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1 Department of Anatomy and Cell Biology, The University of Iowa, Iowa City, Iowa 52242, USA2 Department of Physics, School of Science and Engineering3 Advanced Research Institute for Science and Engineering, Waseda University, 3-4-1 Okubo, Shinjuku-ku, Tokyo 169-8555, Japan
The role of tropomyosin (Tm) in the elementary steps of the cross-bridge cycle in bovine myocardium was investigated. The thin filament was selectively removed using gelsolin (thin filament severing protein), and the actin filament was reconstituted from G-actin. Tm was further reconstituted without troponin (Tn), and the kinetic constants of the elementary steps of the cross-bridge cycle were deduced using sinusoidal analysis at pCa 
4.66, pH 7.00, and 25°C. The association constant of MgATP to cross-bridges (K1) after reconstitution of Tm was 20.7 ± 2.3 mM–1, which was about 2 x the control (untreated) myocardium (9.1 ± 1.3 mM–1). Following reconstitution of Tm, the equilibrium constant of the cross-bridge detachment step (K2), the phosphate (Pi) association constant (K5) and the equilibrium constant of the force-generation step (K4), which significantly changed in the actin filament-reconstituted myocardium, recovered to those of the control myocardium. Active tension after reconstitution of Tm was 0.69 x the control myocardium, a value between the control (1.00 x) and the actin filament-reconstituted myocardium (0.59 x). Tm-reconstituted myocardium was further reconstituted with Tn, and the effect of MgATP on the rate constants (K1, K2) was studied. Following reconstitution with Tn, the myocardium regained the Ca2+-sensitivity and the active tension became 0.83 x the control myocardium. In addition, K1 recovered to the value of the control myocardium with Tn reconstitution. These results indicate that both Tm and Tn enhance the force generated by each cross-bridge, and that Tm is primarily responsible for the change in the kinetic constants of the elementary steps of the cross-bridge cycle.
(Received 17 December 2003;
accepted after revision 20 January 2004;
first published online 23 January 2004)
Corresponding author M. Kawai: Department of Anatomy and Cell Biology, College of Medicine, The University of Iowa, Iowa City, IA 52242, USA. Email: masataka-kawai{at}uiowa.edu
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