Suicide inhibitors/metabolic

In the earlier scheme, I represents a product formed by metabolism of the inhibitor by the enzyme. This product may be released into bulk solvent, or may interact (often covalently) with a suitably reactive component of the enzyme within the active site. This irreversibly inactivated enzyme complex is shown as El". There are two kinetic constants that can be obtained from relatively straightforward experiments with a suicide inhibitor. The Ki value is an equilibrium constant for the initial reversible step, and all the rate constants from the above scheme contribute to its value. The rate of irreversible inactivation of enzyme at a saturating concentration of the suicide inhibitor is given by fcinact. to which only k2> h, and k contribute (Silverman, 1995). At infinitely high concentrations of the inhibitor, the half-Ufe for inactivation is equal to ln2/ l inact ... 

Aromatase inhibitors may be classified into two types. Type 1 aromatase inhibitors bind to the aromatase enzyme irreversibly, so they are called inactivators. In some cases they are dubbed mechanism-based or suicide inhibitors when they are metabolized by the enzyme into reactive intermediates that bind covalently to the active site. Type 1 aromatase inhibitors are usually steroidal in structure as represented by exemestane (1), formestane (13), and atamestane (14). Formestane (13) was launched by Ciba-Geigy in 1992. As formestane (13) is readily and extensively metabohzed when administered orally, it is used as a depot formulation for injection. 

Reactive metabolites of xenobiotics may differ in reactivity, and therefore have varying impact on enzymatic activities in terms of proximity to their origin. For example, some intermediates are highly reactive and directly inhibit the enzyme that leads to their formation. These substances are commonly referred to as suicide inhibitors, for obvious reasons. Some suicide inhibitors, such as piperonyl butoxide (PBO), a pesticide synergist) are common inhibitors of certain CYP isozymes. PBO amplifies the toxicity of certain insecticides by inhibiting the insect s CYP enzymes that are involved in its degradation. It is metabolized to a highly reactive carbene, which forms an inhibitor complex with the heme iron of CYP, as shown in Scheme 3.6. 

Metabolites that are less reactive than suicide inhibitors may impact more distant enzymes, within the same cell, adjacent cells, or even in other tissues and organs, far removed from the original site of primary metabolism. For example, organopho-sphates (OPs), an ingredient in many pesticides, are metabolized by hepatic CYPs to intermediates, which, when transported to the nervous system, inhibit esterases that are critical for neural function. Acetylcholinesterase (AChE) catalyzes the hydrolysis of the ester bond in the neurotransmitter, acetylcholine, allowing choline to be recycled by the presynaptic neurons. If AChE is not effectively hydrolyzed by AChE in this manner, it builds up in the synapse and causes hyperexcitation of the postsynaptic receptors. The metabolites of certain insecticides, such as the phos-phorothionates (e.g., parathion and malathion) inhibit AChE-mediated hydrolysis. Phosphorothionates contain a sulfur atom that is double-bonded to the central phosphorus. However, in a CYP-catalyzed desulfuration reaction, the S atom is... 

Unlike the (3-lactones and (3-lactams, the mode of action of the unusual C-glucosyl nucleoside-based natural product showdomycin is unknown. Nevertheless, this compound has been shown to possess potent antibiotic properties, with results obtained in vitro suggesting a role as a suicide inhibitor of uridine metabolism [11]. Isolated from the bacteria Streptomyces showdoensis, showdomycin contains an electrophilic moiety, malaimide, in place of the base (cf. the structures of uridine or pseudouridine). 

Pluorouracil is a pyrimidine analog which is converted metabolically to its toxic form, fluorodeoxyuridylate (F-dUMP). As cells metabolically activate the drug it acts as a "suicide" inhibitor of Thymidyiate Synthetase. 

Metabolism of a substrate by an enzyme to form a compound that irreversibly inhibits that enzyme. Penicillinase inhibitors, such as clavulanlc acid and 5ul-bactam, are suicide inhibitors. 

A different approach led to the development of vigabatrin (Sabril). Its ability selectively and irreversibly to inhibit GABA-transaminase from metabolically inactive GABA (as a suicide inhibitor) makes it an effective agent (Fig. 12-5), particularly in complex partial seizures. 

An example of this type of ne>v agent may be found in the fluorinated fatty acids, >vhich have been shovm to be taken up by the myocardium (60,61). When these compounds are labeled with attached to an even-numbered carbon atom they are highly toxic because of their ability to be converted to fluorocitrate, which is a suicide inhibitor of aconitase (73). The label thus becomes locked into an intermediate step of the citric acid cycle. When is attached to an odd-numbered carbon, toxicity is dramatically reduced, and )8-oxidation and subsequent metabolism proceeds as with the physiologic FFA substrates. A more detailed understanding of the behavior of such tracer molecules in altered and normal states of myocardial metabolism may provide considerable insight into applying them to the in vivo measurement of metabolic rates. 

In 1992, Dewey and colleagues from BNL studied the effects of a suicide inhibitor of the enzyme that breaks down GABA, an inhibitory neurotransmitter. Inhibition of the enzyme that metabolized GABA resulted in decreased production of GABA. This resulted in decreased inhibition of the dopaminergic neurons, so synaptic dopamine secretion increased. 

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. 

When reactive metabolites are formed by metabolic activation, some of them can escape from the active site and bind to external protein residues or be trapped by reduced glutathione (GSH) or other nucleophiles. The remaining molecules that are not released from the active site will cause the suicide inhibition [7]. The ratio of the number of reactive molecules remaining in the active site and those escaping is a measure of the reactivity of the intermediates formed. The addition of scavengers or GSH to the incubation mixture does not affect and cannot prevent the CYP mechanism-based inhibition. However, GSH can reduce the extent of the nonspecific covalent binding to proteins by those reactive molecules that escape from the active site. In contrast, addition of substrates or inhibitors that compete for the same catalytic center usually results in reduction of the extent of inhibition. 

The abnormal T- and B-cell functions in patients with SCID are the result of ADA dehciency. The ADA gene has been mapped to chromosome 20q.l3, and a number of point and deletion mutations have been identihed in SCID patients [5-7]. ADA catalyses the irreversible deamination of adenosine and 2 -deoxyadenosine to inosine and 2 -deoxyi-nosine as a part of purine nucleoside metabolism. Adenosine and deoxyadeno-sine are suicide inachvators of S-adenosyl-homocysteine (SAH) hydrolase, and lead indirectly to intracellular accumulation of SAH, which is a potent inhibitor of methy-lation reactions. Cellular methylation function is essential for detoxihcation of adenosine and deoxyadenosine. As a result ADA dehciency leads to accumulation to... 

Mechanism-based inhibitors or suicide substrates seem to be particularly prevalent with CYP3A4. Such compounds are substrates for the enzyme, but metabolism is believed to form products that deactivate the enzyme. Several macrolide antibiotics, generally involving a tertiary amine function, are able to inhibit CYP3A4 in this manner (147,148). Erythromycin is one of the most widely used examples of this type of interaction, although there are other commonly prescribed agents that inactivate CYP3A4 (149-151), and a consideration of this phenomenon partially explains a number of interactions that are not readily explained by the conventional in vitro data (152). 

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