Exploitation of methylthioribose kinase in the development of antiprotozoal drugs

Potential chemotherapeutic targets in the purine metabolism of parasites

Parasites are responsible for a wide variety of infectious diseases in human as well as in domestic and wild animals, causing an enormous health and economical blight. Current containment strategies are not entirely successful and parasitic infections are on the rise. In the absence of availability of antiparasitic vaccines, chemotherapy remains the mainstay for the treatment of most parasitic diseases. However, there is an urgent need for new drugs to prevent or combat some major parasitic infections because of lack of a single effective approach for controlling the parasites (e.g., trypanosomiasis) or because some serious parasitic infections developed resistance to presently available drugs (e.g., malaria). The rational design of a drug is usually based on biochemical and physiological differences between pathogens and host. Some of the most striking differences between parasites and their mammalian host are found in purine metabolism. Purine nucleotides can be synthesized by the de novo and/or the so-called "salvage" pathways. Unlike their mammalian host, most parasites studied lack the pathways for de novo purine biosynthesis and rely on the salvage pathways to meet their purine demands. Moreover, because of the great phylogenic separation between the host and the parasite, there are in some cases sufficient distinctions between corresponding enzymes of the purine salvage from the host and the parasite that can be exploited to design specific inhibitors or "subversive substrates" for the parasitic enzymes. Furthermore, the specificities of purine transport, the first step in purine salvage, diverge significantly between parasites and their mammalian host. This review highlights the unique transporters and enzymes responsible for the salvage of purines in parasites that could constitute excellent potential targets for the design of safe and effective antiparasitic drugs.

Affinity purification of 5-methylthioribose kinase and 5-methylthioadenosine/S-adenosylhomocysteine nucleosidase from Klebsiella pneumoniae [corrected]

Two enzymes in the methionine salvage pathway, 5-methylthioribose kinase (MTR kinase) and 5'-methylthioadenosine/ S-adenosylhomocysteine nucleosidase (MTA/SAH nucleosidase) were purified from Klebsiella pneumoniae. Chromatography using a novel 5'-(p-aminophenyl)thioadenosine/5-(p-aminophenyl)thioribose affinity matrix allowed the binding and selective elution of each of the enzymes in pure form. The molecular mass, substrate kinetics and N-terminal amino acid sequences were characterized for each of the enzymes. Purified MTR kinase exhibits an apparent molecular mass of 46-50 kDa by SDS/PAGE and S200HR chromatography, and has a Km for MTR of 12.2 microM. Homogeneous MTA/SAH nucleosidase displays a molecular mass of 26.5 kDa by SDS/PAGE, and a Km for MTA of 8.7 microM. Comparisons of the N-terminal sequences obtained for each of the enzymes with protein-sequence databases failed to reveal any significant sequence similarities to known proteins. However, the amino acid sequence obtained for the nucleosidase did share a high degree of sequence similarity with the putative translation product of an open reading frame in Escherichia coli, thus providing a tentative identification of this gene as encoding an MTA/SAH nucleosidase.

Methionine recycling pathways and antimalarial drug design

5'-Deoxy-5'-(methylthio)adenosine (MTA) is an S-adenosylmethionine metabolite that is generated as a by-product of polyamine biosynthesis. In mammalian cells, MTA undergoes a phosphorolytic cleavage catalyzed by MTA phosphorylase to produce adenine and 5-deoxy-5-(methylthio)ribose-1-phosphate (MTRP). Adenine is utilized in purine salvage pathways, and MTRP is subsequently recycled to methionine. Whereas some microorganisms metabolize MTA to MTRP via MTA phosphorylase, others metabolize MTA to MTRP in two steps via initial cleavage by MTA nucleosidase to adenine and 5-deoxy-5-(methylthio)ribose (MTR) followed by conversion of MTR to MTRP by MTR kinase. In order to assess the extent to which these pathways may be operative in Plasmodium falciparum, we have examined a series of 5'-alkyl-substituted analogs of MTA and the related MTR analogs and compared their abilities to inhibit in vitro growth of this malarial parasite. The MTR analogs 5-deoxy-5-(ethylthio)ribose and 5-deoxy-5-(hydroxyethylthio)ribose were inactive at concentrations up to 1 mM, and 5-deoxy-5-(monofluoroethylthio)ribose was weakly active (50% inhibitory concentration = 700 microM). In comparison, the MTA analogs, 5'-deoxy-5'-(ethylthio)adenosine,5'-deoxy-5'-(hydroxyethylthio)ade nosine (HETA), and 5'-deoxy-5'-(monofluoroethylthio)adenosine, had 50% inhibitory concentrations of 80, 46, and 61 microM, respectively. Extracts of P. falciparum were found to have substantial MTA phosphorylase activity. Coadministration of MTA with HETA partially protected the parasites against the growth-inhibitory effects of HETA. Results of this study indicate that P. falciparum has an active MTA phosphorylase that can be targeted by analogs of MTA.

Synergistic activity of 5-trifluoromethylthioribose and inhibitors of methionine synthesis against Klebsiella pneumoniae

5-Methylthioribose (MTR) is an intermediate in the methionine recycling pathway of organisms containing the enzyme MTR kinase. Analogs of MTR have been proposed as a new class of antimicrobial agents because of their ability to perturb the growth of MTR kinase-containing pathogens through inhibition of methionine salvage or by conversion to toxic products. One such analog, 5-trifluoromethylthioribose (TFMTR), has demonstrated potent inhibitory effects on the growth of Klebsiella pneumoniae (A. G. Gianotti, P. A. Tower, J. H. Sheley, P. A. Conte, C. Spiro, J. H. Fitchen, and M. K. Riscoe, J. Biol. Chem. 265:831-837, 1990). Although the mode of action of TFMTR has yet to be determined, it is believed that the drug is converted to the toxic products trifluoromethionine or carbonothioic difluoride via MTR kinase and the methionine recycling pathway. On the basis of this assumption, we theorized that blocking de novo methionine synthesis would increase dependence on the methionine salvage pathway and lead to an increased rate of synthesis of toxic metabolites from TFMTR. In this report, we show that three separate inhibitors of de novo methionine synthesis (1,2,4-triazole, azaserine, and propargylglycine) act synergistically with TFMTR in inhibiting the growth of K. pneumoniae.