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Several structural and enzymatic properties of myosin from skeletal muscles of neonatal and adult rabbits were compared. Electrophoretic analyses and proteolysis experiments indicated that differences between the two myosin types could be attributed to their heavy subunits. Circular dichroism measurements of subfragment-1 species, and trypsin-digested derivatives showed that the neonatal protein contained less alpha-helices than the adult form. The Mg2(+)-ATPase activity of neonatal myosin was lower than that of adult myosin, especially in the presence of actin. In comparison with adult subfragment-1, it was found that the binding of ATP analogues such as adenosine 5'-beta, gamma-iminotriphosphate and PPi, or that of ATP (as deduced from the apparent KmATP) to neonatal subfragment-1 in the presence of actin was enhanced, while that of ADP was decreased. On the other hand, the association of actin with the ADP - neonatal-subfragment-1 complex was weaker. These features must be expressed in the cyclical actin-myosin association/dissociation steps occurring in ATP hydrolysis, and more particularly in the reassociation of actin with the ATP-hydrolysis-products - myosin complex.
Houadjeto et al. (Wed,) studied this question.
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