Effects of sodium and lithium salts on the conformation of human alpha-thrombin

Chemical modification studies have demonstrated that the ultra-violet difference spectrum of alpha-thrombin produced in the presence of sodium is due primarily to changes in the environment of tyrosine residues. This is based on the observation that the spectrum could be abolished by treatment of alpha-thrombin with tetranitromethane but not with dimethyl-(2-hydroxy-5-nitrobenzyl) sulfonium bromide. Although lithium produces similar (UV) difference spectrum, circular dichroism studies indicate that sodium and lithium induce different conformational transitions. alpha-Thrombin tends to assume a more ordered structure in the presence of sodium whereas lithium has the reverse effect. This inverse behavior is consistent with the effects of these cations on the autolysis rate and thermal stability of the activities of alpha-thrombin.