Salting-In Effects on Urate Oxidase Crystal Design

In this paper, solubility and interactions in solution of the recombinant urate oxidase from Aspergillus flavus, rasburicase, are studied both in the absence and in the presence of salt at a pH close to the pI. An intense salting-in effect is demonstrated first by an increased solubility when various salts are added. Thus, merely adding salt does not induce rasburicase crystallization. Second virial coefficient measurements also confirm this effect by exhibiting repulsive interactions over a large range of salt concentrations. Therefore, the salting-in effect enables the stabilization of rasburicase solution at high concentrations. Moreover, it enables crystals of improved size and habit to be grown when polymer is added to a solution of rasburicase concentrated with salt, or when salt is removed from it. We also show, with the example of high pressure macromolecular crystallography, that salt enables the stabilization of the desired polymorph under the highly concentrated polyethylene glycol conditions required by this technique.