Studies on myosin from red and white skeletal muscles of the rabbit. II. Inactivation of myosin from red muscles under mild alkaline conditions.

The rapid loss of ATPase activity of myosin from the red, or slow, skeletal muscles of the rabbit under mild alkaline conditions has been studied in detail. The loss of ATPase activity is irreversible and consistent with a mechanism involving two !irst order reactions with rate constants of 0.3 and 0.007 min-l. No gross change in molecular structure accompanies inactivation, as judged by the lack of major change in optical rotatory dispersion or sedimentation. Slight increases in the rate of tryptic digestion and reactivity of sulfhydryl groups which may or may not be involved in the loss of enzymic activity are observed after inactivation. It appears that the molecular changes involved in loss of enzymic activity are relatively small and may be localized at or near the catalytic site. Myosin from rabbit cardiac muscle loses ATPase activity at pH 9.0 at the same rate as myosin from red skeletal muscles.