Enzyme inhibition mechanism-based

Yang, J., Jamei, M., Yeo, K.R., Tucker, G. T. and Rostami-Hodjegan, A. (2005) Kinetic values for mechanism-based enzyme inhibition assessing the bias introduced by the conventional experimental protocol. European Journal of Pharmaceutical Sciences, 26 (3-4), 334-340. 

Sometimes CYPs can also produce reactive metabolite species that, instead of undergoing the normal detoxification pathway, can act as irreversible CYP inhibitors, thus causing toxicity. Such reactive metabolites that cause CYP inactivation are called MBI and are described in Chapter 9. Mechanism-based enzyme inhibition is associated with irreversible or quasi-irreversible loss of enzyme function, requiring synthesis of new enzymes before activity is restored. The consequences of MBI could be auto-inhibition of the clearance of the inactivator itself or prolonged inhibition of the clearance of other drugs that are cleared by the same isozyme. There may also be serious immunotoxicological consequences if a reactive intermediate is covalently bound to the enzyme. Therefore, screening of new compounds for MBI is now a standard practice within the pharmaceutical industry. 

Ghanbari, F., Rowland-Yeo, K.. Bloomer. J.C., Clarke, S.E., Lennard, M.S., Tucker, G.T. and Rostami-Hodjegan, A. (2006) A critical evaluation of the experimental design of studies of mechanism based enzyme inhibition, with implications for in vitrO in vivo extrapolation. Current Drug Metabolism, 1, 315-334. 

Terminal Fluoroolefins. Synthesis and Application to Mechanism-Based Enzyme Inhibition ... 

This article describes various approaches to inhibition of enzyme catalysis. Reversible inhibition includes competitive, uncompetitive, mixed inhibition, noncompetitive inhibition, transition state, and slow tight-binding inhibition. Irreversible inhibition approaches include affinity labeling and mechanism-based enzyme inhibition. The kinetics of the various inhibition approaches are summarized, and examples of each type of Inhibition are presented. 

Mechanism-based enzyme inhibition may be one of the more important types of DDIs, and examples of drugs that inhibit GYP enzymes in this fashion include paroxetine inhibition of GYP2D6, verapamil inhibition of GYP3A4, and the inhibition of GYP3A4 by protease inhibitors such as ritonavir. Some of the other drugs that cause mechanism-based inhibition of GYP enzymes include erythromycin, fluvoxamine, and ethinyl estradiol. 

Mechanism-based CYP inhibition or irreversible inhibition, involves permanent inactivation of CYP enzymes during catalysis, where reactive intermediate(s) are formed, leading to apoprotein or heme-ion center modification. Typical characteristics of mechanism-based enzyme inhibition include time-dependent loss of enzyme activity, a rate of inactivation generally following saturation kinetics, enzyme activity that cannot be recovered after... 

SCHEME 163 Proposed kinetic scheme for mechanism-based enzyme inhibition. E, I and P stand for enzyme, inhibitor and produet, respectively, and El is the initial binding of inhibitor to enzyme (the enzyme-inhibitor complex) and El is the aetive form of the complex in whieh the inhibitor is catalyzed to intermediate (reactive metabolite). Ejnact is the inactivated enzyme by the reactive metabolite formed. Inaetivation of the enzyme is an irreversible proeess over the time scale of the experiment. At the given eoneentrations of inhibitor and enzyme, the reactions are governed by the first-order rate eonstants ki, k i, k2,13 and I4, respeetively. 

The functionalized phenaceturates 16 (Fig. 11.10) are substrates of class A and C [3-lactamases, especially the class C enzymes, as observed with the parent unfunctionalized phenaceturates 15. They are also modest inhibitors of these enzymes and the serine DD-peptidase of Streptomyces R61. The inhibition of class C [3-lactamases is turnover dependent, as expected for a mechanism-based inhibitor. Inhibition is not very dependent on the nature of the leaving group, suggesting that the QM is generated in solution after the product phenol has been released from the active site. It therefore... 

Other useful targets for pharmaceutical agents are thymidylate synthase and dihydrofolate reductase, enzymes that provide the only cellular pathway for thymine synthesis (Fig. 22-49). One inhibitor that acts on thymidylate synthase, fluorouracil, is an important chemotherapeutic agent. Fluorouracil itself is not the enzyme inhibitor. In the cell, salvage pathways convert it to the deoxynucleoside monophosphate FdUMP, which binds to and inactivates the enzyme. Inhibition by FdUMP (Fig. 22-50) is a classic example of mechanism-based enzyme inactivation. Another prominent chemotherapeutic agent, methotrexate, is an inhibitor of dihydrofolate reductase. This folate analog acts as a competitive inhibitor the enzyme binds methotrexate with about 100 times higher affinity than dihydrofolate. Aminopterin is a related compound that acts similarly. 

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.

Figure 2.10. Mechanism of enzyme inhibition by saccharin based inhibitors. Only if R contains a leaving group, eg. (15-4), does the inhibitor progress to a dicovalently linked enzyme.

Another nucleoside-derived mechanism-based enzyme inhibitor is Fluoronepla-nocin A [79]. This compound is of interest as a broad-spectrum antiviral drug which acts by irreversible inhibition of S-adenosylhomocystein hydrolase (SAH). In a first enzymatic reaction step the 3 -hydroxy group of the inhibitor is oxidized to the corresponding ketone (Scheme 4.34). This leads to depletion of the biochemical oxidizer nicotinamide adenine dinudeotide (NAD ). In the next step a nucleophilic residue of the enzyme undergoes Michael addition to the /i-fluoro a,/>-unsatu-rated ketone moiety. This is followed by fluoride elimination and thus the inhibitor stays covalently trapped in the active site and disables the enzyme permanently. 

Mechanism-based irreversible inhibition occurs when a reactive metabolic intermediate is formed in situ that can (1) bind covalently with the prosthetic heme through N-alkyllary-lation (e.g., secobarbital), f"2 alkylate the apo-cytochrome (e.g., chloramphenicol or 2-ethy-nylnaphthalene), or (5) cause destruction of the prosthetic heme to products that irreversibly bind to the apocytochrome (e.g. CCI4) (158). These mechanism-based inactivators have primarily been designed and used for the selective inhibition of specific CYP enzymes and elucidating the mechanism of P450 reactions. Some drugs (e.g., aromatase inhibitors)... 

Mechanism-based irreversible inhibition of P450 enzymes represents a serious flaw in any drug candidate because of the potential for clinical drug-drug interactions, as was demonstrated by the withdrawal of mibefradil (Posicor ) from the market. It is... 

Mechanism-based enzyme inactivation kinetics refers to the irreversible inhibition of an enzyme via a catalytically formed reactive intermediate that binds covalently (typically) to the enzyme active site prior to release and causes permanent inactivation of the enzyme. This type of inhibition is also known as... 

Following concurrent administration of two drugs, especially when they are metabolized by the same enzyme in the liver or small intestine, the metabolism of one or both drugs can be inhibited, which may lead to elevated plasma concentrations of the dtug(s), and increased pharmacological effects. The types of enzyme inhibition include reversible inhibition, such as competitive or non-competitive inhibition, and irreversible inhibition, such as mechanism-based inhibition. The clinically important examples of drug interactions involving the inhibition of metabolic enzymes are listed in Table 1 [1,4]. 

Inhibitors of the catalytic activities of enzymes provide both pharmacologic agents and research tools for study of the mechanism of enzyme action. Inhibitors can be classified based upon their site of action on the enzyme, on whether or not they chemically modify the enzyme, or on the kinetic parameters they influence. KineticaUy, we distinguish two classes of inhibitors based upon whether raising the substrate concentration does or does not overcome the inhibition. 

The starting point for much of the work described in this article is the idea that quinone methides (QMs) are the electrophilic species that are generated from ortho-hydro-xybenzyl halides during the relatively selective modification of tryptophan residues in proteins. Therefore, a series of suicide substrates (a subtype of mechanism-based inhibitors) that produce quinone or quinonimine methides (QIMs) have been designed to inhibit enzymes. The concept of mechanism-based inhibitors was very appealing and has been widely applied. The present review will be focused on the inhibition of mammalian serine proteases and bacterial serine (3-lactamases by suicide inhibitors. These very different classes of enzymes have however an analogous step in their catalytic mechanism, the formation of an acyl-enzyme intermediate. Several studies have examined the possible use of quinone or quinonimine methides as the latent... 

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