A kinetic and equilibrium binding study of 1 alpha,25-dihydroxyvitamin D3 with its cytosol receptor from chick intestinal mucosa.

A kinetic and equilibrium binding study of the interaction of 1 alpha,25-dihydroxyvitamin D3 (1 alpha,25-(OH)2D3) with its receptor system from chick intestinal mucosa has been carried out. The equilibrium dissociation constant (Kd) for the binding of 1 alpha,25-(OH)2D3 to the crude cytoplasmic receptor varies from 2.3 x 10(-10) M at 4 degrees C to 5.0 x 10(-10) M at 24 degrees C, yielding values for delta G0, delta H0 and delta S0 for the dissociation process of 12.2 +/- 0.1 kcal/ mol, 6.7 +/- 1.2 kcal/mol, and -19.9 +/- 3.2 cal/degree mol. This suggests that the binding of 1 alpha,25-(OH)2D3 to the receptor may be an entropy-driven process. Kinetic studies on the association and dissociation of the 1 alpha,25-(OH)2D3 . receptor complex revealed that kassoc varied from 0.6 x 10(7) M-1 min-1 at 0 degrees C to 14.5 x 10(7) M-1 min-1 at 30 degrees C and that kdiss varied from 4 x 10(-5) min-1 at 4 degrees C to 4 x 10(-2) min-1 at 30 degrees C. The activation energy for the association process was found to be 17 kcal/mol, while that for the dissociation reaction was 45 kcal/mol.