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

The 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 less than 0.001). These data suggest the existence of two ligand binding sites located in subunits for the 1,25(OH)2D3 receptor.