Androgen Receptors in Prostatic Tissue: Evaluation of Systematic Errors and Comparison of Three Procedures to Correct for Nonspecific Binding Sites in the Binding Assays

In androgen receptor analysis, nonspecific binding can distort the measurement of the binding constants of the receptor ligand complex. We evaluate this influence with a one-ligand two-binding sites model. The binding constants correspond to those of androgen receptors in prostatic tissue. Three procedures which correct for the influence of nonspecific binding sites are compared: difference of bound radioligands measured in both assays (assay 1: only radioligand, assay 2: additional unlabelled ligand in excess), nonlinear regression, and difference of bound radioligands corrected by the ratio of the free radioligands. The subtraction of bound radioligands causes considerable errors in the estimation of the total concentration of receptor binding sites. Nonlinear regression yields the correct result when the influence of nonspecific binding is moderate. The corrected difference of bound ligand gives the correct results even in the range where nonlinear regression fails. We conclude that the latter method corrects for the influence of nonspecific binding as effectively as nonlinear regression and suggest that the free fraction of radioligand should be measured and not discarded, as it is common in the clinical practice.