Characterization of arginine as a solvent additive: a halophilic enzyme as a model protein.

Arginine suppresses the aggregation of proteins. However, little is known about its mechanism. Here we have used HsNDK (Halobacterium salinarum nucleoside diphosphate kinase) to examine the solvent property of arginine. After exposure to 2 M arginine, HsNDK was diluted to a low salt buffer, resulting in fully active protein. Since unfolded HsNDK cannot refold in such low salt buffer, the observed activity indicates that HsNDK was in the native state in 2 M arginine. Enzyme activity was also examined directly in the presence of arginine, showing that it was active in the presence of 1 M arginine and, to less extent, 2 M arginine. Arginine, however, could not support refolding of heat-denatured HsNDK. HsNDK was stable at 40 degrees C for 19 h incubation in the presence of 1M arginine.