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Sedimentation Analysis of a Papain-digested Human Multiple Myeloma G-Globulin

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

The results of certain further characterizations of some immunoglobulin fragments, in this instance Pieces I (or II) and III, of a multiple myeloma G-globulin (HL) are presented here. The submolecules have been separated from the papain-cysteine-protein system by gel filtration on a Sephadex G-200 column which had been equilibrated with cacodylate buffer.

For the physicochemical observations, two solvent media were used: aqueous cacodylate buffer and 8 m urea-cacodylate buffer, both at pH 7 and an ionic strength of approximately 0.1. Sedimentation equilibrium and sedimentation velocity experiments were performed with the protein systems. It was found that the two fragments have substantially the same molecular weight (50,000).

In each case, the sedimentation equilibrium data (apparent weight average molecular weight plotted against the average concentration) suggest that the protein submolecules associate in cacodylate buffer. Values of the equilibrium constant for the process have been computed on the assumption that only dimerization occurs. Association was suppressed in the solvent system containing 8 m urea, and conventional sedimentation behavior was observed.

The course of sedimentation velocity experiments has been explained on the same basis, namely, association in the cacodylate buffer system. It has been shown that it is possible to estimate limiting sedimentation coefficients (s_25,w⁰) for both monomer and dimer which are consistent with the observed sedimentation equilibrium result.

An attempt has been made to obtain a distribution of sedimentation coefficients for Pieces I (or II) and III, but it is recognized that there are difficulties involved.

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