Cooperativity in the binding of 1α,25-dihydroxyvitamin D3 to the chick intestinal receptor

AbstractThe binding of 1,25-dihydroxyvitamin D3 [1,25(OH)2D3] to its chick intestinal receptor does not fit well to the linear regression line of Scatchard's model (r = −0.79; dissociation constant, Kd = 0.51 nM). In fact, a concave ‘hook’ curve describes the data better. By using the Hill analysis the linear fitting was improved (r= 0.99), the Kd was found to be 0.14 nM and the Hill coefficient (nH) 1.42, which indicates a positive cooperativity in the binding of 1,25(OH)2D3 to its receptor. Further we found that Kd and nH are strongly correlated (p < 0.001). These data suggest the existence of two ligand binding sites located in subunits for the 1,25(OH)2D3 receptor