title: Mechanism of regulation of cardiac actin-myosin subfragment 1 by troponin-tropomyosin.
abstract: The effect of Ca2+ on the interaction of bovine cardiac myosin subfragment 1 (S-1) with actin regulated by cardiac troponin-tropomyosin was evaluated. The ratios of actin to troponin and to tropomyosin were adjusted to optimize the Ca2+-dependent regulation of the steady-state actin-activated magnesium adenosinetriphosphatase (MgATPase) rate of myosin S-1. At 25 degrees C, pH 6.9, 16 mM ionic strength, the extrapolated values for maximal adenosine 5'-triphosphate (ATP) turnover rate at saturating actin, Vmax, were 6.5 s-1 in the presence of Ca2+ and 0.24 s-1 in the absence of Ca2+. In contrast to this 27-fold regulation of ATP hydrolysis, there was negligible Ca2+-dependent regulation of cardiac myosin S-1 binding to actin. In the presence of ATP, the dissociation constant of regulated actin and cardiac myosin S-1 was 32 microM in the presence of Ca2+ and 40 microM in the presence of [ethylenebis(oxyethylenenitrilo)]tetraacetic acid. These dissociation constants are indistinguishable from the concentrations of actin needed to reach half-saturation of the myosin S-1 MgATPase rates, 37 microM actin in the presence of Ca2+ and 53 microM in its absence. Although there may be Ca2+-dependent regulation of cross-bridge binding in the intact heart, the present biochemical studies suggest that cardiac regulation critically involves other parts of the cross-bridge cycle, evidenced here by almost complete Ca2+-mediated control of the myosin S-1 MgATPase rate even when the myosin S-1 is actin-bound.
mesh_terms: D008841:Actin Cytoskeleton / Q000378:metabolism / Q000648:ultrastructure; D000199:Actins / Q000378:metabolism*; D000251:Adenosine Triphosphatases / Q000378:metabolism; D000818:Animals; D002122:Calcium Chloride / Q000494:pharmacology; D002417:Cattle; D006706:Homeostasis; D007700:Kinetics; D008274:Magnesium / Q000494:pharmacology; D015636:Magnesium Chloride; D009206:Myocardium / Q000378:metabolism*; D015879:Myosin Subfragments; D009218:Myosins / Q000378:metabolism*; D010446:Peptide Fragments / Q000378:metabolism*; D014335:Tropomyosin / Q000378:metabolism*; D014336:Troponin / Q000378:metabolism*
chemical_list: D000199:Actins; D015879:Myosin Subfragments; D010446:Peptide Fragments; D014335:Tropomyosin; D014336:Troponin; D015636:Magnesium Chloride; D000251:Adenosine Triphosphatases; D009218:Myosins; D008274:Magnesium; D002122:Calcium Chloride