INTERACTION OF POLYAMINES WITH THE CA2+-BINDING PROTEIN PARVALBUMIN

The interaction of spermine, spermidine and putrescine with the Ca2+-binding protein, parvalbumin, was studied at pH 6 and 7, with the help of the intrinsic fluorescence properties of tryptophan and circular dichroic spectroscopy of the protein in the ultraviolet region. Polyamines bind to parvalbumin that is either Ca2+-free or partially saturated with Ca2+, as indicated by a change in the emission maximum and intensity of tryptophan fluorescence. The binding affinities for the interactions are about 4 mM, 8 mM and > 20 mM for spermine, spermidine and putrescine, respectively. No alterations in fluorescence properties were detected when the polyamines were added to fully Ca2+-bound parvalbumin. An increase in the ellipticity of the circular dichroic spectrum in the region where tryptophan absorbs was observed when polyamines were added to Ca2+-free parvalbumin. This finding indicates that polyamine binding affects the segment of the protein where tryptophan is located. Based on these results it is postulated that polyamines bind to the Ca2+-free of partially saturated parvalbumin and stabilize its structure.