A Sub-Picogram Competitive Protein-Binding Method for Biotinyloestrone

A very sensitive competitive protein binding assay for biotinyloestrone. using competition between biotinyloestrone conjugate and 125I-labelled biocytin for avidin is described. The method is based upon the fact that the chemical modification or conjugation to the carboxyl moiety of biotin does not substantially alter the very high Ka of the protein avidin binding its natural ligand biotin.