1μM。在多种测试条件下,当以每毫克蛋白质的ATP水解量表示时,在离子化Ca2+饱和浓度和pH 6.8条件下,心脏微粒体的ATP酶活性比骨骼肌微粒体低3至6倍。在pH 6.8和25℃时,在离子化Ca2+饱和浓度存在下,高浓度MgATP的表观Km值为0.18±0.03mM。在这些条件下,心脏微粒体ATP酶活性的最大速度为每毫克每分钟0.45±0.05μmol,骨骼肌微粒体为1.60±0.05μmol。心脏微粒体E~P的最大稳态水平为每毫克1.3±0.1nmol,明显低于骨骼肌微粒体每毫克4.9±0.2nmol的值,因此,心脏微粒体Ca2+依赖性ATP酶的转换数(计算为ATP酶活性与E~P水平之比)与骨骼肌ATP酶相似。这些发现表明,与骨骼肌微粒体相比,心脏微粒体钙转运速率相对较慢反映了钙泵位点密度较低以及这些位点对Ca2+的亲和力较低,而不是转换率较低。

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