KINFIT II: a nonlinear least-squares program for analysis of kinetic binding data.

We describe a versatile computer program for least-squares fitting of ligand/receptor association and dissociation curves from several experiments simultaneously. The program is designed to handle any number of classes of binding sites reacting with a single ligand that may have two forms: labeled and unlabeled. For a single class of binding sites, the exact, analytical solution is used to generate the computed curves. For more than one class of sites, the computed curves are generated through numerical solution of a set of ordinary differential equations. The parameters determined with this procedure are the on- and off-rate constants and the concentrations of each binding site class. An extensive selection of experimental designs can be processed. The times of observation may be freely chosen, although that choice will affect the quality of the results. Starting with a sample material (e.g., cells, membranes, macromolecules), one can preincubate (to equilibrium) with any combination of labeled and unlabeled ligand. One can then perturb the system by adding any combination of labeled and cold ligand or simply diluting the sample; such an experiment can be continued through several perturbations. A variety of such runs may be combined for analysis as a single data set.