The kinetics of glucocorticoid binding to the soluble specific binding protein of mouse fibroblasts

The kinetics of binding of glucocorticoids to the soluble, specific binding protein of mouse fibroblasts has been examined. The rate at which both potent and weak glucocorticoids achieve binding equilibrium is very slow. Second order rate constants of association range from 3 times 10-5 M- minus 1 min- minus 1 for cortisol to 6.7 times 10-5 M- minus 1 min- minus 1 for triamcinolone acetonide. Studies of the rates of binding at high steroid concentrations suggest that the slow rate of binding may be explained by a two-step mechanism. Active glucocorticoids, regardless of their potency, bind initially in a rapid manner to form a weak complex with the binding protein. The dissociation constant for the weak binding reaction is 0.87 times 10- minus 7 M for triamcinolone acetonide and 2.4 times 10- minus 7 M for cortisol. The weak binding complex becomes converted slowly to a tight complex. The first order rate constants for this conversion and the rate constants of dissociation from the tight complex have been determined for cortisol, dexamethasone and triamcinolone acetonide. The binding affinity of steroids of different biological potency is correlated with their rate of dissociation from this second tight binding state.