Ca2%-binding protein from pig duodenum

The cation-binding properties of the vitamin D-dependent Ca2 +-binding protein from pig duodenum were investigated, mainly by flow dialysis. The protein bound two Ca2 + ions with high affinity, and Mg2 +, Mn2+ and K+ were all bound competitively with Ca2 + at both sites. The sites were distinguished by their different affinities for Mn2 +, the one with the higher affinity being designated A (Kd 0.61 + 0.02 UM) and the other B (Kd 50+ 6 UM). Competitive binding studies allied to fluorimetric titration with Mg2 + showed that site A bound Ca2 +, Mg2 + and K + with Kd values of 4.7 + 0.8 nM, 94+ 18 gM and 1.6 + 0.3mM respectively, and site B bound the same three cations with Kd values of 6.3+ 1.8nM, 127+38,UM and 2.1 +0.6mM. For the binding of these cations, therefore, there was no significant difference between the two sites. In the presence of 1mM-Mg2 + and 150mM-K +, both sites bound Ca2 + with an apparent Kd of 0.5 gLM. The cation-binding properties were discussed relative to those of parvalbumin, troponin C and the vitamin D-dependent Ca2 +-binding protein from chick duodenum.