Purine analogues metabolism

In addition to the enzymes that catalyse the formation of nucleotides and polynucleotides, a large number of catabolic systems exist which operate at all levels of the internucleotide pathways. The ribonucleases and deoxyribonucleases that degrade polynucleotides are probably not significantly involved in purine analogue metabolism, but the enzymes which dephosphorylate nucleoside 5 -monophosphates are known to attack analogue nucleotides and may be of some importance to their in vivo activity. Phosphatases of low specificity are abundant in many tissues [38], particularly the intestine [29]. Purified mammalian 5-nucleotidases hydrolyse only the nucleoside 5 monophosphates [28] and... 

XANTHINE OXIDASE AND ALDEHYDE OXIDASE IN PURINE AND PURINE ANALOGUE METABOLISM... 

In the sections that follow, the metabolism of the various purine analogues is presented first, followed by a discussion of the current status of our knowledge about the mechanism of action of these compounds. Next the distribution and effects of these drugs on the host and invading organisms is considered with some concluding remarks on the ever present problem of resistance. 

A comparison of EDso values for feedback inhibition and for growth inhibition in H.Ep.-2 cells in culture is shown in Table 2.3. It is readily apparent that for most of the purine analogues listed, the correlation between feedback inhibition and cytotoxicity is good. The few discrepancies may be due to the fact that these particular compounds are not feedback inhibitors, but metabolism (or lack of it) of the analogue in question may be important. In comparing these ED50 values, the difference in experimental conditions for cytotoxicity determination (long... 

The subject of the incorporation of anticancer agents into macromolecules [13] and other compounds [336] has been reviewed. A number of purine analogues are incorporated into nucleic acid, but the incorporation of these compounds requires that they be anabolized to nucleoside mono-, di-, and triphosphates, and it is difficult to separate the metabolic effects of the nucleoside phosphates from the metabolic effects of the fraudulent polynucleotides. 

Azaguanine was the first purine analogue shown to be incorporated into polynucleotides [337] and, since its primary metabolic effect is on protein synthesis, the incorporation into RNA is considered the basis for its biologic activity [338]. In microbial systems 8-aza-adenine, 8-azahypoxanthine, 8-azaxanthine, and 5(4)-amino-l/f-l, 2, 3-triazole-4(5)-carboxamide are all incorporated into RNA as 8-azaguanylic acid [336]. 

Thus there are a number of metabolic events that are interfered with by the various purine analogues. The biological consequences of these interferences are discussed in the sections that follow. 

Much information on the mechanism of action and cross-resistance of purine analogues has been obtained in bacteria, some of which are quite sensitive to certain of these compounds in vitro. There is a great deal of variation in response of the various bacteria to a particular agent and of a particular bacterium to the various cytotoxic purine analogues. Some, if not most, of these differences are probably due to differences in the anabolism of the various compounds. Despite the fact that certain purine analogues have quite a spectrum of antibacterial activity in vitro, none has been useful in the treatment of bacterial infections in vivo because their toxicity is not selective—the metabolic events whose blockade is responsible for their antibacterial activity are also blocked in mammalian cells and thus inhibition of bacterial growth can only be attained at the cost of prohibitive host toxicity. In contrast, the sulpha drugs and antibiotics such as penicillin act on metabolic events peculiar to bacteria. 

It is of historical interest that Tetrahymena gelii, whose metabolism has been described in detail [387], is inhibited by 8-azaguanine [388] and other purine analogues [389, 390]. Of more importance to chemotherapy is the fact that pathogenic protozoa such as the trypanosomes respond in vitro to a number of... 

The mechanism of inhibition of these protozoal infections by the most active drugs, puromycin and the aminonucleoside, is not known. Puromycin and nucleocidin both interfere with protein synthesis, but the aminonucleoside does not. It is known to be demethylated to 3 -amino-3-deoxyadenosine, which is phosphorylated and interferes with nucleic acid metabolism (see above). Whether puromycin must be converted to the aminonucleoside before it can inhibit protozoa has not been established. Some purine analogues known to interfere with nucleic acid metabolism, however, are less effective as antiprotozoal agents, even in vitro, perhaps because their effects are primarily on the de novo pathway which many, if not all, protozoa do not use [406]. 

The reason for the selective toxicity of 6-mercaptopuiine remains to be established, but two factors may be of primary importance. 6-Mercaptopurine is anabolized primarily, if not exclusively, to the monophosphate level, and it is readily catabolized by xanthine oxidase, an enzyme that is low in most cancer cells compared to normal cells. Another factor that must be considered is the metabolic state of the target cells. Actively proliferating leukaemia cells are more sensitive to 6-mercaptopurine, as they are to all antimetabolites, than cells in the so-called Gq or stationary phase. Although this does not explain the difference between 6-mercaptopurine and other purine analogues, it may explain the ineffectiveness of 6-mercaptopurine against solid tumours, most of the cells of which are in the non-dividing state. 

Konigk, E. (1978) Purine nucleotide metabolism in promastigotes of Leishmania tropica inhibitory effect on allopurinol and analogues of purine nucleosides. Tropenmed. Parasitol. 29 435-438. 

In addition to metabolizing some aldehydes, aldehyde oxidase also oxidizes a variety of azaheterocycles but not thia- or oxaheterocycles. Of the various purine nucleosides metabolized by aldehyde oxidase, the 2-hydroxy- and 2-amino derivatives are more efficiently metabolized, and for the N -substituents, the typical order of preference is the acyclic nucleosides is as follows 9-[(hydroxy-alkyloxy)methyl]-purines) > 2 -deoxyribofuranosyl > ribofuranosyl > arabinofuranosyl > H. The kinetic rate constants for purine analogues revealed that the pyrimidine portion of the purine ring system is more important for substrate affinity than the imidazole portion. Aldehyde oxidase is inhibited by potassium cyanide and menadione (synthetic vitamin K). 

Pyrazolopyrimidines are purine analogues in which the nitrogen and carbon of the imidazole ring are inverted. The best known member of this group, allopurinol (4-hydroxypyrazolo(3,4-d)pyrimidine HPP), is a structural analogue of hypoxanthine. Its major metabolic conversions in mammalian cells are to oxipurinol, via xanthine oxidase, and to allopurinol ribonucleoside (HPPR)l. 

R. W. Brockman, "Metabolism and Mechanisms of Action of Purine Analogues, "A Collection of Papers Presented at the Twenty-Second Annual Symposium on Fundamental Cancer Research, 1968, The Williams and Wilkins Company, Baltimore, 1968. 

Antimetabolites interfere with normal metabolic pathways. They can be grouped into folate antagonists and analogues of purine or pyrimidine bases. Their action is limited to the S-phase of the cell cycle and therefore they target a smaller fraction of cells as compared with alkylating agents. 

The enzymatic conversion of many analogues of the naturally occurring purines directly to their biologically active form, the ribonucleotides, in vivo [5, 8, 10, 13, 39] underlines the importance of these enzymes to the drug action of this class of compounds. 2-Aminoadenine (2, 6-diaminopurine, I) [107], 2-fluoroadenine (II) [108], 4-aminopyrazolo [3, 4-d] pyrimidine (VIll) [109]. and 2- and 8-aza-adenine (IX and X) [ 110, 111] have all been shown to be substrates for the adenine phosphoribosyltransferase [J12, 113]. Extensive studies on the metabolism of 2-aminoadenine (I) in E. coli [114, 115], L cells [116], and mice [117] have also shown its conversion by this enzyme to the ribonucleotide. 

The pyrazolo[3, 4-d] pyrimidines are substrates for and inhibitors of xanthine oxidase [266, 267]. 4-Hydroxypyrazolo[3,4-d] pyrimidine was first investigated for its ability to protect 6-mercaptopurine and other analogues from oxidation by xanthine oxidase [384], but it also inhibits the oxidation of the natural purines, hypoxanthine, and xanthine. Its profound effect on uric acid metabolism made it an obvious choice for the treatment of gout and its utility in the control of this disease has been demonstrated [385, 386]. 

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