The evaluation of non-hyperbolic ligand binding and substrate saturation data using variable Hill coefficients.

A fitting method for saturation curves is described that may be applied to any kind of ligand binding process, regardless of whether positive, negative or mixed cooperativity is involved. The procedure yields 'best values' of maximal saturation (or maximal velocity with enzymes); in addition, an empirical function is calculated that represents the concentration dependence of the Hill slope (i.e. the slope of the corresponding Hill plot). Since the method does not require the maximal saturation to be known in advance, it can be successfully applied to systems which, due to experimental limitations, may not be saturated with ligand. It is shown that in these cases values of maximal saturation are obtained that are considerably more reliable than those estimated by customary methods. Moreover, it is demonstrated that the concentration dependence of the Hill slope, the other main result of the proposed procedure, may offer useful evidence as to the molecular events leading to non-hyperbolicity and thus provide a rational basis for selecting more specific models to describe the properties of the system studied.