The selective inhibition of an aminopeptidase from Aeromonas proteolytica (AAP), a dinuclear Zn2+ hydrolase, by 8-quinolinol (8-hydroxyquinoline, 8-HQ) derivatives is reported. We previously reported on the preparation of 8-HQ-pendant cyclens as Zn 2+ fluorophores (cyclen is 1,4,7,10-tetraazacyclododecane), in which the nitrogen and phenolate of the 8-HQ units (as well as the four nitrogens of cyclen) bind to Zn 2+ in a bidentate manner to form very stable Zn 2+ complexes at neutral pH (Kd = 8-50 fM at pH 7.4). On the basis of this finding, it was hypothesized that 8-HQ derivatives have the potential to function as specific inhibitors of Zn2+ enzymes, especially dinuclear Zn 2+ hydrolases. Assays of 8-HQ derivatives as inhibitors were performed against commercially available dinuclear Zn 2+ enzymes such as AAP and alkaline phosphatase. 8-HQ and the 5-substituted 8-HQ derivatives were found to be competitive inhibitors of AAP with inhibition constants of 0.16-29 lM at pH 8.0. The nitrogen at the 1-position and the hydroxide at the 8-position of 8-HQ were found to be essential for the inhibition of AAP. Fluorescence titrations of these drugs with AAP and an X-ray crystal structure analysis of an AAP-8-HQ complex (1.3-Å resolution) confirmed that 8-HQ binds to AAP in the ''Pyr-out'' mode, in which the hydroxide anion of 8-HQ bridges two Zn2+ ions (Zn1 and Zn2) in the active site of AAP and the nitrogen atom of 8-HQ coordinates to Zn1 (Protein Data Bank code 3VH9).
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