Cleavage of amyloid precursor protein (APP) by the beta- and gamma-secretases generates the amino and carboxy termini, respectively, of the A beta amyloidogenic peptides A beta40 and A beta42--the major constituents of the amyloid plaques in the brain parenchyma of Alzheimer's disease patients. There is evidence that the polytopic membrane-spanning proteins, presenilin 1 and 2 (PS1 and PS2), are important determinants of gamma-secretase activity: mutations in PS1 and PS2 that are associated with early-onset familial Alzheimer's disease increase the production of A beta42 (refs 4-6), the more amyloidogenic peptide; gamma-secretase activity is reduced in neuronal cultures derived from PS1-deficient mouse embryos; and directed mutagenesis of two conserved aspartates in transmembrane segments of PS1 inactivates the ability of gamma-secretase to catalyse processing of APP within its transmembrane domain. It is unknown, however, whether PS1 (which has little or no homology to any known aspartyl protease) is itself a transmembrane aspartyl protease or a gamma-secretase cofactor, or helps to colocalize gamma-secretase and APP. Here we report photoaffinity labelling of PS1 (and PS2) by potent gamma-secretase inhibitors that were designed to function as transition state analogue inhibitors directed to the active site of an aspartyl protease. This observation indicates that PS1 (and PS2) may contain the active site of gamma-secretase. Interestingly, the intact, single-chain form of wild-type PS1 is not labelled by an active-site-directed photoaffinity probe, suggesting that intact wild-type PS1 may be an aspartyl protease zymogen.

Progressive cerebral amyloid beta-protein (A beta) deposition is believed to play a central role in the pathogenesis of Alzheimer's disease (AD). Elevated levels of A beta(42) peptide formation have been linked to early-onset familial AD-causing gene mutations in the amyloid beta-protein precursor (A beta PP) and the presenilins. Sequential cleavage of A beta PP by the beta- and gamma-secretases generates the N- and C-termini of the A beta peptide, making both the beta- and gamma-secretase enzymes potential therapeutic targets for AD. The identity of the A beta PP gamma-secretase and the mechanism by which the C-termini of A beta are formed remain uncertain, although it has been suggested that the presenilins themselves are novel intramembrane-cleaving gamma-secretases of the aspartyl protease class [Wolfe, M. S., Xia, W., Ostaszewski, B. L., Diehl, T. S., Kimberly, W. T., and Selkoe, D. J. (1999) Nature 398, 513-517]. In this study we report the identification of L-685,458 as a structurally novel inhibitor of A beta PP gamma-secretase activity, with a similar potency for inhibition of A beta(42) and A beta(40) peptides. This compound contains an hydroxyethylene dipeptide isostere which suggests that it could function as a transition state analogue mimic of an aspartyl protease. The preferred stereochemistry of the hydroxyethylene dipeptide isostere was found to be the opposite to that required for inhibition of the HIV-1 aspartyl protease, a factor which may contribute to the observed specificity of this compound. Specific and potent inhibitors of A beta PP gamma-secretase activity such as L-685,458 will enable important advances toward the identification and elucidation of the mechanism of action of this enigmatic protease.

The pharmaceutical industry remains solely reliant on synthetic chemistry methodology to prepare compounds for small-molecule drug discovery programmes. The importance of the physicochemical properties of these molecules in determining their success in drug development is now well understood but we present here data suggesting that much synthetic methodology is unintentionally predisposed to producing molecules with poorer drug-like properties. This bias may have ramifications to the early hit- and lead-finding phases of the drug discovery process when larger numbers of compounds from array techniques are prepared. To address this issue we describe for the first time the concept of lead-oriented synthesis and the opportunity for its adoption to increase the range and quality of molecules used to develop new medicines.

␥-Secretase is a multi-component enzyme complex that performs an intramembranous cleavage, releasing amyloid-␤ (A␤) peptides from processing intermediates of the ␤-amyloid precursor protein. Because A␤ peptides are thought to be causative for Alzheimer's disease, inhibiting ␥-secretase represents a potential treatment for this neurodegenerative condition. Whereas inhibitors directed at the active center of ␥-secretase inhibit the cleavage of all its substrates, certain non-steroidal antiinflammatory drugs (NSAIDs) have been shown to selectively reduce the production of the more amyloidogenic A␤(1-42) peptide without inhibiting alternative cleavages. In contrast to the majority of previous studies, however, we demonstrate that in cell-free systems the mode of action of selected NSAIDs and their derivatives, depending on the concentrations used, can either be classified as modulatory or inhibitory. At modulatory concentrations, a selective and, with respect to the substrate, noncompetitive inhibition of A␤(1-42) production was observed. At inhibitory concentrations, on the other hand, biochemical readouts reminiscent of a nonselective ␥-secretase inhibition were obtained. When these compounds were analyzed for their ability to displace a radiolabeled, transition-state analog inhibitor from solubilized enzyme, noncompetitive antagonism was observed. The allosteric nature of radioligand displacement suggests that NSAID-like inhibitors change the conformation of the ␥-secretase enzyme complex by binding to a novel site, which is discrete from the binding site for transition-state analogs and therefore distinct from the catalytic center. Consequently, drug discovery efforts aimed at this site may identify novel allosteric inhibitors that could benefit from a wider window for inhibition of ␥ (42)-cleavage over alternative cleavages in the ␤-amyloid precursor protein and, more importantly, alternative substrates.According to the "amyloid cascade hypothesis" an enhanced production or decreased clearance of toxic amyloid-␤ (A␤) 1 peptides is thought to be the cause of Alzheimer's disease (AD) (1). A␤ peptides are processing products (2) of the type I transmembrane protein ␤-amyloid precursor protein (␤APP) (3), which has undergone sequential cleavages by ␤-and ␥-secretase enzymes. A common denominator (reviewed by Hardy (4)) for mutations causative of familial AD (FAD) has been revealed, being abnormalities in the metabolism of ␤APP that appear to lead to an elevation of the production of the A␤(1-42) peptide species. This C-terminally elongated A␤ peptide is more prone to aggregation than the shorter and more abundant A␤(1-40) species. Consequently, the prevention of A␤ production by inhibiting either of the proteases required for processing of ␤APP is currently viewed as a promising approach toward a therapy for AD. The membrane-bound aspartyl protease ␤-site ␤APP-cleaving enzyme 1 (5, 6) is the major ␤-secretase required for the generation of A␤ peptides. ␤-Site ␤APP-cleaving enzyme 1 has been shown to cleave within th...

Originally reported in Liebigs Annalen in 1953 (then called Justus Liebigs Annalen der Chemie), the Wittig reaction has evolved to include the Horner–Wittig and Horner–Wadsworth–Emmons reactions and several other variations. Today, these reactions constitute some of the most powerful processes for the construction of carbon–carbon bond frameworks, facilitating the chemical synthesis of myriads of organic molecules both in research laboratories and industrial settings. The Wittig and related reactions were proven particularly useful in the field of total synthesis, where they enabled the construction of highly complex structures and had a significant impact in shaping the art to its present state of sophistication. In this article the authors focus, after a brief introduction, on total syntheses from their own laboratories that employ such processes. These examples, together with the numerous others adorning the chemical literature, illustrate amply the importance of the Wittig and related reactions and point to their continuing prominence in organic synthesis.