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).