title: Enzymatic properties of the heavy meromyosin subfragment of cardiac myosin from normal and thyrotoxic rabbits.
abstract: Myosin from the hearts of thyrotoxic animals (myosin-T) exhibits elevated Ca2+-ATPase activity. To clarify the physiological significance of this increased activity, we have investigated the steady state kinetics of the interaction of actin and MgATP with the double-headed heavy meromyosin subfragment of cardiac myosin from thyrotoxic rabbits (HMM-T). The enhanced Ca2+-ATPase activity of myosin-T was completely retained in HMM-T. The Vmax for actin-activated MgATP hydrolysis by HMM-T (1.08 +/- 0.10 mumol of Pi/mg/min). Under physiological ionic conditions, the Vmax was 0.14 +/- 0.02 mumol of Pi/mg/min as compared with the normal value of 0.08 +/- 0.01 mumol of Pi/mg/min. Furthermore, the salt dependence of Vmax and Kapp for the actin-activated ATPase of HMM-T differed markedly from normal and resembled that usually associated with the single-headed (S1) cleavage product of myosin. These results suggest that the changes in enzymatic properties of myosin-T are responsible for the increased speed of contraction observed in the hearts of thyrotoxic animals. Also, the alteration in the interaction of HMM-T with actin suggests that a loss of cooperativity between the myosin heads may occur.
mesh_terms: D000251:Adenosine Triphosphatases / Q000378:metabolism; D000818:Animals; D002118:Calcium / Q000378:metabolism; D006980:Hyperthyroidism / Q000139:chemically induced / Q000378:metabolism*; D007700:Kinetics; D008297:Male; D009206:Myocardium / Q000201:enzymology; D015879:Myosin Subfragments / Q000378:metabolism*; D009218:Myosins / Q000378:metabolism*; D009994:Osmolar Concentration; D011817:Rabbits; D013974:Thyroxine
chemical_list: D015879:Myosin Subfragments; D000251:Adenosine Triphosphatases; D009218:Myosins; D013974:Thyroxine; D002118:Calcium