Comparative studies on amino and thiol groups in myosins from different sources

Myosins from rabbit white and red skeletal, rabbit heart, fish skeletal and chicken gizzard muscles, as well as from human platelets were subjected to trinitrophenylation by trinitrobenzene sulfonate and alkylation by N-ethyl maleimide which affected their amino and thiol groups, respectively. The blocking of amino groups was carried out in the presence or in the absence of Mg · ADP and was followed both spectrophotometrically and enzymatically. Essential amino groups, whose modification thoroughly changes the enzymic characteristics of myosin, were found in heart and in all skeletal muscle myosins but were absent in myosins from chicken gizzard muscle and from human platelets. The reaction of these amino groups was highly retarded in the presence of Mg · ADP. Alkylation of thiols led to loss of the K⁺-activated ATPase (ATP phosphohydrolase, EC 3.6.1.3) in all myosins. However, the rate of loss of activity varied from one myosin to another and, for a given myosin, was affected by the presence of nucleotides and by the value of the ionic strength. The change in Ca²⁺activated ATPase activity (ATP phosphohydrolase, EC 3.6.1.3)on alkylation was influenced by the presence of Mg · ADP during the reaction. In the absence of this nucleotide, the Ca²⁺-ATPase activity increased and reached a plateau as a consequence of modification. The extent of activation largely depended on the origin of the myosin. When alkylation was carried out in the presence of Mg · ADP, the Ca²⁺-ATPase activity as a function of time exhibited a maximum but the descending part of the curve was absent in myosins from heart and gizzard muscles.