DISSOCIATION OF CHICKEN EGG-WHITE MACROGLOBULIN INTO SUBUNITS IN ACID . HYDRODYNAMIC, SPECTROPHOTOMETRIC, AND OPTICAL ROTATORY MEASUREMENTS

The high molecular weight globulin from egg white, variously referred to as “line 18,” “component 18,” and “ovomacroglobulin,” has been characterized by physical techniques including sedimentation velocity and equilibrium, diffusion, viscosity, optical rotatory dispersion, and ultraviolet absorption. The native macroglobulin is a glycoprotein of mol wt 6.5 × 105. It appears to have little α-helix content. Spectrophotometric titrations show that most of its phenolic chromophores are abnormal. The transition temperature, Tm, of the native globulin at neutral pH is approximately 60°. In acid solution, the macroglobulin dissociates into two sub-units of equal weight which have essentially the same frictional ratio (1.6) as the native molecule. The pH dependence of subunit formation was determined by hydrodynamic and optical methods. The change in ultraviolet absorption parallels the appearance of subunit, but changes in optical rotation do not. The change in the ultraviolet absorption spectrum observed upon separation of subunits and solvent perturbation difference spectra of both the native protein and its subunits indicates that, upon dissociation, 24 phenolic and 2 indole chromophores per subunit become newly exposed to solvent. © 1969, American Chemical Society. All rights reserved.