JBC, Vol. 252, Issue 19, 6925-6929, Oct, 1977
Enzymatic properties of the heavy meromyosin subfragment of cardiac myosin from normal and thyrotoxic rabbits
S. K. Banerjee, E. G. Kabbas and E. Morkin
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.
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