Abstract
Myosin was isolated from purified cardiac myofibrils and characterized for K<sup>+</sup>-ATPase and Ca<sup>2+</sup>-ATPase activities. The light chain (LC) fraction was obtained by exposing myosin to pH 11.5 followed by (NH<sub>4</sub>)<sub>2</sub>SO<sub>4</sub> fractionation. Sodium dodecyl sulphate (SDS) gel electrophoresis revealed two components with molecular weights of 30 000 (LC<sub>1</sub>) and 20 000 (LC<sub>2</sub>) daltons. No detectable cyclic AMP-dependent protein kinase activity was found. Cyclic AMP markedly stimulated phosphorylation of the myosin light chain fraction in the presence of added protein kinase. Alkaline phosphatase dephosphorylated the phosphorylated light chains. LC<sub>2</sub> was phosphorylated by cyclic AMP-dependent protein kinase.
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