2-Guanidinoethylmercaptosuccinic Acid, also knowns as GEMSA, is a potent inhibitor of enkephalin convertase (CPE; Ki = 8.8 nM), known as a carboxypeptidase B-like processing enzyme. The compound has been shown to be a model candidate for use in affinity chromatography of the enzyme. Studies show that GEMSA can influence the endogenous opioid peptide system and have a dominant affect on the central nervous system. 2-Guanidinoethylmercaptosuccinic acid is an inhibitor of carboxypeptidase N, polypeptide 1 (CPN cat).