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Thimet oligopeptidase (TOP) is a thiol- andmetallo-dependent peptidase and has been shown to beone of the β-secretase candidates. TOPexpressed in COS cells cleaved amyloid precursorprotein (APP) at the β-secretase site, and wefound a proteolytic product of APP called secretedform of APP by β-secretase (sAPPβ) in theconditioned media. Here we demonstrate thatsAPPβ was increased in conditioned media whenTOP was coexpressed in COS cells with APP and treatedwith an ADAM inhibitor SI-27. In addition, althoughTOP expressed in COS cell was localized at nuclei orGolgi apparatus, it exclusively colocalized at Golgiapparatus when APP was coexpressed with TOP.