Peptide aldehyde inhibitors of the chymotrypsin-like activity of the proteasome (CLIP) such as Nacetyl-Leu-Leu-Nle-H (or ALLN) have been shown previously to inhibit the secretion of -amyloid peptide (A) from cells. To evaluate more fully the role of the proteasome in this process, we have tested the effects on A formation of a much wider range of peptide-based inhibitors of CLIP than published previously. The inhibitors tested included several peptide boronates, some of which proved to be the most potent peptide-based inhibitors of -amyloid production reported so far. We found that the ability of the peptide aldehyde and boronate inhibitors to suppress A formation from cells correlated extremely well with their potency as CLIP inhibitors. Thus, we conclude that the proteasome may be involved either directly or indirectly in A formation. Key Words: Alzheimer's disease -Amyloid--Amyloid peptide -Proteasome -Inhibitor. J. Neurochem. 73, 195-204 (1999).The accumulation of the 39 -43-residue -amyloid peptide (A) (Glenner and Wong, 1984) in senile plaques and in the walls of cerebral blood vessels is an important pathological feature of Alzheimer's disease. A is derived by proteolytic processing of the -amyloid precursor protein (APP), an integral type 1 membranespanning glycoprotein. There is considerable interest in the proteolytic enzymes involved in this process, as the formation and aggregation of A are likely to be key pathological events in the development of Alzheimer's disease (Hardy and Allsop, 1991). Early studies (Esch et al., 1990;Anderson et al., 1991) showed that the majority of APP is cleaved by ␣-secretase at the Lys 16 -Leu 17 bond in the middle of the A sequence, to produce a large, soluble, N-terminal fragment, which is referred to here as sAPP␣. An alternative pathway of APP processing has been implicated in the formation of A, involving an initial cleavage by -secretase at the Met 596 -Asp 597 bond (numbered according to APP-695) to derive the N terminus of A (Seubert et al., 1993), followed by a second ␥-secretase-mediated cleavage within the membrane-spanning domain of APP to release the C terminus of A. Several forms of A of different length have been described, with A40 and A42 being prominent species (see, for example, Asami-Odaka et al., 1995). The longer forms of A seem to be particularly important in the pathogenesis of Alzheimer's disease (Younkin, 1995).The identity of the ␣-, -, and ␥-secretases is unknown, although the effects of various class-specific protease inhibitors suggest that ␣-secretase is a zinc metalloproteinase, with similarities to the angiotensin converting enzyme secretase and other related membrane-associated secretases (Parvathy et al., 1998). As far as -secretase is concerned, the proteasome has been implicated (Ishiura et al., 1989), as have several chymotrypsin-like serine proteinases (Nelson et al., 1993;Sahasrabudhe et al., 1993;Savage et al., 1994), the metallopeptidase thimet (McDermott et al., 1992), and Received November 4, 1...