Sedimentation Analysis of a Multiple Myeloma G-Globulin and One of Its Mercaptoethylamine Reaction Products

From the ¹ From the Department of Chemistry, University of Wisconsin, Madison, Wisconsin 53706

Descriptions of some thermodynamic properties and transport behavior in solution of an intact multiple myeloma G-globulin and of one of the products of its reaction with mercaptoethylamine have been provided. For the sedimentation equilibrium experiments, the parent G-globulin was dissolved in three solvent systems: cacodylate buffer, its 8 m urea solution and its 6 m guanidine hydrochloride solution, all at pH 7. Molecular weight and second virial coefficient data were also obtained for the mercaptoethylamine reaction product in 6 m urea solvent. Sedimentation velocity experiments were performed with the same solutions.

The concentration dependence, both of apparent molecular weight and of sedimentation coefficient, for the parent molecule in 6 m guanidine hydrochloride is quite normal. The same may be said of the corresponding behavior of the submolecule in 6 m urea buffer solution.

On the other hand, suitable interpretations of sedimentation equilibrium and sedimentation transport data for the solutions of the G-globulin in cacodylate and 8 m urea buffers seem to require the use of an association mechanism. A simplified theory, based on the assumption that only dimerization occurs in solution, is presented which makes use of a combination of the two kinds of information. In this way, it has become possible to provide a value for the dimerization constant and to assign limiting sedimentation coefficients for monomer and dimer. For the reasons given, the dimerization is believed to take place in an end-to-end fashion.

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