Although a number of studies have examined amyloid precursor protein (APP) mRNA levels in Alzheimer's disease (AD), no clear consensus has emerged as to whether the levels of transcripts for isoforms containing a Kunitz protease inhibitory (KPI)-encoded region are increased or decreased in AD. Here we compare AD and control brain for the relative amounts of APP protein containing KPI to APP protein lacking this domain. APP protein was purified from the soluble subcellular fraction and Triton X-100 membrane pellet extract of one hemisphere of AD (n ‫؍‬ 10), normal (n ‫؍‬ 7), and neurological control (n ‫؍‬ 5) brains. The amount of KPI-containing APP in the purified protein samples was determined using two independent assay methods. The pathological hallmark of Alzheimer's disease (AD) 1 is the deposition of amyloid as cerebrovascular, diffuse and neuritic plaques (within the brain extracellular space), and neurofibrillary tangles (within neurons). The principal component of extracellular amyloid is a 4-kDa peptide, the A␤ protein (1, 2) (also called the ␤A4 protein). The A␤ peptide is not expressed as a functional protein entity (3) but is released by the processing of a much larger transmembrane protein, the amyloid protein precursor (APP). The pathogenesis of AD is thought to involve the disregulated expression or abnormal processing of APP.APP is encoded by a single 18-exon gene on chromosome 21 (4 -6). Exons 7, 8, and 15 of the APP gene can be alternatively spliced to produce multiple isoforms. In brain the predominant isoform transcripts demonstrated to date are APP 695 , APP 751 , and APP 770 (7-9). These transcripts code for species containing 695, 751, and 770 amino acids, respectively. The isoforms APP 751 and APP 770 both contain a Kunitz protease inhibitory (KPI) motif that APP 695 lacks. APP 770 contains an additional OX.2 domain (7, 10). The secreted form of APP 751 is identical to protease nexin II, a plasma serine protease inhibitor (11). In addition to KPI and OX.2 domains several other structural features have been identified on APP, including binding domains for heparin (12), zinc (13,14) and copper (15), and N-linked carbohydrate attachment sites (10).Normal catabolism of APP involves proteolytic cleavage of full-length membrane-associated forms within the extracellular domain of the A␤ region and release of soluble COOHterminal truncated species (sAPP) (16,17). The proteases that release sAPP have yet to be identified but have been named the ␣-and ␤-secretases. The ␣-secretase cleaves within the A␤ sequence of APP and its products are non-amyloidogenic. The ␤-secretase cleavage site is the NH 2 terminus of the A␤ domain. The proteolytic activities that release intact A␤ from the transmembrane domain of APP (COOH terminus of A␤) have been designated ␥-secretases. The catabolic pathway for A␤ generation is unclear but probably involves internalization of full-length APP from the cell surface and degradation in endosomal-lysosomal complexes (18 -20). In cultured hamster cells one route for A␤ p...

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