Effect of the Thyroid State on the Enzymatic Characteristics of Cardiac Myosin

The effect of thyroid state on the activity of myosin adenosinetriphosphatase (ATPase) was examined in the rat and the rabbit. Cardiac myosin from thyroxinetreated rabbits showed enzymatic properties characterized by high Ca-activated ATPase activity, low activating energy, lower rate of inactivation at alkaline pH, and no activation by Af-ethylmaleimide compared with the same properties in the normal rabbit; thyroidectomy did not affect the enzymatic properties of rabbit cardiac myosin. These findings suggest a difference in the myosin molecule at or near the active site, involving some sulfhydryl groups, between hyperthyroid and euthyroid rabbits. However, rat cardiac myosin showed a pattern of activity in the euthyroid state similar to that of the hyperthyroid rabbit and changed to the euthyroid type after thyroidectomy. These changes were specific for cardiac myosin, since no change was observed in skeletal myosin. It is unlikely that there are major differences in the myosin molecule associated with the two types of activity, since similar proportion and amino acid composition of the subunits of cardiac myosin were observed in the different thyroid states. Thus, we concluded that the administration of thyroxine to the rabbit stimulates the synthesis of new cardiac myosin with altered enzymatic properties and that synthesis of this type of cardiac myosin is maintained by the normal level of thyroid hormone in the rat.