Suicide substrate inhibitors

Thus, it would be reasonable to expect that the acetylenic group would be metabolized to a carboxylic acid. While the acid metabolite has not been detected, ethinyl estradiol is a suicide substrate inhibitor of CYP2B6 consistent with a reactive ketene being formed as an intermediate (142). In addition, there is a rearrangement product (4) shown in Figure 4.75. 

Noncompetitive, irreversible inhibition requiring in situ metabolism to a reactive intermediate (mechanism-based inactivators, also known as suicide substrate inhibitors). 

Covey, D.F. and W.F. Hood (1982). A new hypothesis based on suicide substrate inhibitor studies for the mechanism of action of aromatase. Cancer Res. 42, 3327-3333. 

Further biotransformations of A VPA involve both the liver microsomal cytochrome P-450 enzymes and the fatty acid 3-oxidation pathway (Fig. 32.28). The mixed-function oxidase system metabolizes the unsaturated metabolite to a y-butyrolactone derivative through a chemically reactive entity that is a suicide-substrate inhibitor of cytochrome P-450. The alkylation of the prosthetic haem by means of the radical occurs prior to the formation of the epoxide. Thus the epoxide is not involved in the cytochrome P-450 inhibition. 

The mechanism-based irreversible inhibitors also have been termed as suicide substrates, inhibitors, Trojan horse inhibitors, or latent alkylating... 

Zou AR Ma YH, Sui ZH, Ortiz de Montellano PR, Clark JE, Masters BS, Roman RJ (1994) Effects of 17-octadecynoic acid, a suicide-substrate inhibitor of cytochrome P450 fetty acid i--hydroxylase, on renal function in rats. J Pharmacol Exp Therap 268 474 81... 

In this chapter, mechanism-based inhibition is discussed in its broadest sense, where an inhibitor is converted by the enzyme catalytic mechanism to form an enzyme-inhibitor complex. Other terms used in the literature for mechanism-based inhibitors include suicide inhibitors, suicide substrate inhibitors, alternate substrates, substrate inhibitors, and enzyme inactivators, as well as irreversible, catalytic, or cat inhibitors. The terms alternate substrate inhibition and suicide inhibition are used here to describe the two major subclasses of mechanism-based inhibition. 

AT,r,=41 0.7 IIM. These rates were also pH dependent, with Ka = 6.58, in reasonable agreement with the catalytic Ka value for a serine protease. The actual inactivation rate was determined from rescue experiments. At various times t following addition of suicide substrate inhibitor to enzyme, 10 vaM of the nucleophile )6-mercaptoethanol was added. This nucleophile reacted rapidly with excess ynenol lactone, allowing any enzyme not inactivated to deacylate to regenerate active enzyme, as shown in Fig. 13.2. The inactivation rates were also saturable, giving 4 or /Tinact = 0.0037 0.0001 s and inact = 0.63 0.08 (xM. Gel filtration of the enzyme-inhibitor mixture before full inactivation could occur, followed by dilution into assay conditions, allowed determination of the deacylation rate, 3 = 0.0056 s The pH dependence of this rate was also determined and found to have a Ka value of 7.36. This value was in excellent agreement with the catalytic p a value, providing further evidence for the role of enzyme catalysis in the mechanism of inactivation. 

Herbicidal Inhibition of Enzymes. The Hst of known en2yme inhibitors contains five principal categories group-specific reagents substrate or ground-state analogues, ie, rapidly reversible inhibitors affinity and photo-affinity labels suicide substrate, or inhibitors and transition-state, or reaction-intermediate, analogues, ie, slowly reversible inhibitors (106). 

Suicide substrate and reaction intermediate inhibitors promise the highest degree of specificity and have drawn increased attention (106). 

Active site directed P-lactam-derived inhibitors have a competitive component of inhibition, but once in the active site they form an acyl en2yme species which follows one or more of the pathways outlined in Figure 1. Compounds that foUow Route C and form a transiendy inhibited en2yme species and are subsequendy hydroly2ed to products have been termed inhibitory substrates or competitive substrates. Inhibitors that give irreversibly inactivated P-lactamase (Route A) are called suicide inactivators or irreversible inhibitors. The term progressive inhibitor has also been used. An excellent review has appeared on inhibitor interactions with P-lactamases (28). 

The efficiency of inactivation by covalent bond formation vs release of the reactive species into solution has been described by its partition ratio. The most efficient inactivators have catalytic partition ratios of 0, in which case each inhibitor molecule leads to inactivation of the enzyme. To this date, many of these inhibitors have been designed, and alternative names like suicide substrate, Trojan Horse inactivator, enzyme induced inactivator, inhibitor, and latent inactivator have been used for this class of inhibitors. A number of comprehensive reviews are available (26—32). 

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... 

Finally, coumarin derivatives may act as general inhibitors of serine proteases or as specific inhibitors of human leukocyte elastase, depending on the nature of the substituents, through two distinct mechanisms, suicide substrates (a-chymotrypsin)... 

SCHEME 11.3 Postulated mechanisms for the inhibition of serine proteases by coumarin derivatives. NuH nucleophile. Pathway a suicide-type inactivation (suicide substrate). Pathway b transient inactivation by formation of a stable acyl-enzyme (alternate substrate-inhibitor). 

Suicide substrates and quiescent affinity labels, unlike the other types of inhibitors discussed in this chapter, form covalent bonds with active site nucleophiles and thereby irreversibly inactivate their target enzymes. A suicide substrate,191 also described by Silverman in a comprehensive review1101 as a mechanism-based inactivator, is a molecule that resembles its target enzyme s true substrate but contains a latent (relatively unreactive) electrophile. When the target enzyme attempts to turn over the... 

Somehow, Seebach ends his superb review, in which more than 500 references are quoted, with a rather optimistic message "that organic synthesis continues to react forcefully and with vitality to new challenges, still ready to pursue old dreams", and he refers to some exciting new targets such as supramolecular structures inhibitors, suicidal substrates and flustrates monoclonal antibodies and... 

These mediators probably play significant roles in CNS functions consistent with the stimulant effects of MAO inhibitors on mood and psychomotor drive and their use as antidepressants in the treatment of depression (A). Tranylcypromine is used to treat particular forms of depressive illness as a covalently bound suicide substrate, it causes long-lasting inhibition of both MAO isozymes, (MAOa, MAOb). Moclobemide reversibly inhibits MAOa and is also used as an antidepressant. The MAOb inhibitor selegiline (deprenyl) retards the cat-obolism of dopamine, an effect used in the treatment of parkinsonism (p. 188). 

Irreversible inhibition with based-mechanism inhibitors (suicide-substrates)... 

Irreversible Inhibition with Mechanism-Based inhibitors (Suicide Substrates)... 

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