Pteridine reductase (PTR) catalyzes the reduction of 5,6,7,8-tetrahydrobiopterin (THBP) to biopterin. PTR is part of pterin and folate metabolism. NADP is the hydrogen acceptor in this reduction. PTR is essential for growth of trypanosomatid protozoans like Leishmania parasites, thus PTR inhibitors like methotrexate (MTX) or triaminoquinazoline (TAQ) are tested as potential drugs.