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J. Biochem, 1988, Vol. 104, No. 1 102-107
© 1988 Japanese Biochemical Society

research-article

The Steady State Intermediate of Scallop Smooth Muscle Myosin ATPase and Effect of Light Chain Phosphorylation. A Molecular Mechanism for Catch Contraction

Masayuki Takahashi, Hitoshi Sohma1 and Fumi Morita2

Department of Chemistry, Faculty of Science, Hokkaido University Kita-ku, Sapporo, Hokkaido 060

1 Present address: Univerisity of Arizona, Tucson, Arizona, U.S.A

2 To whom correspondence should be addressed

The ATP-induced difference UV-absorption spectrum of myosin isolated from the opaque portion of scallop smooth muscle (opaque myosin) was Ca2+ at 40 mM KC1 and 1.5 M sucrose. On adding sucrose to 1.5 M, the turbidity of myosin decreased to 24% and the characteristic two forms of the difference spectrum, the ATP-form and ADP-form (Morita, F. (1967) J. Biol. Chem. 242,4501–4506), were distinguishable. In the presence of Ca2+ the difference spectrum was the ATP-form first and then decayed intothe ADP-form with .the depletion of ATP. In the absence of Ca2+ however, only the ADP-form was observed. The ADP-form observed in the absence of Ca2+ returned to the ATP-form when the regulatory light chain-a (RLC-a), one of the regulatory light chains of opaque myosin, was phospho rylated. These results suggest that the main intermediate at the steady state of opaque myosin ATPase is converted depending on the concentration of Ca2+ from E in the presence of Ca to EADP in the absence of Ca2+It changes to EADPPin the absence of Ca2+ on the phosphorylation of RLC-a. Consistent results were obtained by measuring the ATP induced Trp-fiuorescence increase of opaque myosin in the absence of sucrose. Since the opaque portion of scallop smooth muscle is known to be responsible for catch contraction (Ruegg, J.C. (1961) Proc. R. Soc. London Ser. B 154, 224–249), these findings lead us to suppose that the opaque myosin in vivo may stay in the E.ADP complex during the catch state. It changes to EADPP by the phosphorylation of RLC-a, which may terminate the catch state.


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