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Researchers are homing in on the elusive enzyme that makes the final cut that releases the Alzheimer peptide, amyloid -peptide (A), from its transmembrane amyloid precursor protein (APP). Two groups have independently shown that the active site of the elusive enzyme---secretase--is contained in a transmembrane protein dubbed presenilin. Presenilin threads through the membrane in a serpentine fashion. Shortly after being synthesized, it is cleaved by an unknown mechanism to form two subunits. Mutations in presenilin are associated with familial Alzheimer's disease.
A is generated after -secretase, which was identified by several pharmaceutical companies last year, cleaves APP at a site outside the membrane and -secretase snips it within the membrane. The peptide is either 40 or 42 amino acids long depending on where -secretase makes its cut. Both forms are produced normally in small amounts, although their function is not known. In Alzheimer's disease, the ratio of A40 to A42 is skewed in favor of A42, and that triggers the peptides to form fibrillar deposits in amyloid plaque in the brain. Although -secretase has a name and a function, the enzyme itself has never been isolated. Evidence that presenilin might be -secretase began to appear last year when author showed that mutations in two aspartyl residues in presenilin inhibited -secretase activity in cells growing in culture.Earlier this year, Gardell and colleagues precipitated presenilin with an antibody to it and showed that the precipitate displays -secretase activity . Both groups have shown that inhibitors intended to bind to the active site of -secretase bind directly to presenilin. compound is a substrate-based -secretase inhibitor that exploits a rationally designed difluoroalcohol group the team previously had shown to be an analog of -secretase's transition state. To find out whether the inhibitor would bind to presenilin, the team attached a bromoacetamide to one end of the compound, reasoning that bromoacetamide would undergo nucleophilic attack by residues near the active site to form a covalent bond with presenilin. At the other end, they tacked on biotin so the complex could be easily detected with streptavidin, which binds to biotin. In various experiments they confirmed that the derivatized inhibitor, dubbed BrA-1-Bt, bound to each of two presenilin subunits and inhibited -secretase activity. inhibitors are transition-state analogs. They would therefore be expected to bind selectively to active forms of -secretase, the researchers point out. Compounds that would inhibit either - or -secretase could potentially block A production and be useful in treating Alzheimer's disease. Indeed, -secretase inhibitors are already under development. Not much is known about -secretase, though, other than its role in cleaving APP, Wolfe notes. -Secretase, on the other hand, is involved in processing the Notch protein that's crucial for embryonic development. That "raises a red flag" about whether -secretase could be blocked without affecting Notch activity, he says. But he believes "the answer is 'yes,' because you don't have to completely shut down -amyloid production to get a therapeutic effect." Cholesterol-lowering drugs don't completely shut down cholesterol synthesis, he notes. Likewise, -secretase inhibitors could reduce A production without depleting the enzyme
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