Analysis of allosterism in functional assays.

The theoretical basis for analyzing the effects of an allosteric modulator on the response to an agonist is described. The effects of an allosteric modulator on the concentration-response curve to an agonist can be attributed to changes in the observed dissociation constant and intrinsic efficacy of the agonist-receptor complex. Each of these two changes can be represented by a coefficient or factor. It is possible to estimate the ratio of the coefficient of change in agonist efficacy divided by that for the agonist dissociation constant. This ratio is designated as the relative activity (RA) of the agonist in the presence of the allosteric modulator. The RA value can be estimated for each concentration of allosteric modulator by nonlinear regression analysis, regardless of the shape of the concentration-response curve. Regression analysis of the RA values against the concentration of allosteric modulator yields estimates of the dissociation constant (K(A)) of the allosteric modulator and the maximal RA value. If the concentration-response curve to the agonist obeys a logistic function and the allosteric modulator influences the maximal response, it is possible to distinguish between the maximal change in affinity from that of efficacy. If the agonist concentration-response curve obeys a logistic equation with a Hill slope of 1, the RA values can be estimated easily from the agonist EC(50) and E(max) values. This analysis illustrates the utility of the RA value in quantifying allosteric effects.