Pteridine salvage throughout the Leishmania infectious cycle: implicationsfor antifolate chemotherapy

Protozoan parasites of the trypanosomatid genus Leishmania are pteridine au xotrophs. and have evolved an elaborate and versatile pteridine salvage net work capable of accumulating and reducing pteridines. This includes biopter in and folate transporters (BT1 and FT1), pteridine reductase (PTR1), and d ihydrofolate reductase-thymidylate synthase (DHFR-TS). Notably, PTR1 is a n ovel alternative pteridine reductase whose activity is resistant to inhibit ion by standard antifolates. In cultured promastigote parasites, PTR1 can f unction as a metabolic by-pass under conditions of DHFR inhibition and thus reduce the efficacy of chemotherapy. To test whether pteridine salvage occ urred in the infectious stage of the parasite, we examined several pathogen ic species of Leishmania and the disease-causing amastigote: stage that res ides within human macrophages. To accomplish this we developed a new sensit ive HPLC-based assay for PTR1 activity. These studies established the exist ence of the pteridine salvage pathway throughout the infectious cycle of Le ishmania, including amastigotes. In general, activities were not well corre lated with RNA transcript levels, suggesting the occurrence of at least two different modes of post-transcriptional regulation. Thus, pteridine salvag e by amastigotes may account for the clinical inefficacy of antifolates aga inst leishmaniasis, and ultimately provide insights into how this may be ov ercome in the future. (C) 2001 Elsevier Science B.V. All rights reserved.