The biosynthesis of uridine 5′-monophosphate in Giardia lamblia

The biosynthesis of uridine 5′monophosphate in Giardia lamblia. International Journal for Parasitology 17: 805–812. Giardia lamblia trophozoites were shown to rely on salvage enzymes for their pyrimidine requirements. Of the six enzymes responsible for the pyrimidine de novo biosynthetic pathway, only some non-specific “Carbamyl phosphate synthetase-II-type” activity was observed. Thymidine, uridine and uracil but not thymine, aspartate or orotate were incorporated into nucleotide pools. Nucleoside phosphorylases, thymidine kinase and an active cytosolic UPRTase were responsible for the salvage of pyrimidine bases and nucleosides, consistent with an absence of thymidylate synthetase. The major pathway of pyrimidine salvage appeared to be via the UPRTase. For this enzyme, the K m for uracil was 17.9 ± 5.1 μM and 5-fluorouracil and UMP were shown to be competitive inhibitors with respect to uracil with K i values of 0.56 and 0.2 mM respectively. UPRTase was also inhibited by TTP and activated by GTP. In vitro cultures of G. lamblia were inhibited by 5-FU with an ld 50 of 0.4 Him.Partial purification of the phosphorylases provided evidence that the pyrimidine nucleoside phosphorylases were a single enzyme capable of utilizing uridine, deoxyuridine and thymidine (and corresponding bases) as substrates. It is suggested that, as these enzymes and the UPRTase differ significantly from those in the mammalian host, they can be viewed as potential targets at which to direct chemotherapeutic agents.