Pyrimidine salvage in Giardia lamblia.

Abstract

We have found that the anaerobic protozoan parasite Giardia lamblia is incapable of de novo pyrimidine metabolism, as shown by its inability to incorporate orotate, bicarbonate, and aspartate into the pyrimidine nucleotide pool. Results from high performance liquid chromatography of pyrimidine and pyrimidine nucleoside pulse-labeled nucleotide pools and enzyme assays suggest that the parasite satisfies its pyrimidine nucleotide needs predominantly through salvage of uracil by a cytoplasmic uracil phosphoribosyltransferase. Exogenous uridine and cytidine are primarily converted to uracil by the action of uridine hydrolase and cytidine deaminase before incorporation into nucleotide pools. Direct salvage of cytosine occurs to a relatively limited extent via cytosine phosphoribosyltransferase. G. lamblia relies on salvage of exogenous thymidine for ribosylthymine monophosphate (TMP) synthesis, accomplished primarily through the action of a 100,000 g-pelletable thymidine phosphotransferase.

Biology ofGiardia lamblia

“…6). Uridine is probably imported by the thymidine transporter (62,89) and broken down into uracil by a hydrolase (13) or phosphorylase (181). Cytidine and uridine can also be imported by a broad-specificity transporter (63), but most cytidine probably enters the synthetic pathway for UTP synthesis rather than being converted more directly to CTP (Fig.…”

“…In addition, G. lamblia (13), as well as T. vaginalis (340) and Tritrichomonas foetus (341), lack pyrimidine synthetic pathways and depends on salvage pathways for obtaining both purine and pyrimidine nucleosides.…”

“…lamblia trophozoites also depend on the salvage of exogenous thymidine, cytidine, and uridine for the synthesis of the pyrimidine nucleotides (13,190) (Fig. 6).…”

“…Pyrimidine ribonucleoside salvage pathways. The enzymes are labeled as follows: 1, uracil phosphoribosyltransferase (UPRTase) (58); 2, uridine/thymine phosphorylase (181) (a uridine hydrolase [13] has not been confirmed); 3, uridine phosphotransferase (kinase) (13) (not confirmed [336]); 4, UMP kinase (151); 5, UDP kinase (151); 6, CTP synthetase (151, 187); 7, CDP kinase (151); 8, cytosine phosphoribosyltransferase (CPRTase) (13) (low level of activity [151]); 9, cytidine hydrolase (13); 10, cytidine deaminase (336). The initial pyrimidine salvage pathway was described in (reference) and has been updated from more recent literature (see the references cited for individual enzymes).…”

Current Therapeutics, Their Problems, and Sulfur-Containing-Amino-Acid Metabolism as a Novel Target against Infections by “Amitochondriate” Protozoan Parasites

“…The ATP generation in these parasites occurs exclusively through substrate-level phosphorylation, despite differences in their life cycles and pathogenic properties (216,217). As obligatory parasites, these organisms have a reduced ability for the de novo synthesis of essential building blocks of DNA and proteins, including nucleic acid precursors (7,10,322) and amino acids (7,233,247). As a consequence, certain metabolic pathways either are missing in these organisms or are divergent from those of mitochondriate organisms.…”