We have investigated the functional relationship between metalloendopeptidase EC 3.4.24.15 (MP24.15) and the amyloid precursor protein involved in Alzheimer's disease (AD) and discovered that the enzyme promotes A␤ degradation. We show here that conditioned medium (CM) of MP24.15 antisense-transfected SKNMC neuroblastoma has significantly higher levels of A␤. Furthermore, synthetic-A␤ degradation was increased or decreased following incubation with CM of sense or antisense-transfected cells, respectively. Soluble A␤1-42 was degraded more slowly than soluble A␤1-40, while aggregated A␤1-42 showed almost no degradation. Alzheimer's disease (AD)1 is a progressive neurodegenerative disorder and the most common form of dementia in the elderly (1). Evidence indicates that accumulation of amyloid-␤ (A␤) deposits in senile plaques and in cerebrovasculature is associated with the pathophysiology of AD (2). The A␤ peptide is composed of 40 -42 amino acids (3). The events leading to its formation from the transmembrane amyloid precursor protein (APP) involve proteolytic cleavage by enzymes that have been termed: 1) ␤-secretase, which cleaves at the amino terminus of A␤, and 2) ␥-secretase, which cleaves at the carboxyl terminus. In the senile plaques A␤ is associated with a number of proteins, including ␣ 1 -antichymotrypsin (ACT) (4), which is a serine proteinase inhibitor of the serpin family as well as an acute-phase protein (5). Thus, we hypothesized 10 years ago that ACT may play a role in APP processing (4, 6).Soluble A␤ peptide can be detected in the conditioned media of a variety of cultured mammalian cells in vitro (7-9), as well as in serum and cerebrospinal fluid in vivo (10). The majority of secreted A␤ is 40 amino acids in length (A␤1-40), but approximately 10% of all A␤ is 42 amino acids in length (A␤1-42) (11). Little is known about how secreted A␤ is degraded and cleared from the extracellular milieu. The excessive cerebral accumulation of A␤ that occurs in AD could be explained in part by a decreased ability of the brain to degrade and clear A␤. If neural cells can be shown to release specific A␤-degrading proteinases, changes in the activity of such proteinases and/or their upregulation could represent a therapeutic approach to AD.We have explored the role of the metalloendopeptidase MP24.15 in the degradation of A␤. MP24.15 is a thiol-dependent enzyme that was purified to homogeneity from AD brain as a candidate ␤-secretase (12). McDermot et al. (13) also identified a partially purified MP24.15 as a potential ␤-secretase using synthetic peptide substrates. In vitro, MP24.15 has been shown to be involved in the inactivation of a number of neuropeptides, including somatostatin, bradykinin, substance P, and neurotensin (14 -16). The cDNA coding for MP24.15 was subsequently cloned from a human brain library (17,18). In an attempt to further examine the ␤-secretase properties of MP24.15 under physiologic conditions, we transfected human neuroblastoma cells with MP24.15 cDNA in the sense and antisense directi...