Rabbit Cardiac Myosin. I. Physical and Chemical Characterization of the Native Molecule

Abstract

Rabbit cardiac myosin, isolated from frozen tissue, was effectively purified by batchwise treatment with DEAE-cellulose in addition to using dilution–precipitation techniques. An extensive experimental program was subsequently carried out with respect to the enzymic, amino acid, optical, and physicochemical properties of native cardiac myosin. This program has included the following: examination of the effects of pH and varying concentrations of ATP, CaCl2, MgCl2, and PCMB on its ATPase activity; measurement of its circular dichroic spectrum in solvent buffers, at different pH or containing ATP in the absence or presence of Ca2+ or Mg2+ ions; study of the concentration dependence of its viscosity and sedimentation velocity at low temperatures; and investigation of its molecular weight by the Archibald method and low- and high-speed sedimentation equilibrium. The results of these studies were consistent with the interpretation that cardiac myosin is comprised of highly asymmetric, semi-rigid molecules with a molecular weight in the order of 4.7 × 105, which display non-ideality even in solvent buffers of high ionic strength at neutral pH. In addition, computer analysis of the high-speed sedimentation equilibrium data has provided evidence for the presence of a self-association reaction at low protein concentration. Even though the specific ATPase activity of cardiac myosin was found to be approximately one-third that reported for skeletal myosin in all cases, it was concluded, on the basis of the essentially analogous physical and chemical properties of rabbit cardiac and skeletal myosin, that the two proteins are very similar in terms of molecular size, shape, and secondary structure.