Inability of [3H]estriol to induce maximal cooperativity of the estrogen receptor.

The interaction of [3H]estriol with the partially purified estrogen receptor from calf uterus shows positive cooperativity that is dependent upon receptor concentration and temperature. At a receptor concentration of 1 nM and 25 degrees C the [3H]estriol-receptor cooperativity was low, the Hill coefficient (nH) was 1.03 +/- 0.02 however, with increasing receptor concentrations the receptor's cooperativity increased until at approximately intracellular receptor concentration (20 nM) the nH = 1.20 +/- 0.04. At 0 degrees C and a receptor concentration of 10 nM the [3H]estriol-receptor interaction was highly cooperative, the Scatchard plot was convex and nH = 1.58 +/- 0.04 while at 30 degrees C the Scatchard plot approached linearity and nH = 1.03 +/- 0.02. In comparison, [3H]estradiol was capable of inducing, at 0 or 30 degrees C and at a receptor concentration of 1 nM or greater, maximal receptor cooperativity, nH = 1.63. These data demonstrate: (a) the receptor's conformation and binding mechanism change in a specific manner with temperature, so that receptor analysis at 0 degrees C does not necessarily reflect the receptor's properties at biologically relevant temperatures; (b) the dependence of the receptor's cooperativity on receptor concentration, which suggests interaction between dissociable subunits; and (c) the lower cooperativity induced by estriol, in comparison with estradiol, which indicates estriol is less efficient in shifting the receptor toward a higher affinity or the activated state of the receptor.