A diverse range of membrane proteins of Type 1 or Type II topology also occur as a circulating, soluble form. These soluble forms are often derived from the membrane form by proteolysis by a group of enzymes referred to collectively as 'secretases' or 'sheddases'. The cleavage generally occurs close to the extracellular face of the membrane, releasing physiologically active protein. This secretion process also provides a mechanism for down-regulating the protein at the cell surface. Examples of such post-translational proteolysis are seen in the Alzheimer's amyloid precursor protein, the vasoregulatory enzyme angiotensin converting enzyme, transforming growth factor-alpha, the tumour necrosis factor ligand and receptor superfamilies, certain cytokine receptors, and others. Since the proteins concerned are involved in pathophysiological processes such as neurodegeneration, apoptosis, oncogenesis and inflammation, the secretases could provide novel therapeutic targets. Recent characterization of these individual secretases has revealed common features, particularly sensitivity to certain metalloprotease inhibitors and upregulation of activity by phorbol esters. It is therefore likely that a closely related family of metallosecretases controls the surface expression of multiple integral membrane proteins. Current knowledge of the various secretases are compared in this Review, and strategies for cell-free assays of such proteases are outlined as a prelude to their ultimate purification and cloning.

The two hallmarks of Alzheimer's disease (AD) are the presence of neurofibrillary tangles (NFT) made of aggregates of the hyperphosphorylated tau protein and of amyloid plaques composed of amyloid-β (Aβ) peptides, primarily Aβ1-40 and Aβ1-42. Targeting the production, aggregation, and toxicity of Aβ with small molecule drugs or antibodies is an active area of AD research due to the general acceptance of the amyloid cascade hypothesis, but thus far all drugs targeting Aβ have failed. From a review of the recent literature and our own experience based on in vitro, in silico, and in vivo studies, we present some reasons to explain this repetitive failure.

Genetic discoveries of Alzheimer’s disease are the drivers of our understanding, and together with polygenetic risk stratification can contribute towards planning of feasible and efficient preventive and curative clinical trials. We first perform a large genetic association study by merging all available case-control datasets and by-proxy study results (discovery n = 409,435 and validation size n = 58,190). Here, we add six variants associated with Alzheimer’s disease risk (near APP, CHRNE, PRKD3/NDUFAF7, PLCG2 and two exonic variants in the SHARPIN gene). Assessment of the polygenic risk score and stratifying by APOE reveal a 4 to 5.5 years difference in median age at onset of Alzheimer’s disease patients in APOE ɛ4 carriers. Because of this study, the underlying mechanisms of APP can be studied to refine the amyloid cascade and the polygenic risk score provides a tool to select individuals at high risk of Alzheimer’s disease.

The pattern of solubilization of nine kidney microvillar ectoenzymes by a range of detergents distinguished two classes of membrane proteins: those released from the membrane by bacterial phosphatidylinositol-specific phospholipase C and those not so released. The latter group of transmembrane proteins were solubilized efficiently (greater than 80%) by all the detergents examined. In contrast, proteins released by phosphatidylinositol-specific phospholipase C were solubilized effectively only by octyl glucoside, 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulphonate and sodium deoxycholate. Octyl glucoside solubilized the amphipathic forms of the ectoenzymes examined, suggesting that this may be a useful detergent in the purification of glycosyl-phosphatidylinositol-anchored ectoenzymes.

Peptidyl-dipeptidase A (angiotensin converting enzyme; ACE, EC 3.4.15.1), has been purified from pig kidney and striatum by affinity chromatography employing the selective inhibitor lisinopril as ligand. The inclusion of a 2.8 nm spacer arm improved the yield of the enzyme compared with the 1.4 nm spacer arm described in previous work. Two forms of striatal ACE (Mr 180,000 and 170,000), but only a single form of kidney ACE (Mr 180,000), were isolated by this procedure. Both forms of striatal ACE were recognized by a polyclonal antibody to kidney ACE. No significant differences in substrate specificity or inhibitor sensitivity between kidney and striatal ACE could be detected. In particular, the amidated neuropeptide, substance P, was hydrolysed identically by both preparations and no significant hydrolysis of the related tachykinin peptides neurokinin A and neurokinin B could be detected. After chemical or enzymic deglycosylation, kidney and both forms of striatal ACE migrated identically on sodium dodecyl sulphate/polyacrylamide-gel electrophoresis with an apparent Mr of 150,000. We suggest that the two detectable forms of ACE in pig brain are not isoenzymes but are the result of differential glycosylation in different cell types in the brain. It appears that ACE, unlike endopeptidase-24.11, does not have the general capacity to hydrolyse and inactivate the tachykinin peptides at a significant rate in brain.