Mechanism-based inhibition stabilizers

Figure 8 Irreversible inhibitors of proteases. Serine and cysteine proteases can be acylated by aza-peptides, which release an alcohol, but cannot be deacylated due to the relative unreactivity of the (thio) acyl-enzyme intermediate. Reactive carbons, such as the epoxide of E64, can alkylate the thiol of cysteine proteases. Phosphonate inhibitors form covalent bonds with the active site serine of serine proteases. Phosphonates are specific for serine proteases as a result of the rigid and well-defined oxyanion hole of the protease, which can stabilize the resulting negative charge. Mechanism-based inhibitors make two covalent bonds with their target protease. The cephalosporin above inhibits elastase [23]. After an initial acylation event that opens the p-lactam ring, there are a number of isomerization steps that eventually lead to a Michael addition to His57. Therefore, even if the serine is deacylated, the enzyme is completely inactive.

Figure 8.23. Mechanism-Based (Suicide) Inhibition. Monoamine oxidase, an enzyme important for neurotransmitter synthesis, requires the cofactor FAD (flavin adenine dinucleotide). AA -Dimethylpropargylamine inhibits monoamine oxidase by covalently modifying the flavin prosthetic group only after the inhibitor is first oxidized. The N-5 flavin adduct is stabilized by the addition of a proton.

The patent literature contains numerous examples of enzyme stabilization schemes based on the use of various chemical additives. Common themes are to either lower the amount of water in the formulation while retaining solubility of other components, or to add specific enzyme stabilizer/inhibitors. Some examples are highlighted in Table 4, and many others can be found in the review by Crutzen and Douglass. Few published articles have addressed the mechanisms of enzyme stabilization by additives. One study examines protease stabilization by carboxylic acid salts and another discusses the use of borate in conjunction with propylene glycol to inhibit protease activity. ... 

Since pyrethrins are highly photolytic, antioxidants are often added to preparations to stabilize formulations antioxidants adjoin include pyrocatechol, pyrogal-lol, hydroquinone, and l-benzene-azo-2-naphthol. Practically, all pyrethrins and many pyrethroids are commonly combined to additives (including synergists), some formulations include additional insecticides, insect repellents, or both, and many contain hydrocarbon solvents [3] to enhance their insecticidal activity. Pyrethrin and pyrethroid sprays may also be water based or be alcohol or petroleum based, which increases the overall toxicity. It is known that concomitant use of pyrethrins and pyrethroids with synergists such as piperonyl butoxide, A -octyl bicycloheptene dicarboximide, sulfoxide, sesamin, sesame oil, sesamolin, isosafrole, and organophosphorus compounds or carbamates may increase toxicity by mechanisms involving inhibition of microsomal oxidation [4]. 

Paclitaxel and docetaxel have been shown to act as spindle poisons, causing cell division cycle arrest, based on a unique mechanism of action.7-10 These drugs bind to the P-subunit of the tubulin heterodimer, the key constituent protein of cellular microtubules (spindles). The binding of these drugs accelerates the tubulin polymerization, but at the same time stabilizes the resultant microtubules, thereby inhibiting their depolymerization. The inhibition of microtubule depolymerization between the prophase and anaphase of mitosis results in the arrest of the cell division cycle, which eventually leads to the apoptosis of cancer cells. 

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