Myosins from White and Dark Muscles of Mackerel

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Abstract
A comparative study was performed on the myosins isolated from the white and dark muscles of the mackerel, Pneumatophorus japonicus japonicus. The myosin from white muscle had three light chain subunits of molecular weights of 26,500, 20,000, and 17,500. The myosin from dark muscle had only two light chains of molecular weights of 24,000 and 19,000. The former and the latter light chain patterns resemble those of the fast and slow muscle myosins of rabbit, respectively. Both mackerel myosins did not differ from each other significantly in sedimentation coefficient (about 6S) and amino acid composition. The Ca2+-ATPase activity of dark muscle myosin was higher than that of white muscle myosin and the EDTA-ATPase activity of the former was lower than that of the latter irrespective of ionic strength at pH 7.0. The effects of Ca2+ and EDTA on both the kinetic constants, Km and Vmax, were similar for the two mackerel myosins. The pH optima of Ca2+-ATPase activity were observed at around 6.5 and 9.5 for white muscle myosin and only at around 6.0 for dark muscle myosin. Actin, whether from white or dark muscle, enhanced the Mg2+-ATPase activity of each myosin significantly. This ATPase activity of synthetic actomyosin from the white muscle was about 6 times as great as that from the dark muscle counterpart. The inactivation rate constant of ATPase activity of white and dark muscle myosins was 1.3×10−5 and 3.5×10−6s−1 at 0°C, and 1.1×10−1 and 3.3×10−3s−1 at 30°C, respectively, indicating that the dark muscle myosin was 3.3–3.7 times more thermostable than the white muscle myosin.

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