Discovery of a sensitive, selective, and tightly binding fluorogenic substrate of bovine plasma amine oxidase.

We report a novel fluorogenic substrate of bovine plasma amine oxidase (BPAO), namely, (2-(6-(aminomethyl)naphthalen-2-yloxy)ethyl)trimethylammonium (ANETA), which displays extremely tight binding to BPAO (K(m) 183 +/- 14 nM) and yet is metabolized fairly quickly (k(cat) 0.690 +/- 0.010 s(-1)), with the aldehyde turnover product (2-(6-formylnaphthalen-2-yloxy)ethyl)trimethylammonium serving as a real time reporting fluorophore of the enzyme activity. This allowed for the development of a fluorometric noncoupled assay that is 2 orders of magnitude more sensitive than the spectrophotometric benzylamine assay. The discovery of ANETA involved elaboration of the lead compound 6-methoxy-2-naphthalenemethaneamine by structure-based design, which recognized the ancillary cation binding site of BPAO as the most significant structural features controlling binding affinity. Structure-based design further ensured a high level of selectivity: ANETA is a good substrate of BPAO but is not a substrate of either porcine kidney diamine oxidase (pkDAO) or rat liver monoamine oxidase (MAO-B). ANETA represents the first highly sensitive, selective, and tight binding fluorogenic substrate of a copper amine oxidase that is able to respond directly to the enzyme activity in real time.