Influence of monovalent cations on rat alpha- and beta-parvalbumin stabilities

The mammalian genome encodes both alpha- and beta-parvalbumin isoforms. The rat beta-parvalbumin (aka "oncomodulin") is more stable than the alpha isoform at physiological pH and ionic strength, despite its substantially higher charge density and truncated C-terminal helix [Henzl, M. T., and Graham, J. S. (1999) FEBS Lett. 442, 241-245]. Reasoning that solvent interactions could contribute to this unexpected finding, we have examined the stabilities of the Ca(2+)-free alpha- and beta-parvalbumins as a function of Na(+) and K(+) concentration. Differential scanning calorimetry data suggest that, at physiological pH and ionic strength, the beta isoform binds roughly 2 equiv of Na(+) or a single equivalent of K(+) with moderate affinity. Under comparable conditions, the alpha isoform apparently binds just 1 equiv of Na(+) and essentially no K(+). Isothermal titration calorimetry experiments suggest that the bound monovalent ions occupy the EF-hand motifs. In 0.15 M K(+), at pH 7.4, the stability of the apo-beta-parvalbumin exceeds that of the alpha isoform by approximately 2.6 kcal/mol at 37 degrees C and by approximately 3.0 kcal/mol at 25 degrees C. The latter value represents a substantial fraction of the difference in Ca(2+)-binding free energies measured in vitro for the two proteins. Significantly, however, these results do not completely explain the paradoxical stability of the beta isoform, which maintains its higher melting temperature under all conditions examined.