Purine and Pyrimidine Metabolism of Leishmania mexicana mexicana and Other Parasitic Protozoa

Hassan, Husain Fadil (1986) Purine and Pyrimidine Metabolism of Leishmania mexicana mexicana and Other Parasitic Protozoa. PhD thesis, University of Glasgow.

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Abstract

Purine and pyrimidine nucleotides are synthesized either de novo from small precursors or via salvage pathways from preformed purines and pyrimidines. Most parasitic protozoa are incapable of synthesizing purines de novo and so are dependent on salvage pathways. This contrasts markedly with the situation in mammalian host where both de novo synthesis and salvage pathways are significant. In contrast to the situation with purines, most parasitic protozoa are able to synthesize pyrimidines de novo, although salvage pathways are present. I have studied the presence of these pathways in a variety of parasitic protozoa. The results of this study indicate that Leishmania mexicana mexicana is similar to other pathogenic protozoa in that it lacks the ability to synthesize purines de novo and, therefore, depends upon salvage of preformed purines from its environment for survival. The enzymes involved in the purine salvage pathways of cultured pro mastigotes and isolated amastigotes of L. m. mexicana were examined. Differences between the enzyme content of the amastigote and promastigote forms of this species were found to be quantitative rather than qualitative. However, significant differences were observed between the enzymes present in L. m. mexicana and those present in donovani. Enzymes detected in L. m. mexicana included adenine deaminase, guanine deaminase, adenosine deaminase, two distinct nucleosidases (one active on nucleosides, the other on deoxynucleosides) and nucleotidases (more active on 3'- than 5f-nucleotides). Phosphorylase, phosphoribosyltransferase and nucleoside kinase activities were also detected. Nucleotide interconverting enzymes were present. Cell fractionation studies of promastigotes revealed the distribution of the enzymes in different cell fractions. This study established that the leishmanial mlcrobody-like organelle, the glycosorae, contains xanthine phosphoribosyltransferase, an enzyme not found in mammalian cells. 3'-AMP nucleotidase and 5'-AMP nucleotidase, as well as acid phosphatase activities, were shown to be present on the external surface of both amastigotes and promastigotes. With regard to pyrimidine metabolism, both amastigotes and promastigotes of L. m. mexicana contained cytidine deaminase, uridine nucleosidase, uridine posphorylase and thymidine phosphorylase, whereas only promastigotes contained deoxycytidine deaminase and thymidine nucleosidase. Interestingly, the promastigotes contained 20 times more orotate phosphoribosyltransferase activity than amastigotes. A survey of purine- and pyrimidine-metabolising enzymes in promastigotes of L. m. amazonensis, L. donovani and tarentolae, culture forms of Crithidia fasciculata, Herpetomonas muscarum rauscarum and H. m. ingenoplastis and procyclic trypomastigote of Trypanosoma brucei brucei have also been carried out in this study. Several common features between trypanosomatids were observed, including the presence of nucleosidase, catabolic phosphorylase, phosphoribosyltransferases, kinases and cytidine deaminase activities and the apparent absence of AMP deaminase, anabolic phosphorylase and cytosine deaminase. Significant differences between these species were also discovered, notably in adenine and adenosine metabolism. Extracts of Entamoeba histolytica have also been surveyed for purine salvage enzymes. The results obtained suggest that this pathogen largely relies on nucleoside phosphorylase and nucleoside kinase activities for salvage synthesis of purine nucleotides and so it differs from Leishmania species which appear to depend more upon phosphoribosyltransferases for synthesis of nucleotides. Entamoeba histolytica also differs from Leishmania in lacking enzymes that are involved in nucleotide interconversions. The results obtained with Acanthamoeba species indicate that they are capable of de novo synthesis of purine nucleotides. In this respect, they appear to be unique amongst parasitic protozoa studied to date. Acanthamoeba species can also salvage purines and the results suggest that hypoxanthine is of central importance in purine metabolism. Finally, the survey of enzymes potentially involved in the salvage of purines in extracts of Babesia divergens suggested that phosphoribosyltransferases and nucleoside kinases may be together responsible for the production of both adenine and guanine nucleotides. My studies on purine and pyrimidine metabolism of parasitic protozoa have revealed more details of the enzymes involved including a variety of differences between these species. In addition, it has been confirmed that there are important differences between these parasites and mammalian cells, differences that may be exploitable by antiprotozoal drugs.

Item Type: Thesis (PhD)
Qualification Level: Doctoral
Keywords: Parasitology
Date of Award: 1986
Depositing User: Enlighten Team
Unique ID: glathesis:1986-77476
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 14 Jan 2020 11:53
Last Modified: 14 Jan 2020 11:53
URI: http://theses.gla.ac.uk/id/eprint/77476

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