Enzymes ornithine decarboxylase

The 26S proteasome also degrades non-ubiquitylated proteins [71]. The short-lived enzyme ornithine decarboxylase (ODC) and the cell-cycle regulator p21Cip provide well documented examples of ubiquitin-independent proteolysis by the 26S en-... 

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

Let us now consider the synthesis of tetrazole 8.27, an inhibitor of the enzyme ornithine decarboxylase, which catalyses the conversion of ornithine 8.25 to diamine 8.26. 

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. 

Like purine metabolism, the polyamine biosynthetic pathway has served as another paradigm for rational therapeutic intervention in parasitic disease. Polyamine synthesis in T. brucei and Leishmania, like that in mammalian cells, is initiated by the enzyme ornithine decarboxylase (ODCase), although T. cruzi may synthesize polyamines by... 

Pyrrolidine alkaloids are derived via the intermediacy of putrescine (1). Putrescine is derived by decarboxylation of arginine or ornithine the enzyme ornithine decarboxylase (E.C. 4.11.1.17) has been detected in tobacco roots (Leete, 1980) (also see Chapter 30). Arginine decarboxylase (E.C. 4.1.1.19) first converts arginine to agmatine, and subsequently to car-bamoylputrescine, and then to putrescine see Fig. 28.7 of Chapter 28). In several studies, arginine, rather than ornithine, appears to be the major precursor (Leete, 1990). 

Ornithine decarboxylase is a pyridoxal dependent enzyme. In its catalytic cycle, it normally converts ornithine (7) to putrisine by decarboxylation. If it starts the process with eflornithine instead, the key imine anion (11) produced by decarboxylation can either alkylate the enzyme directly by displacement of either fluorine atom or it can eject a fluorine atom to produce viny-logue 12 which can alkylate the enzyme by conjugate addidon. In either case, 13 results in which the active site of the enzyme is alkylated and unable to continue processing substrate. The net result is a downturn in the synthesis of cellular polyamine production and a decrease in growth rate. Eflornithine is described as being useful in the treatment of benign prostatic hyperplasia, as an antiprotozoal or an antineoplastic substance [3,4]. 

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]. 

Aminooxy compounds can be viewed as O-ethers of HA and are discussed here in terms of their potential anticancer activity. Thus, l-aminooxy-3-aminopropane (APA) is a potent reversible inhibitor of mammalian ornithine decarboxylase (ODC) and of S-adenosylmethionine decarboxylase (SAMDC) , and 6 -(5 -deoxy-5 -adenosyl)methylsul-fonium 0-ethylhydroxylamine (AMA) was reported to be an efficient reversible inhibitor of the latter enzyme . 

Figure 33. Synthesis of alkaloids from ornithine. Aikaioids are derived via putrescine or glutamic semialdehyde. At least two enzymes, ODL (Ornithine decarboxylase) or PDL (Pyrroline...

Eflornithine (difluoromethyl ornithine, Ornidyl) is a unique antiprotozoal agent in that its mode of action involves inhibition of a specific enzyme, ornifhine decarboxylase. In eukaryofes, decarboxylafion of ornifhine is required for biosynfhesis of polyamines, which are im-porfanf in cell division and differenfiafion. 

Ornithine decarboxylase activity. Fixed oil (4.5%), Clupeidae brevortia tyrannus (4%), and Zea mays (1.5%) fixed oil (7.5%), Clupeidae brevortia tyrannus (1%), and Zea mays (1.5%) fixed oil (8.5%) and Zea mays (1.5%), administered orally to mice for 1 year, were active vs benzoyl peroxide-induced ornithine decarboxylase activity Oil, administered to 30 ultraviolet (UV)-irradiated Senear and SKH-1 mice at doses of 1/14% (A diet), 7.9/7.1% (B diet), and 15/0% (C diet) corn oil/coco-nut oil for 6 weeks, produced no increase in enzyme activity. The level of ornithine decarboxylase activity in the UV-irradiated mice fed diet A was significantly higher than in mice fed the B or C diet. In the SKH-1 mice, ornithine decarboxylase activity was increased by 3 weeks and was significantly higher in mice fed diet C than in mice fed diet A. There was no significant effect of dietary fat on UV-induced skin tumor incidence ". 

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. 

Enzymes found in both parasites and humans, but essential to parasite life while being nonessential or less essential to human life (e.g., purine nucleoside kinase, ornithine decarboxylase)... 

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 such as spermine and spermidine, involved in DNA packaging, are derived from methionine and ornithine by the pathway shown in Figure 22-30. The first step is decarboxylation of ornithine, a precursor of arginine (Fig. 22-10). Ornithine decarboxylase, a PLP-requiring enzyme, is the target of several powerful inhibitors used as pharmaceutical agents (Box 22-2). ... 

In mammalian cells, ornithine decarboxylase undergoes rapid turnover—that is, a constant round of enzyme degradation and synthesis. In some trypanosomes, however, the enzyme—for reasons not well understood—is stable, not readily replaced by newly synthesized enzyme. An inhibitor of ornithine decarboxylase that binds permanently to the enzyme would thus have little effect on human cells, which could rapidly replace inactivated enzyme, but would adversely affect the parasite. 

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]. 

Ornithine Decarboxylase Assays. The double-chamber assay system of Moskal and Basu (59) was used to measure enzyme activity in the form of L C] carbon dioxIHe evolution. The assay conditions of O Brien and Diamond (60) were used and consisted of the following components (in micromoles, unless otherwise stated) in a total volume of 100 jul sodium phosphate buffer, pH 7.2, 5.0 EDTA, 1.0 dithiothreitol, 5.0 pyridoxal-5 -monophosphate, 0.2 L-ornithine (specific activity 0.5 x 10° cpm/-jumole), 0.1 and protein, 0.1-0.5 mg. Incubations were carried out at 37°C for 60 min, and the reactions were terminated by the addition of 200 jul of 2M sodium citrate followed by a post-incubation period of 3 hours at 37°C to insure maximal release of radiolabeled carbon dioxide. 

---