title: Actin-induced local conformational change in the myosin molecule. III. Reactivity of S2 thiol and DTNB-reactive thiols of porcine cardiac myosin.
abstract: The reactivity of thiol groups in cardiac myosin obtained from porcine ventricles was compared  with that in rabbit skeletal myosin. 1. The specific thiol group, S2, of cardiac myosin became reactive to N-ethylmaleimide (NEM) upon the addition of Mg2+-ATP or Mg2+-ADP, as in the case of skeletal myosin, although to a lesser degree. Enhancement of the reactivity of S2 resulting from cooperative interaction between F-actin and Mg2+-ATP, but not Mg2+-ADP, which was first found in skeletal myosin, was also observed in cardiac myosin. 2. The total number of 5,5'-dithiobis(2-nitrobenzoate) (DTNB)-reactive thiols decreased in the presence of Mg2+ADP and decreased further in the presence of Mg2+-ATP for both myosins, in contrast to the concomitant increase in the reactivity of S2. The decrement was smaller in cardiac myosin than in skeletal myosin. 3. The DTNB-reactive thiols were classified into three groups with respect to their reactivity. The third or most slowly reacting thiol group, was found to be remarkably susceptible to Ca2+ at physiological concentrations (10(-8)-10(-6) M) in the presence of Mg2+-ADP for cardiac myosin, but in striking contrast, in the presence and relatively long-lived intermediate of myosin ATPase, the structure of which is sensitive to Ca2+, may be different in cardiac and skeletal myosins.
mesh_terms: D000199:Actins / Q000494:pharmacology*; D000244:Adenosine Diphosphate / Q000494:pharmacology; D000255:Adenosine Triphosphate / Q000494:pharmacology; D000818:Animals; D002118:Calcium / Q000494:pharmacology; D004228:Dithionitrobenzoic Acid / Q000494:pharmacology; D007700:Kinetics; D008274:Magnesium / Q000494:pharmacology; D009206:Myocardium / Q000378:metabolism*; D009218:Myosins; D011487:Protein Conformation; D011817:Rabbits; D013438:Sulfhydryl Compounds / Q000378:metabolism*; D013552:Swine
chemical_list: D000199:Actins; D013438:Sulfhydryl Compounds; D000244:Adenosine Diphosphate; D000255:Adenosine Triphosphate; D004228:Dithionitrobenzoic Acid; D009218:Myosins; D008274:Magnesium; D002118:Calcium