Prosphatase binding assay

Abstract

Disclosed is a binding assay for proteases and phosphatases, which contain cysteine in their binding sites or as a necessary structural component for enzymatic binding. The sulfhydryl group of cysteine is the nucleophilic group in the enzyme's mechanistic proteolytic and hydrolytic properties. The assay can be used to determine the ability of new, unknown ligands and mixtures of compounds to competitively bind with the enzyme versus a known binding agent for the enzyme, e.g., a known enzyme inhibitor. By the use of a mutant form of the natural or native wild-type enzyme, in which serine, or another amino acid, e.g., alanine, replaces cysteine, the problem of interference from extraneous oxidizing and alkylating agents in the assay procedure is overcome. The interference arises because of oxidation or alkylation of the sulfhydryl, --SH (or --S.sup.-), in the cysteine, which then adversely affects the binding ability of the enzyme. Specifically disclosed is an assay for tyrosine phosphatases and cysteine proteases, including caspases and cathepsins, e.g., Cathepsin K(O2), utilizing scintillation proximity assay (SPA) technology. The assay has important applications in the discovery of compounds for the treatment and study of, for example, diabetes, immunosuppression, cancer, Alzheimer's disease and osteoporosis. The novel feature of the use of a mutant enzyme can be extended to its use in a wide variety of conventional colorimetric, photometric, spectrophotometric, radioimmunoassay and ligand-binding competitive assays.