Ornithine decarboxylase enzyme inhibition

Biosynthesis of polyamines is essential for growth and multiplication of T. brucei, hence discovery of drug candidates that inhibit enzymes in the polyamine biosynthesis pathway represent an attractive approach to development of trypanocides. The consequences of gene knockout of ornithine decarboxylase (ODC), the target of eflornithine (3), have been further characterized and suggest that new inhibitors of this enzyme may be particularly effective [18]. 

Ornithine decarboxylase catalyzes the conversion of ornithine into putrescine. Like other polyamines, the latter is involved in the regulation of cell development. Inhibition of this enzyme has been an important goal in medicinal chemistry. In this context, difluoroornithine has been shown to be an excellent inhibitor... 

S.2.3.3 Treatment of Trypanosomiasis The difluoromethylornithine (DFMO), eflomithine is a mechanism-based inhibitor of ornithine decarboxylase— a pyridoxal-dependent key enzyme of the polyamine s biosynthesis from ornithine. Fluorine atoms are essential for the inhibition process (cf. Chapter 7). Eflornithine was first clinically developed for cancer, but its development has been abandoned for this indication. The activity of eflornithine on trypanosomes was then discovered. Now, despite its very low bioavailability, eflornithine is the best therapy for sleeeping sickness (trypanosomiasis)—in particular, at the cerebral stage—due to Trypanosoma brucei gambiense parasite. Eflornithine is registered with orphan drug status and is distributed by the WHO. 

Phillips MA, Coffino P, Wang CC. Cloning and sequencing of the ornithine decarboxylase gene from Trypanosoma brucei. Implications for enzyme turnover and selective difluoromethylornithine inhibition. JBiol Chem 1987 262 8721-8727. 

Polyamines are of another significance for the hydrogenosomes inhibition of putrescine-synthesizing enzyme ornithine decarboxylase by 1,4-diamino-... 

Genetic factors influence the rate of not only synthesis of proteins but also their breakdown, i.e., the rate of turnover. As we have seen in Chapter 10, some enzymes are synthesized as inactive proenzymes which are later modified to active forms, and active enzymes are destroyed, both by accident and via deliberate hydrolytic pathways. Protein antienzymes may not only inhibit enzymes but may promote their breakdown.35 An example is the antienzyme that controls ornithine decarboxylase, a key enzyme in the synthesis of the polyamines that are essential to growth.36,37 As with all cell constituents, the synthesis of enzymes and other proteins is balanced by degradation. 

Ornithine decarboxylase is specifically inhibited by the enzyme-activated inhibitor a-difluoromethyl-ornithine, which can cure human infection with Trypanosoma brucei (African sleeping sickness) by interfering with polyamine synthesis.243-2443 In combination with inhibitors of spermidine synthase or S-adenosylmethionine decarboxylase,245 it can reduce polyamine levels and growth rates of cells. Another powerful inhibitor that acts on both ornithine and adenosylmethionine decarboxylases is the hydroxy-lamine derivative l-aminooxy-3-aminopropane 246... 

The mechanisms by which antitumor-promoters suppress the tumor promotion are not known, but may be due to the following effects (i) inhibition of polyamine metabolism (ii) inhibition of arachidonic acid metabolism (iii) protease inhibition (iv) induction of differentiation (v) inhibition of oncogene expression (vi) inhibition of PKC and (vii) inhibition of oxidative DNA damage [3,6,91]. The polyamine content of cells is correlated to their proliferative, and often, their neoplastic capabilities. A key enzyme in the polyamine biosynthetic pathway, ornithine decarboxylase (ODC), catalyzes the convertion of ornithine to putrescine. Phorbol ester promoters such as TPA cause increased ODC activity and accumulation of polyamines in affected tissues. Diacylglycerol activated PKC, and the potent tumor promoter, TPA, binds to, and activates PKC, in competition with diacylglycerol. PKC stimulation results in phosphorylation of regulatory proteins that affect cell proliferation. Some chemopreventive agents have inhibitory activity towards PKC. Refer to recent review articles for further discussion [3,6,91]. 

As already mentioned (see section 3.1) lipid peroxides can also break down non-enzymically to yield a variety of carbonyls, such as the hydroxyalkenals [54]. These aldehydes, and in particular 4-hydroxynonenal (HNE), can react with thiol and amino groups of nearby proteins, affecting several enzymic activities [55], These effects however appear to occur at HNE concentrations greater than 10 [lM. At low non-toxic concentrations other effects have been observed which have considerable relevance to cell proliferation. These include the stimulation of adenyl-cyclase and phospholipase C activity in liver membranes [56,57] and an inhibition of ornithine-decarboxylase activity [58] and the expression of globin genes and the protooncogene c-myc in K562 murine leukaemia cells [59]. 

Tetracycline targets protein synthesis in Plasmodium via a similar mechanism to that seen in bacteria inhibition of chain elongation and peptide bond formation. Eflornithine interferes with the metabolism of the amino acid ornithine in T. brucei gambiense by acting as a suicide substrate for the enzyme ornithine decarboxylase. 

Efiornithine, Eflomithine acts ty inhibiting the enzyme ornithine decarboxylase (ODC) in the hair follicles of the human skin (93). The enzyme is necessary for the synthesis of polyamines. Animal data indicates that inhibiting ODC inhibits cell division and synthetic function and therefore inhibits hair growth. It is postulated that eflomithine causes irreversible inhibition of the enzyme. 

Scheme 4.36 The antiprotozoal drug Eflornithine acts by inhibiting ornithine decarboxylase (OD) box), forming covalent bonds to the enzyme and to the cofactor pyridoxal phosphate (PLP ...

Induction of ornithine decarboxylase is another feature of epidermal exposure to the phorbol diester. Induction of this enzyme is also inhibited by... 

NO has a cytostatic effect by inhibiting ATP synthesis [99] via Kreb s cycle (aconitase inhibition, [100]), glycolysis (GADPH inhibition) and mitochondrial respiration (NAD ubiquinone oxydoreductase and succinate ubiquinone oxydoreductase inhibitions, [101]). Another pathway is the ornithine decarboxylase inhibition. This enzyme is implicated in polyamine production necessary to cell proliferation and its activity is inhibited by NO in human colon cancer cells HT-29 and Caco-2 [102]. Furthermore NO directly inactivates ribonucleotide reductase [103] of TA3 cancer cells (murine breast cancer cells) [104]. This enzyme controlling DNA synthesis catalyses desoxyribonucleotides synthesis, and its inhibition blocks cells in S phase. This inhibition is rapid and reversible in K562 and TA3 cells [105]. 

Eflomithine inhibits ornithine decarboxylase it irreversibly inhibits both mammalian and try-panosomal enzymes, thereby preventing the synthesis of polyamines needed for cell division. The parasite and human enzymes are equally susceptible to eflomithine, but the mammalian enzyme is turned over rapidly, whereas the parasite enzyme is stable. T. brucei rhodesiense cells are less sensitive to eflomithine inhibition than T. brucei gambiense cells, and effective levels generally cannot be achieved clinically. 

DEMO is a suicide irrhibitor of ornithine decarboxylase. Although it also inhibits mammalian ornithine decarboxylase, DMFO is less toxic to the host because of more rapid turnover and replacement of the irreversibly inhibited enzyme itr the host than in parasites. The answer rs (A). The anticoccidial 4-amitroquinolines inhibit mitochorrdrial respiration itr eimeria species, probably through itrteractiorr with a comporrerrt between NADH oxidase atrd C54ochrome b in the electron transport chaitr. The answer is (A). 

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