Studies on Platelet Proteins. IV. Some Physical and Chemical Properties of Thrombosthenin

The contractile platelet protein thrombosthenin has been isolated with butanol and purified further by repeated precipitation and gel filtration. Thrombosthenin thus isolated shows all the typical properties of this protein as reported in the literature. 1 M NaCl-tris, pH 7.2 has been found to be a better solvent than 0.6 M KCl as far as stability, resolution and aggregation of the protein are concerned. In the ultracentrifuge, 0.7 per cent thrombosthenir in 0.6 M KCl shows three hypersharp boundaries with sedimentation coefficients of 36 S, 56 S and 83 S. However, at low protein concentration only the 36 S boundary is observed. In 1 M NaCl, all these components break down producing a single kinetic unit of 18 S which dissociates further to 7 S. The 7 S component retains the ATP-sensitivity typical of thrombosthenin, suggesting it to be the monomeric unit of this protein. A similar hypersharp boundary with similar sedimentation coefficient to purified thrombosthenin has been noted in platelet ghosts solubilized in presence of detergent. A combination of sedimentation and viscosity data leads to an approximate molecular weight 8.9 x 10⁵ for thrombosthenin.