Covalent chromatography and salt-promoted thiophilic adsorption

Covalent chromatography on 3-(2-pyridyl disulfido)-2-hydroxypropyl agarose, abbreviated PyS2, turns out to involve more complex interactions than has been supposed heretofore. Unexpectedly, the sorption is highly salt dependent. The relative affinities for serum proteins have therefore been determined in the absence and presence of different types of salts at different salt concentrations and with different degrees of ligand substitution on the adsorbent. In the presence of water-structuring salts the PyS2-gel shows an adsorption pattern for serum proteins resembling that of the "thiophilic" T-gel (J. Porath; F. Maisano, and M. Belew (1985) FEBS Lett. 185, 306-310). Superimposed on thiophilic adsorption we have found, as expected, covalent attachment of thiol-containing proteins. Also the thiol-disulfide exchange increases from 4-5% in the absence of potassium sulfate or sodium chloride up to about 40% of the applied serum proteins when such a water-structuring salt is present. We have thus shown that the interaction of a protein with the ligand is greatly facilitated by a water-structuring salt--and in this case the product is a covalently as well as a thiophilically immobilized protein. A cautious interpretation of protein interaction phenomena is justified whenever ligands containing sulfide, disulfide, or pi-electron-rich structures such as aromatic moieties are involved.