The pH-dependency of myosin ATPases from yellowtail ordinary and dark muscles

Myosins were isolated from the ordinary (white) and dark (red) muscles of yellowtail, Seriola quinqueradiata, and the pH-dependency of ATPase activity, along with some physicochemical properties, was examined. The ordinary muscle myosin contained three kinds of light chain (A1, DTNB and A2 light chains), the molecular weights of which were 28,000, 20,000, and 16,000, respectively. The dark muscle myosin possessed only two kinds of light chain (D1 and D2), the molecular weights of which were 26,000 and 20,000 respectively. These ordinary and dark muscle myosins resemble the fast and slow muscle myosins of the higher vertebrate, respectively, in light chain pattern. The pH optima of the ordinary muscle myosin Ca2+-ATPase activity appeared at 6-6.5 and 9-10, irrespective of whether the enzyme reaction was started by the addition of ATP to the preincubated reaction mixture containing myosin (method I), or vice versa (method II). In the case of the dark muscle myosin, a small peak appeared at around pH 8.5 on the alkaline side when the activity was assayed by method I, whereas a prominent peak appeared at around 9.5 when it was assayed by method II, suggesting instability of this myosin under alkaline conditions. In connection with this, the reaction mixture at pH 9.5 showed a very small and slow increase in turbidity, suggesting a change in the physical state of myosin. The ordinary muscle myosin exhibited approximately three times higher actin-activated Mg2+-ATPase activity than the dark muscle myosin. Superprecipitation activity was also higher in the former than the latter actomyosin. However, both actomyosins showed similar pH-superprecipitation activity profiles.