Regulatory Light Chain Contents and Molecular Species of Myosin in Catch Muscle of Scallop1

Myosin purified from the smooth muscle of scallop adductor contains two kinds of regulatory light chain, regulatory light chain a (RLC-a) and regulatory light chain b (RLC-b) (Kondo, S. & Morita, F. (1981) J. Biochem. 90, 673). The myosin was fractionated by salting out with ammonium sulfate, and samples containing the two regulatory light chains with different molar ratios were obtained. ATPase activities of the myosin fractions were determined. From the analysis of the dependence of ATPase activity on molar ratio of the two regulatory light chains, we concluded that myosin purified from the smooth muscle of scallop contains three species of myosin having different combinations of regulatory light chains: one has two RLC-a (aa), another has two RLC-b (bb), and the third one each of RLC-a and RLC-b (ab). The order of ATPase activities of these three myosin species was estimated as (aa) < (bb) < (ab). Distribution of the two regulatory light chains in the smooth muscle from the inside, translucent portion to the outside, opaque portion was examined by means of one- and two-dimensional gel electrophoreses. The content of RLC-b was about 1 mol per mol of SH-light chain independent of the portion of muscle. The content of RLC-a was markedly dependent on the portion of muscle—about 0.2 mol per mol of SH-light chain in the innermost portion and 0.7 mol per mol of SH-light chain in the outside, opaque portion. The sum of both regulatory light chain contents was about 1.5 mol per mol of SH-light chain in the opaque portion where the catch contraction is notable. Myosin species in the catch muscle are discussed.