THE ROLES OF PTERIDINE REDUCTASE-1 AND DIHYDROFOLATE REDUCTASE-THYMIDYLATE SYNTHASE IN PTERIDINE METABOLISM IN THE PROTOZOAN PARASITE LEISHMANIA-MAJOR

Trypanosomatid protozoans depend upon exogenous sources of pteridines (pterins or folates) for growth. A broad spectrum pteridine reductase (PTR1) was recently identified in Leishmania major, whose sequence pla ces it in the short chain alcohol dehydrogenase protein family althoug h its enzymatic activities resemble dihydrofolate reductases. The prop erties of PTR1 suggested a role in essential pteridine salvage as well as in antifolate resistance, To prove this, we have characterized fur ther the properties and relative roles of PTR1 and dihydrofolate reduc tase-thymidylate synthase in Leishmania pteridine metabolism, using pu rified enzymes and knockout mutants, Recombinant L. major and Leishman ia tarentolae, and native L. major PTR1s, were tetramers of 30-kDa sub units and showed similar catalytic properties with pterins and folates (pH dependence, substrate inhibition with H(2)pteridines), Unlike PTR 1, dihydrofolate reductase-thymidylate synthase showed weak activity w ith folate and no activity with pterins, Correspondingly, studies of p tr1(-) and dhfr-ts(-) mutants implicated only PTR1 in the ability of L . major to grow on a wide array of pterins, PTR1 exhibited 2000-fold l ess sensitivity to inhibition by methotrexate than dihydrofolate reduc tase-thymidylate synthase, suggesting several mechanisms by which PTR1 may compromise antifolate inhibition in wild-type Leishmania and line s bearing PTR1 amplifications, We incorporate these results into a com prehensive model of pteridine metabolism and discuss its implications in chemotherapy of this important human pathogen.