Membrane-bound proteases, inhibitors

Regardless of toxic risks, various inhibitors of pancreatic and brush border membrane-bound proteases are listed in Table 5.2 and Table 5.3, respectively. Beside this overview, a classification of inhibitory agents based on their chemical structure is shown below. 

Mucoadhesive polymers exhibiting strong complexing properties are capable of inhibiting intestinal brush border membrane-bound proteases through a far distance inhibitory effect [65]. In vivo, the mucoadhesive polymer is separated from the brush border membrane by a mucus layer [30]. Although there is no direct contact between polymer- and membrane-bound enzymes, it could be shown that inhibition takes place. The exploitation of this far distance effect seems to be a very promising alternative to small molecular mass inhibitors, which are currently used as inhibitors of brush border membrane-bound proteases. 

A Streptomyces inhibitor blocked sporulation of B. subtilis presumably by selective inhibition of a membrane bound protease. The latter was postulated to degrade the B. subtilis protease inhibitor and thus indirectly modulate the level of intracellular protease activity and consequently, sporulation. Burnett et al. [5] found that a substantial fraction of the B. subtilis intracellular protease activity was cryptic i.e. antigen was in excess of activity early in sporulation when protease inhibitor activity was highest. This correlation was also consistent with a function for the inhibitor in modulating intracellular protease. 

Bacillus subtllls by MAPI, a serine protease Inhibitor, and Interaction of MAPI with membrane bound protease, Agrlc. Biol. Chem., 48 365 (1984). 

These must be worthwhile objectives and the recent identification by a number of research groups (see Skovronsky and Lee 2000 for description and details) of P-secretase as the membrane-bound aspartyl protease (RACE), S-site APP cleaving enzyme, paves the way for developing possible chemical inhibitors of its activity for experimental and clinical evaluation, although that remains for the future. 

The urokinase-plasminogen activator system consists of three main components, urokinase-plasminogen activator (uPA, a 53kDa serine protease), the uPA membrane-bound receptor (uPAR), and the uPA inhibitors, PAI-1 and PAl-2 5 3XB2,185... 

BACE-1 is a membrane-anchored aspartic acid protease that is localized to the acidic compartments of endosomes and lysosomes in the CNS and has an optimal enzymatic activity at around pH 5. As a consequence, a BACE-1 inhibitor needs to be able to cross the blood-brain barrier and to have a significant non-protein bound fraction in order to reach the active site of the enzyme. This makes traditional aspartic protease inhibitors, which typically are large and peptidic, unsuitable as BACE-1 inhibitors. Moreover, the BACE-1 active site is extended, shallow and hydrophilic (Fig. 2) [99]. Therefore, the development of potent, selective, orally active, and brain penetrant low MW compounds has been a big challenge for the pharmaceutical industry [101, 102],... 

In some instances, it might be necessary to supplement the 0.32M sucrose solution, pH 7.4, with 1 mAf EDTA, 0.25 mM dithiothreitol (DTT), and/or a cocktail of protease inhibitors. Membrane-bound phospholipases and proteases can be activated during cell disruption. EDTA chelates metal ions (calcium and magnesium) that activate certain phospholipases. DTT is a reducing agent that prevents oxidation of functionally important sulfhydryl groups. Cell disruption may also cause release of proteases from lysosomes. Protease attack on membrane proteins can be prevented by addition of protease inhibitors, such as phenylmethylsulfonyl fluoride (PMSF an inhibitor of serine proteases) and/or E-64 (an inhibitor of cysteine proteases), to the sucrose solution... 

In this connection, it is noteworthy that some cellular proteins, including hoimones and other secretory polypeptides, are produced by limited cleavage of precursors. The proteases involved are not fully characterized, but seem to be located on membrane-bound structures (1J, 27, 28), have optimiim activities at neutral to slightly alkaline pH, and are sensitive to phosphofluoridate or other serine esterase inhibitors (12, 27, 29). 

A further means of inactivating the proteases of the clotting cascade is provided by a number of protease inhibitors which are present in circulating blood. The two most important are antithrombin, which binds tightly to thrombin and can also scavenge Factors IXa, X and XI when free in solution, and a-antitrypsin, which will inactivate Factor Xg even when it is bound to a phospholipid membrane. Activated clotting intermediates may also be removed by the liver and to a lesser extent by other tissues. 

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