The Journal of Biochemistry
Online ISSN : 1756-2651
Print ISSN : 0021-924X
Calcium Regulation in Squid Mantle and Scallop Adductor Muscles
Kunihiko KONNOKen-ichi ARAIMikiharu YOSHIDAShizuo WATANABE
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1981 Volume 89 Issue 2 Pages 581-589

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Abstract

Calcium binding to the regulatory light chain was studied by an equilibrium dialysis method, and it was found that calcium binding to the regulatory light chain in an isolated form was qualitatively different from that in a bound form, i.e., in myosin. This finding can acount for our previous observation (1979) that the calcium or strontium concentration required for inducing difference spectra in the regulatory light chain (in an isolated form) was higher than that required for activating ATPase or for superprecipitation of actomyosin (in a bound form).
Most of the findings obtained by Asada et al. (1979) and Ashiba et al. (1980) for clam foot myosin were confirmed with squid mantle myosin and scallop adductor myosin. Therefore, the following properties are probably not confined to clam foot myosin but are common to myosins from molluscan muscles. (1) The Mg-ATPase activity of myosin alone was sensitive to calcium. (2) Removal of the regulatory light chain resulted in a reversible loss of superprecipitation ability. (3) As the ATP concentration increased, the ATPase activity of actomyosin changed in a biphasic manner, whereas that of myosin alone changed in a monophasic manner.
Two of our observations appear to favor the suggestion of Asada et al. and Ashiba et al. that the primary action of calcium is to activate myosin-ATPase rather than to induce actin-myosin bindings. (1) Desensitized myosin (free from the regulatory light chain) behave exactly like untreated myosin in the presence of calcium. For example, as the ATP concentration increased, the ATPase activity of myosin alone and that of actomyosin changed qualitatively and quantitatively in the same way as those of untreated myosin and acto-untreated myosin in the presence of calcium. (2) At KCl concentrations between 0.2M and 0.4M, actin-activation of myosin (in the presence of calcium) was absent whereas calcium sensitivity of myosin-ATPase was still detectable.

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