Aminopeptidase A (APA) is a highly selective peptidase, which cleaves the N-terminal Glu or Asp residues of biologically active peptides, and has therefore been proposed to be involved in angiotensin II and CCK8 metabolism. Highly potent and selective APA inhibitors are consequently required to study the physiological regulation of these two peptides. Using, as a model, Glu-thiol (4-amino-5-mercaptopentanoic acid), which was the first efficient APA inhibitor described but is however equipotent on APA (0.14 microM) and aminopeptidase N (APN) (0.12 microM), several beta-amino thiol inhibitors have been synthesized. In these molecules, the length of the side chain was varied and the carboxylate group of Glu-thiol was replaced by other negatively charged groups, such as phosphonate, sulfonate, hydroxamate, and thiol. The inhibitory potency of one of these compounds, 22h (S)-3-amino-4-mercaptobutanesulfonate, was found to be nearly 100-fold better for APA than for APN, with an affinity (0.29 microM) almost equivalent to that of Glu-thiol. Hence, this compound is the first selective APA inhibitor reported, and as such, it should be an interesting probe to explore the physiological involvement of APA in the metabolism of neuropeptides like angiotensin II and CCK8.