Some Properties of the Actomyosin-ATP System of Cardiac Muscle I

概要

論文の詳細を見る
The interaction between actin, myosin and ATP is generally accepted as an elementary process in the contraction of skeletal muscle 2) . The myofibril of skeletal muscle consists of double arrays of myosin and actin filaments, and the contraction of skeletal muscle takes place by the sliding of these two types of filaments past each other 1) . Weber et al. 3) and Tonomura et al. 4, 5) have shown that the action of the actomyosin type of ATPase [ATP phosphohydrolase, EC 3. 6. 1.3] is necessary for the contraction of skeletal muscle models. Spiro et al. 6) have reported that the myofibril of cardiac muscle consists of two kinds of filaments quite similar to those of skeletal muscle, and that during contraction these two kinds of filaments slide past each other like those of skeletal muscle. In the present work, therefore, kinetic studies were made on ATPase of dog cardiac myosin B (natural actomyosin, i. e. a complex of actin and myosin), and superprecipitation of dog cardiac myosin B induced by ATP was followed by the turbidity method and by electron microscopy, in order to investigate the actin-myosin-ATP interaction in cardiac muscle. The following results were obtained. Electron Microscopic Study Actin and myosin filaments of cardiac myosin B were dissociated from each other in the presence of a high concentration of ATP, where myosin B showed aclearing response, while in the presence of a low concentration of ATP, where myosin B showed superprecipitation, many large aggregates were formed, apparently due to congregation of myosin and actin filaments with each other. Kinetics of Superprecipitation and Clearing Response Followed by the Turbidity Method The superprecipitation and clearing response of cardiac myosin B induced by various concentrations of ATP were investigated by the turbidity method 7-9) under the following conditions: 0.82mg cardiac myosin B/ml, 0.05 M KCl, 2 mM MgCl2, 0.1 mM CaCl2, 0.266 mg pyruvate kinase [ATP: pyruvate phosphotransferase, EC 2.7. 1.40]/ml, 0.5 mM phosphoenolpyruvate and 20 mM Tris-maleate buffer at pH 7.0 and 25°C. The specific activity of pyruvate kinase was usually 70-90 or 180-220 μmoles pyruvate/mg/min in the presence of 0.24 or 2mM ADP in 0.075 M KCl-8 mM MgCl2-0.8mM phosphoenolpyruvate-20mM Tris-HCl buffer at pH 7.5 and 25°C. The extent (increment of optical density) and rate 7-9) of superprecipitation increased with increase in concentration of ATP and reached the almost constant levels at the ATP concentra-concentrations above 5μM. At the ATP concentrations above 20EM, the extent adecreased with the increase in ATP concentration, and the clearing response occurrednd when the ATP concentration was above the critical level of about 0.5mM. In the absence of the kinase system, the extent of superprecipitation was dependent on the ATP concentration from 2 to 20μM, though, within this range, the extent in the presence of the kinase was independent of the ATP concentration. Kinetics of ATPase of Cardiac Myosin B The rate of ATP-hydrolysis under the same conditions as for superprecipitation was examined in the presence of 1mM CaCl2 and 1mM EGTA*. Cardiac myosin B ATPase followed the Michaelis-Menten kinetics until the critical concentration of ATP was reached, above which it decreased with the increase in ATP concent-ration. The critical concentration was 20μM ATP in the presence of EGTA and 100μM in the presence of CaCl2, and the substrate inhibition in the latter was far less marked than in the former. The maximum rates, Vm, of cardiac myosin B ATPase determined from the 1/v uersus1/[ATP] plot over the concentration range of ATP from 1 to 20μM were 300μmoles ADP/g/min in the presence of CaC12 and 66μmoles ADP/g/min in the presence of EGTA, and the Michaelis constants, Km, were 30 and 6μM, respectively.
Japan Society of Clinical Chemistryの論文

Some Properties of the Actomyosin-ATP System of Cardiac Muscle I

スポンサーリンク

概要

著者

関連論文

スポンサーリンク