Quin 2: the dissociation constants of its Ca2+ and Mg2+ complexes and its use in a fluorimetric method for determining the dissociation of Ca2+-protein complexes

Abstract

Fluorimetric or spectrophotometric titrations with the appropriate cations gave Kd values of 2.9 .+-. 0.2 nM and 89 .+-. 5 .mu.M respectively for the Ca2+ and Mg2+ complexes of quin 2 at pH 7.5. Mixtures of quin 2 and vitamin D-dependent Ca2+-binding protein from pig duodenum were titrated fluorimetrically with Ca2+ in the absence or presence of Mg2+. These measurements were used with the Kd values of the Ca2+ and Mg2+ complexes of quin 2 to obtain Kd or apparent Kd values for Ca2+-protein complexes ranging from 5 nM to 5 .mu.M with good accuracy.