Stephen W. LaFon

Purine metabolism in Trypanosoma cruzi

Culture forms of Trypanosoma cruzi are incapable of synthesizing purines de novo from formate, glycine, or serine and require an exogenous purine for growth. Adenine, hypoxanthine, guanine, xanthine and their respective ribonucleosides are equal in their abilities to support growth. Radiolabeled purine bases, with the exception of guanine, are stable and are converted to their respective ribonucleotides directly by phosphoribosyltransferase activity. Guanine is both converted to its ribonucleotide and deaminated to xanthine. Purine nucleosides are not hydrolysed to any extent but are converted to their respective ribonucleotides. This conversion may involve a rete-limiting ribonucleoside cleaving activity or a purine nucleoside kinase or phosphotransferase activity. The apparent order of salvage efficiency for the bases and their respective ribonucleosides is adenine greater than hypoxanthine greater than guanine greater than xanthine.

Purine and pyrimidine salvage pathways in Leishmania donovani

Leishmania donovani, grown in culture, salvaged radiolabeled purine bases which were distributed into adenine and guanine ribonucleotides and into the RNA of these cells. De novo synthesis of purines in L. donovani does not occur [J. J. Marr, R. L. Berens and D. J. Nelson, Biochim. biophys. Acta 544, 360 (1978)]. [8-14C]Adenine was rapidly deaminated to hypoxanthine via the action of an adenine aminohydrolase (EC 3.5.4.2). [8-14C]Guanine was also rapidly deaminated by guanase (EC 3.5.4.3) to form zanthine in these cells. Therefore, the formation of nucleotides of hypoxanthine and xanthine are the first committed steps of purine salvage in L. donovani. While purines are efficiently conserved by this parasite, the salvage of pyrimidines is not so dramatic. [2-14C]Orotic acid was converted to OMP and then incorporated into the pyrimidine nucleotides and into RNA, indicating the existence of the later steps of de novo pyrimidine synthesis. [6-14C]Thymidine was salvaged by L. donovani, being incorporated into the thymine deoxyribonucleotides and into DNA. The major pathway of thymidine metabolism in this parasite, however, was cleavage of the deoxyriboside linkage to form thymine, probably via the action of a thymidine phosphorylase (EC 2.4.2.4).

Inosine analogs as chemotherapeutic agents for African trypanosomes: metabolism in trypanosomes and efficacy in tissue culture.

Certain purine analogs, the pyrazolopyrimidines, are effective chemotherapeutic agents against Leishmania spp. and Trypanosoma cruzi both in vitro and in some clinical models. Heretofore they have not been effective against the African trypanosomes; this suggested that these organisms were not comparable to the other pathogens with respect to their purine metabolism. We have studied the efficacy and metabolism of the pyrazolopyrimidine bases allopurinol and thiopurinol, their respective ribonucleosides, and the C-nucleosides formycin B and 9-deazainosine in Trypanosoma brucei subsp. gambiense and Trypanosoma brucei subsp. rhodesiense. The efficacy of these compounds was dependent on the purine content of the culture medium. The C-nucleosides were the most effective, with 90% effective doses for formycin B and 9-deazainosine of 0.01 and 2 micrograms/ml, respectively. Metabolism was the same in both the bloodstream and culture forms and identical to that reported for Leishmania spp. and T. cruzi. Both agents were phosphorylated to the ribonucleotide and then aminated to produce adenine nucleotide analogs. Growth inhibition studies were performed with three inosine analogs (allopurinol riboside, formycin B, and 9-deazainosine) on trypomastigotes grown in bone marrow tissue culture. Both C-nucleosides eradicated the infection at a concentration of 0.25 micrograms/ml. Unlike formycin B, 9-deazainosine is not known to be aminated by mammalian cells and appears to be relatively nontoxic in three different mammalian tissue culture systems. This nucleoside was very active against all pathogenic leishmaniae and trypanosomes investigated and is worthy of further study.

Atovaquone suspension in HIV-infected volunteers : Pharmacokinetics, pharmacodynamics, and TMP-SMX interaction study

Study Objective. To evaluate the pharmacokinetics and safety of atovaquone suspension in volunteers infected with the human immunodeficiency virus ((HIV).

6-Thiopurine Riboside Analogs: Their Toxicity and Metabolism in Leishmania Donovani and Mammalian Cells

The purine nucleoside analog, allopurinol riboside, is cytotoxic to several Leishmania sp1. This nucleoside was metabolized to its 5′-monophosphate by L. donovani which, in turn, was aminated to form adenine nucleotide analogs. The aminopyrazolopyrimidine 5′-triphosphate thus formed was incorporated into the RNA of the parasite. Several purine nucleoside analogs, structurally similar to allopurinol riboside. have differential cytotoxicity to Leishmania donovani and mouse L cells2. The cytotoxic action of these compounds correlates with their conversion to ATP and/or GTP analogs by these organisms3.