Bovine Platelet Myosin

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Abstract
The properties of bovine platelet myosin were investigated with regards to its Mg- and Ca-ATPase [EC 3.6.1.3] activities. The Mg-ATPase exhibited an initial burst of phosphate liberation, the size of which was slightly larger than 1 mol P1/480,000 g protein The apparent Km value of the Mg-ATPase for ATP at a high ionic strength was very low (0.1 μM). A Lineweaver-Burk plot of Ca-ATPase at a high ionic strength was almost linear and the allo-stericity suggested by Malik et al. ((1974) Biochem. Biophys. Res. Commun. 61, 1071) was not observed. The apparent Km value of Ca-ATPase for ATP was 10 μM at a constant CaCI2 concentration of 2 itim or 54 μM at a constant Ca:ATP ratio of 1:1. In the presence of 1 mM EDTA at a high ionic strength, the apparent Km value for ATP was 0.25 mM, and in the virtual absence of EDTA, 0.1 mM. At a high ionic strength, both Mg- and Ca-ATPase activities showed typical U-shaped pH dependences at 25°C or 0°C. While Mg-ATPase activity at a low ionic strength and 25°C showed a similar U-shaped dependence, the activity measured at 0°C exhibited a slightly different dependence in a neutral pH region. The difference suggested a difference in rate limiting steps of the enzyme reaction, which was clearly shown with a bend at 4–6°C in an Arrhenius plot of Mg-ATPase activity at a low ionic strength. KCl concentration dependences of Mg- and Ca-ATPase activities were different from that of Ca-ATPase described previously and were similar to those for smooth muscle myosin; ATPase activities increased with increasing KC1 concentration. The difference between previous and present results would be due to the difference in assay temperatures. The Mg-activated enzyme activity at a high ionic strength showed broad substrate specificity as described previously for Ca-activated enzyme activity. While urea brought about activation of both Mg- and Ca-ATPase activities by 50–80%, modification with NEM did not enhance Ca-ATPase activity. On the contrary, Mg-ATPase was activated by NEM by 56%.