Mitochondrial function in Babesia bovis.

A variety of anti-mitochondrial drugs that had previously been found to inhibit the growth of the malarial parasite Plasmodium falciparum were tested on Babesia bovis in vitro. Several of these drugs were found to be non-toxic towards B. bovis. However, those drugs that were found to inhibit babesial growth included compounds (shown in parentheses) that have the following putative mitochondrial targets in the parasite: ATP synthetase complex (rhodamine 123, oligomycin, Janus Green); ATP-ADP translocase (bongkrekic acid); electron transport (rotenone, n-heptyl-4-hydroxyquinoline-N-oxide (HQNO), antimycin A); ubiquinone (CoQ) function (BW58C, menoctone); protein synthesis (tetracycline); and the proton pump (CCCP). We have also investigated the effects of some of these drugs on pyrimidine biosynthesis de novo by following the incorporation of [14C]bicarbonate into pyrimidine nucleotides and into the pyrimidine moieties of nucleic acids. The ubiquinone analogues BW58C and menoctone inhibited this pathway in the nM-microM range of concentrations. Inhibitors of electron transport (antimycin A and oligomycin) and an uncoupler (CCCP) were also effective inhibitors of pyrimidine biosynthesis de novo. We conclude that B. bovis has a functional mitochondrion that contributes significantly to pyrimidine biosynthesis de novo and to the overall energy metabolism of the parasite.     

De novo pyrimidine nucleotide synthesis in the preimplantation mouse embryo

Utilizing phosphonacetyl- -aspartate (PALA), the transition state analog which specifically inhibits aspartate carbamyl transferase, we have shown that the preimplantation mouse embryo in culture has a functioning de novo pyridmidine biosynthetic pathway. This pathway accounts for some of the carbon dioxide fixation into nucleic acids previously described. Inhibition of de novo pyrimidine nucleotide synthesis during 2-cell to 8-cell development does not prevent morula development, but does prevent blastocyst development in nearly all embryos. Inhibition of the morula to blastocyst transition is most likely caused by a diminished pyrimidine nucleotide pool. Both de novo and salvage pathways appear active from the 2-cell embryo through blastocyst formation.     

Flux through the de novo pyrimidine pathway in vivo. Effect of N-phosphonacetyl-L-aspartate, a potent inhibitor of aspartate transcarbamylase

The activity of the de novo pyrimidine biosynthetic pathway has been measured in resistant and sensitive murine tumors in vivo following a single intraperitoneal dose of N-phosphonacetyl-L-aspartate (PALA) (400 mg/kg). For these studies, we utilized a gas chromatograph-mass spectrometric technique which enabled measurement of 13C incorporation from 13CO2 into the uracil nucleotide pool (sigma uracil) of tumors in situ. Flux through the de novo pathway was 75-85% inhibited 1 h after PALA treatment in both sensitive (Lewis lung carcinoma) and the resistant (L1210) tumors, but there was a lag time before this inhibition was reflected in reduced sigma uracil pools. The activity of the pathway in the Lewis lung carcinoma tumors remained maximally depressed (5-15% of control activity) for up to 48 h after the dose of PALA. In contrast, flux through the pathway of L1210 tumors remained 80% inhibited for up to 4 h following PALA administration, but recovered to 70% of control activity between 4 and 12 h after PALA treatment. Recovery of the remaining 30% of control activity in the L1210 tumor was at a much slower rate requiring between 12 and 48 h after PALA treatment to regain full activity of the pathway. This recovery of flux through the de novo pyrimidine biosynthetic pathway did not correlate with the measurement of recovery of aspartate transcarbamylase activity in similarly treated tumors. These data argue strongly in favor of the importance of the de novo biosynthetic pathway, rather than salvage mechanisms, for determining in vivo sensitivity or resistance of these tumors to PALA treatment.     

Antipyrimidine effects of five different pyrimidine de novo synthesis inhibitors in three head and neck cancer cell lines

The pyrimidine de novo nucleotide synthesis consists of 6 sequential steps. Various inhibitors against these enzymes have been developed and evaluated in the clinic for their potential anticancer activity: acivicin inhibits carbamoyl-phosphate-synthase-II, N-(phosphonacetyl)-L- aspartate (PALA) inhibits aspartate-transcarbamylase, Brequinar sodium and dichloroallyl-lawsone (DCL) inhibit dihydroorotate-dehydrogenase, and pyrazofurin (PF) inhibits orotate-phosphoribosyltransferase. We compared their growth inhibition against 3 cell lines from head-and-neck-cancer (HEP-2, UMSCC-14B and UMSCC-14C) and related the sensitivity to their effects on nucleotide pools. In all cell lines Brequinar and PF were the most active compounds with IC50 (50% growth inhibition) values between 0.06–0.37 µM, Acivicin was as potent (IC50s 0.26-1 µM), but DCL was 20-31-fold less active. PALA was most inactive (24–128 µM). At equitoxic concentrations, all pure antipyrimidine de novo inhibitors depleted UTP and CTP after 24 hr exposure, which was most pronounced for Brequinar (between 6–10% of UTP left, and 12–36% CTP), followed by DCL and PF, which were almost similar (6–16% UTP and 12–27% CTP), while PALA was the least active compound (10–70% UTP and 13–68% CTP). Acivicin is a multi-target inhibitor of more glutamine requiring enzymes (including GMP synthetase) and no decrease of UTP was found, but a pronounced decrease in GTP (31–72% left). In conclusion, these 5 inhibitors of the pyrimidine de novo nucleotide synthesis varied considerably in their efficacy and effect on pyrimidine nucleotide pools. Inhibitors of DHO-DH were most effective suggesting a primary role of this enzyme in controlling pyrimidine nucleotide pools     

RNA synthesis in starfish embryos: developmental consequences of its inhibition by formycin

Embryos of the starfish Asterina pectinifera were examined with regard to their ability to undergo the early events of embryonic development in the presence of formycin, an analogue of adenosine and a reported inhibitor of RNA synthesis. It was shown that in normal embryos the pool of ribonucleoside 5'-triphosphates increased during the period of blastula formation. The increase of the UTP pool was blocked nearly completely by 25 micrograms/ml formycin, and that of the CTP pool was inhibited partially by the same concentration of the drug. On the other hand, the pools of ATP and GTP were the same for both control and formycin-treated embryos. The development of embryos cultured in the presence of 25 micrograms/ml formycin stopped at the early blastula stage. Addition of 100 micrograms/ml each of uridine and cytidine to cultures of embryos that had been placed in 25 micrograms/ml formycin at the onset of blastulation allowed gastrulation to occur, suggesting that the developmental arrest produced by formycin is due primarily to the inhibition of pyrimidine nucleotide biosynthesis.     

Characterization of pyrimidine metabolism in the cellular slime mold, Dictyostelium discoideum

The arginine-independent, de novo biosynthetic pathway of pyrimidines in Dictyostelium discoideum is initiated by a class II carbamoyl-phosphate synthetase (EC 6.3.5.5) specific for pyrimidine biosynthesis which utilized L-glutamine as its N donor and was partially inhibited by both UTP and CTP. The second step in the de novo pathway was provided by an unregulated aspartate transcarbamoylase (EC 2.1.3.2) which primarily appeared as a multimeric enzyme of 105 kilodaltons. The next enzyme, dihydroorotase (EC 3.5.2.3), was approximately 90-100 kilodaltons. Although the early enzymatic activities of the pyrimidine pathway appeared to reside in independent protein complexes, various unstable molecular species were observed. These structural variants may represent proteolytic fragments of a multienzyme complex. In addition to de novo synthesis, the amoeba demonstrated the capacity for salvage utilization of uracil, uridine, and cytidine. Upon starvation on a solid substratum, axenically grown amoebas began a concerted developmental program accompanied by a restructuring of nucleotide metabolism. The absolute levels of the ribonucleotide pools droppedby 98% within 30 h; however, both the adenylate energy charge and the GTP/ATP ratios were maintained for 50 h after the initiation of development. The maintenance of these metabolic energy parameters required the tight cell-cell contact necessary for development, and the capacity for pyrimidine metabolism was maintained throughout developmental morphogenesis.     

Pyrimidine synthesis in Burkholderia cepacia ATCC 25416

K. LI AND T.P. WEST. 1995. Pyrimidine synthesis in the food spoilage agent Burkholderia cepacia ATCC 25416 was investigated. The five de novo pathway enzymes of pyrimidine biosynthesis were found to be active in B. cepacia ATCC 25416 and growth of this strain on uracil had an effect on the de novo enzyme activities. The in vitro regulation of aspartate transcarbamoylase activity in B. cepacia ATCC 25416 was studied and its activity was inhibited by PP_i, ATP, GTP, CTP and UTP. The enzymes cytidine deaminase, uridine phosphorylase and cytosine deaminase were found to be active in the salvage of pyrimidines in ATCC 25416. Overall, de novo pyrimidine synthesis in B. cepacia ATCC 25416 was regulated at the level of enzyme activity and its pyrimidine salvage enzymes differed from those found in B. cepacia ATCC 17759.     

The effect of glutamine concentration on the activity of carbamoyl-phosphate synthase II and on the incorporation of [3H]thymidine into DNA in rat mesenteric lymphocytes stimulated by phytohaemagglutinin.

The maximum catalytic activities of carbamoyl-phosphate synthase II, a limiting enzyme for pyrimidine nucleotide synthesis, are very much less than those of glutaminase, a limiting enzyme for glutamine utilization, in lymphocytes and macrophages; and the flux through the pathway for pyrimidine formation de novo is only about 0.4% of the rate of glutamine utilization by lymphocytes. The Km of synthase II for glutamine is about 16 microM and the concentration of glutamine necessary to stimulate lymphocyte proliferation half-maximally is about 21 microM. This agreement suggests that the importance of glutamine for these cells is provision of nitrogen for biosynthesis of pyrimidine nucleotides (and probably purine nucleotides). However, the glutamine concentration necessary for half-maximal stimulation of glutamine utilization (glutaminolysis) by the lymphocytes is 2.5 mM. The fact that the rate of glutamine utilization by lymphocytes is markedly in excess of the rate of the pathway for pyrimidine nucleotide synthesis de novo and that the Km and 'half-maximal concentration' values are so different, suggests that the glutaminolytic pathway is independent of the use of glutamine nitrogen for pyrimidine synthesis.     

Biosynthesis and scavenging of pyrimidines by pathogenic mycobacteria

Mycobacterium microti incorporated a wide range of exogenously supplied pyrimidines into its nucleic acids. M. avium incorporated a relatively narrow range of pyrimidines but both M. avium and M. microti when recovered after growth in vivo incorporated a slightly wider range of pyrimidines than the same strains grown in vitro. M. microti and M. leprae could not take up uridine nucleotides directly but could utilize the pyrimidines by hydrolysing them to uridine and then taking up the uridine. Pyrimidine biosynthesis, judged by the ability to incorporate carbon from CO2 or aspartate into pyrimidines was readily detected in non-growing suspensions of M. microti and M. avium harvested from Dubos medium, which does not contain pyrimidines. The biosynthetic activity was diminished in mycobacteria grown in vivo when there is likely to be a source of pyrimidines which they might use. Relative activities for pyrimidine biosynthesis de novo in M. microti were 100 for cells isolated from Dubos medium, 6 for cells isolated from Dubos medium containing the pyrimidine cytidine and 11 from cells recovered after growth in mice. In contrast, relative activities for a scavenging reaction, uracil incorporation, were 100, 71 and 59, respectively. Three key enzymes in the pathway of pyrimidine biosynthesis de novo were detected in M. microti and M. avium. Two, dihydroorotate synthase and orotate phosphoribosyltransferase appeared to be constitutive in M. microti and M. avium. Aspartate transcarbamoylase activity was higher in these mycobacteria grown in vivo than in Dubos medium but it was repressed in M. microti or M. avium grown in Dubos medium in the presence of 50 microM-pyrimidine. Aspartate transcarbamoylase was strongly inhibited by the feedback inhibitors ATP, CTP and UTP. Enzymes for scavenging pyrimidines were detected at low specific activities in all mycobacteria studied. Activities of phosphoribosyltransferases, enzymes that convert bases directly to nucleotides, were not related to the ability of intact mycobacteria to take up pyrimidine bases while activities of pyrimidine nucleoside kinases were generally related to the ability of intact mycobacteria to take up nucleosides. Phosphoribosyltransferase activity for uracil, cytosine, orotic acid and--in organisms grown in Dubos medium with 50 microM-uridine-thymine, as well as kinases for uridine, deoxyuridine, cytidine and thymidine were detected in M. microti. However, M. avium only contained uracil and orotate phosphoribosyltransferase, uridine, cytidine and thymidine kinase, and additionally deoxyuridine kinase when grown axenically with 50 microM-uracil, reflecting its more limited abilities in pyrimidine scavenging.     

Cytotoxic mechanisms of glutamine antagonists in mouse L1210 leukemia

The glutamine antagonists, acivicin (NSC 163501), azaserine (NSC 742), and 6-diazo-5-oxo-L-norleucine (DON) (NSC 7365), are potent inhibitors of many glutamine-dependent amidotransferases in vitro. Experiments performed with mouse L1210 leukemia growing in culture show that each antagonist has different sites of inhibition in nucleotide biosynthesis. Acivicin is a potent inhibitor of CTP and GMP synthetases and partially inhibits N-formylglycineamidine ribotide (FGAM) synthetase of purine biosynthesis. DON inhibits FGAM synthetase, CTP synthetase, and glucosamine-6-phosphate isomerase. Azaserine inhibits FGAM synthetase and glucosamine-6-phosphate isomerase. Large accumulations of FGAR and its di- and triphosphate derivatives were observed for all three antagonists which could interfere with the biosynthesis of nucleic acids, providing another mechanism of cytotoxicity. Acivicin, azaserine, and DON are not potent inhibitors of carbamyl phosphate synthetase II (glutamine-hydrolyzing) and amidophosphoribosyltransferase in leukemia cells growing in culture although there are reports of such inhibitions in vitro. Blockade of de novo purine biosynthesis by these three antagonists results in a "complementary stimulation" of de novo pyrimidine biosynthesis.     

A Novel Mechanism for Resistance to the Antimetabolite N-Phosphonoacetyl-l-Aspartate by Helicobacter pylori

The mechanism of resistance to N-phosphonoacetyl-l-aspartate (PALA), a potent inhibitor of aspartate carbamoyltransferase (which catalyzes the first committed step of de novo pyrimidine biosynthesis), in Helicobacter pylori was investigated. At a 1 mM concentration, PALA had no effects on the growth and viability of H. pylori. The inhibitor was taken up by H. pylori cells and the transport was saturable, with a K_m of 14.8 mM and a V_max of 19.1 nmol min⁻¹ μl of cell water⁻¹. By ³¹P nuclear magnetic resonance (NMR) spectroscopy, both PALA and phosphonoacetate were shown to have been metabolized in all isolates of H. pylori studied. A main metabolic end product was identified as inorganic phosphate, suggesting the presence of an enzyme activity which cleaved the carbon-phosphorus (C-P) bonds. The kinetics of phosphonate group cleavage was saturable, and there was no evidence for substrate inhibition at higher concentrations of either compound. C-P bond cleavage activity was temperature dependent, and the activity was lost in the presence of the metal chelator EDTA. Other cleavages of PALA were observed by ¹H NMR spectroscopy, with succinate and malate released as main products. These metabolic products were also formed when N-acetyl-l-aspartate was incubated with H. pylori lysates, suggesting the action of an aspartase. Studies of the cellular location of these enzymes revealed that the C-P bond cleavage activity was localized in the soluble fraction and that the aspartase activity appeared in the membrane-associated fraction. The results suggested that the two H. pylori enzymes transformed the inhibitor into noncytotoxic products, thus providing the bacterium with a mechanism of resistance to PALA toxicity which appears to be unique.Helicobacter pylori has been established as the causative agent of chronic gastritis and a significant proportion of duodenal and gastric ulcers (14). Recently, the World Health Organization classified H. pylori as a group 1 carcinogen, owing to its role in the development of gastric cancer (10). The failure of some regimens in the treatment of H. pylori infection has motivated work in our laboratory directed at characterizing the physiology of the bacterium, with the aim of discovering potential sites for therapeutic intervention, including nucleotide biosynthetic pathways (24, 25).Earlier studies on the uptake of nucleotide precursors by H. pylori showed that there was relatively little acquisition of pyrimidine nucleotide precursors by the salvage of preformed bases and nucleosides (24). Uracil, a commonly salvaged pyrimidine base, is also not required for the growth of this bacterium (34), suggesting that the majority of its pyrimidine nucleotides are synthesized through the de novo pathway. In contrast, humans can utilize the de novo or salvage pathway for the synthesis of pyrimidine nucleotides. Inhibitors of H. pylori de novo pyrimidine biosynthesis may therefore be potentially effective therapeutic drugs, as the host could still efficiently acquire its nucleotide requirements by salvage. This potential was demonstrated earlier by the finding that the inhibition of de novo pyrimidine biosynthesis at the second enzyme of this pathway, dihydroorotase, resulted in the killing of H. pylori cells (35).Aspartate carbamoyltransferase (ACTase) (EC 2.1.3.2) catalyzes the first committed step in the de novo formation of pyrimidine nucleotides and is a key regulatory enzyme in bacteria (8). N-Phosphonoacetyl-l-aspartate (PALA) is a synthetic, transition state bisubstrate analogue of the intermediate of the ACTase-catalyzed reaction (5). PALA belongs to a group of organophosphorus compounds known as phosphonates, characterized by their extremely stable carbon-phosphorus (C-P) bond in place of the more common carbon-oxygen-phosphorus ester bond (39), which confers on them the advantage of inherent stability. Natural phosphonates are found in phosphonolipids, glycolipids, glycoproteins, and polysaccharides of many different organisms. PALA and other synthetic phosphonates have been produced for use as herbicides, antibacterial agents (1, 28), and even as agents of chemical warfare (38).PALA is a potent inhibitor of the ACTase-catalyzed reaction in a range of prokaryotic and eukaryotic organisms, including Escherichia coli (5), Pyrococcus abyssi (33), and Leishmania donovani (29), and in mammalian cells (36). Owing to its stability and toxic effects on a key regulatory enzyme, PALA has been employed as an antitumor agent to inhibit the growth of rapidly proliferating cancer cells (9, 36). The inhibitor was also suggested as a possible antimetabolite for the protozoan pathogen L. donovani due to its cytotoxic effects on this organism (29). However, we have not found any detailed studies investigating the effects of PALA on the viability of bacterial cells. Recent results indicated that PALA is a potent inhibitor of ACTase activity in H. pylori, with 50% inhibition of enzyme activity observed at 0.1 μM PALA, and that PALA binds to the enzyme over 2,500 times more tightly than carbamoyl phosphate (3). This finding suggested that ACTase in H. pylori was a potential target for therapeutic intervention. However, initial results in our laboratory showed that PALA did not have inhibitory effects on the growth and viability of the bacterium.The aim of this work was to elucidate the mechanism(s) for H. pylori resistance to the potentially toxic effects of PALA. The effects on growth and viability, the transport of the inhibitor into whole cells, and the metabolic fate of this compound inside the cell were investigated by radiotracer analyses and nuclear magnetic resonance (NMR) spectroscopy.     

Separate pyrimidine-nucleotide pools for messenger-RNA and ribosomal-RNA synthesis in HeLa S3 cells.

Kinetic analyses of mRNA and 28-S RNA labeling [3H]uridine revealed distinctly different steady-state specific radioactivities finally reached for uridine in mRNA and 28-S RNA when exogenous [3H]uridine was kept constant for several cell doubling times. While the steady-state label of (total) UTP and of uridine in mRNA responded to the same extent to a suppression of pyrimidine synthesis de novo by high uridine concentrations in the culture medium, uridine in 28-S RNA was scarcely influenced. Similar findings were obtained with respect to labeling of cytidine in the various RNA species due to an equilibration of UTP with CTP [5-3H]Uridine is also incorporated into deoxycytidine of DNA, presumably via dCTP. The specific radioactivity of this nucleosidase attained the same steady-state value as UTP, uridine in mRNA and cytidine in mRNA. The data indicate the existence of two pyrimidine nucleotide pools. One is a large, general UTP pool comprising the bulk of the cellular UTP and serving nucleoplasmic nucleic acid formation (uridine and cytidine in mRNA, deoxycytidine in DNA). Its replenishment by de novo synthesis can be suppressed completely by exogenous uridine above 100 muM concentrations. A second, very small UTP (and CTP) pool with a high turnover provides most of the precursors for nucleolar RNA formation (rRNA). This pool is not subject to feedback inhibition by extracellular uridine to an appreciable extent. Determinations of (total) UTP turnover also show that the bulk of cellular RNA (rRNA) cannot be derived from the large UTP pool.     

Genetic Analysis of Metabolic Crosstalk and Its Impact on Thiamine Synthesis in Salmonella Typhimurium

The first five steps in de novo purine biosynthesis are involved in the formation of the 4-amino-5-hydroxymethyl-2-methyl pyrimidine (HMP) moiety of thiamine. We show here that the first enzyme in de novo purine biosynthesis, PurF, is required for thiamine synthesis during aerobic growth on some but not other carbon sources. We show that PurF-independent thiamine synthesis depends on the recently described alternative pyrimidine biosynthetic (APB) pathway. Null mutations in zwf (encoding glucose-6-P dehydogenase), gnd (encoding gluconate-6-P dehydrogenase), purE (encoding aminoimidazole ribo-nucleotide carboxylase), and purR (encoding a regulator of gene expression) were found to affect the function of the APB pathway. A model is presented to account for the involvement of these gene products in thiamine biosynthesis via the APB pathway. Results presented herein demonstrate that function of the APB pathway can be prevented either by blocking intermediate formation or by diverting intermediate(s) from the pathway. Strong genetic evidence supports the conclusion that aminoimidazole ribotide (AIR) is an intermediate in the APB pathway.     

Effect of plasma concentrations of uridine on pyrimidine biosynthesis in cultured L1210 cells

The concentration of uridine in the media of cultured L1210 cells was maintained within the concentration range found in plasma (1 to 10 microM) to determine if such concentrations are sufficient to satisfy the pyrimidine requirements of a population of dividing cells and to determine if cells utilize de novo and/or salvage pathways when exposed to plasma concentrations of uridine. When cells were incubated in the presence of N-(phosphonacetyl)-L-aspartate to block de novo biosynthesis, plasma concentrations of uridine maintained normal cell growth. De novo pyrimidine biosynthesis, as determined by [14C]sodium bicarbonate incorporation into uracil nucleotides, was affected by the low concentrations of uridine found in the plasma. Below 1 microM uridine, de novo biosynthesis was not affected; between 3 and 5 microM uridine, de novo biosynthesis was inhibited by approximately 50%; and above 12 microM uridine, de novo biosynthesis was inhibited by greater than 95%. Inhibition of de novo biosynthesis correlated with an increase in the uracil nucleotide pool. The de novo pathway was much more sensitive to the uracil nucleotide pool size than was the salvage pathway, such that when de novo biosynthesis was inhibited by greater than 95% the uracil nucleotide pool continued to expand and the cells continued to take up [14C]uridine. Thus, the pyrimidine requirements of cultured L1210 cells can be met by concentrations of uridine found in the plasma and, when exposed to such physiologic concentrations, L1210 cells decrease their dependency on de novo biosynthesis and utilize their salvage pathway. Circulating uridine, therefore, may be of physiologic importance and could be an important determinant in anti-pyrimidine chemotherapy.     

Control of pyrimidine biosynthesis in human lymphocytes. Inhibitory effect of guanine and guanosine on induction of enzymes for pyrimidine biosynthesis de novo in phytohemagglutinin-stimulated lymphocytes.

Human peripheral lymphocytes were incubated with Phaseolus vulgaris phytohemagglutinin. The induction of glutamine-utilizing carbamyl phosphate synthetase (EC 2.7.2.5) and aspartate transcarbamylase (EC 2.1.3.2) for pyrimidine biosynthesis de novo and the induction of uridine kinase were observed as described previously (Ito, K., and Uchino, H. (1971) J. Biol. Chem. 246, 4060-4065; Ito, K., and Uchino, H. (1973) J. Biol. Chem. 248, 389-392; Lucas, Z.J. (1967) Science 156, 1237-1240). By the addition of 1 mM guanine to the culture, the induction of the former two enzymes was inhibited, while that of uridine kinase was not, and even accelerated. An increase in the rate of [14C] bicarbonate incorporation into the acid-soluble uridine nucleotides via the de novo pathway for pyrimidine biosynthesis after phytohemagglutinin stimulation was inhibited by guanine, the incorporation rate being almost at the level of the control culture without phytohemagglutinin. Guanosine had a similar effect on pyrimidine biosynthesis. The induction of the three enzymes mentioned above was completely inhibited by adenine (1 mM). Guanine and guanosine seem to have a unique inhibitory effect on the induction of glutamine-utilizing carbamyl phosphate synthetase and aspartate transcarbamylase.     

Pyrimidine biosynthetic pathway of Pseudomonas fluorescens

Pyrimidine biosynthesis in Pseudomonas fluorescens strain A126 was investigated. In this study, de novo pyrimidine biosynthetic pathway mutant strains were isolated using both conventional mutagenesis and transposon mutagenesis. The resulting mutant strains were deficient for either aspartate transcarbamoylase, dihydroorotase or orotate phosphoribosyltransferase activity. Uracil, uridine or cytosine could support the growth of every mutant strain selected. In addition, the aspartate transcarbamoylase mutant strains could utilize orotic acid to sustain their growth while the orotidine-5'-monophosphate decarboxylase mutant strains grew slowly upon uridine 5'-monophosphate. The wild-type strain and the mutant strains were used to study possible regulation of de novo pyrimidine biosynthesis in P. fluorescens. Dihydroorotase specific activity more than doubled after the wild-type cells were grown in orotic acid relative to unsupplemented minimal-medium-grown cells. Starving the mutant strains of pyrimidines also influenced the levels of several de novo pyrimidine biosynthetic pathway enzyme activities.     

Pathway of thiamine pyrophosphate synthesis in Micrococcus denitrificans.

The pathway of thiamine pyrophosphate (TPP) biosynthesis, which is formed either from exogeneously added thiamine or from the pyrimidine and thiazole moieties of thiamine, in Micrococcus denitrificans was investigated. The following indirect evidence shows that thiamine pyrophosphokinase (EC 2.7.6.2) catalyzes the synthesis of TPP from thiamine: (i) [35S]thiamine incubated with cells of this microorganism was detected in the form of [35S]thiamine; (ii) thiamine gave a much faster rate of TPP synthesis than thiamine monophosphate (TMP) when determined with the extracts; and (iii) a partially purified preparation of the extracts can use thiamine, but not TMP, as the substrate. The activities of the four enzymes involved in TMP synthesis from pyrimidine and thiazole moieties of thiamine were detected in the extracts of M. denitrificans. The extracts contained a high activity of the phosphatase, probably specific for TMP. After M. denitrificans cells were grown on a minimal medium containing 3 mM adenosine, which causes derepression of de novo thiamine biosynthesis in Escherichia coli, the activities of the four enzymes involved with TMP synthesis, the TMP phosphatase, and the thiamine pyrophosphokinase were enhanced two- to threefold. These results indicate that TPP is synthesized directly from thiamine without forming TMP as an intermediate and that de novo synthesis of TPP from the pyrimidine and thiazole moieties involves the formation of TMP, followed by hydrolysis to thiamine, which is then converted to TPP directly. Thus, the pathway of TPP synthesis from TMP synthesized de novo in M. denitrificans is different from that found in E. coli, in which TMP synthesized de novo is converted directly to TPP without producing thiamine.     

Pyrimidine Salvage Pathways In Toxoplasma Gondii

ABSTRACT. Pyrimidine salvage enzyme activities in cell-free extracts of Toxoplasma gondii were assayed in order to determine which of these enzyme activities are present in these parasites. Enzyme activities that were detected included phosphoribosyltransferase activity towards uracil (but not cytosine or thymine), nucleoside phosphorylase activity towards uridine, deoxyuridine and thymidine (but not cytidine or deoxycytidine), deaminase activity towards cytidine and deoxycytidine (but not cytosine, cytidine 5'-monophosphate or deoxycytidine 5'-monophosphate), and nucleoside 5'-monophosphate phosphohydrolase activity towards all nucleotides tested. No nucleoside kinase or phosphotransferase activity was detected, indicating that T. gondii lack the ability to directly phosphorylate nucleosides. Toxoplasma gondii appear to have a single non-specific uridine phosphorylase enzyme which can catalyze the reversible phosphorolysis of uridine, deoxyuridine and thymidine, and a single cytidine deaminase activity which can deaminate both cytidine and deoxycytidine. These results indicate that pyrimidine salvage in T. gondii probably occurs via the following reactions: cytidine and deoxycytidine are deaminated by cytidine deaminase to uridine and deoxyuridine, respectively; uridine and deoxyuridine are cleaved to uracil by uridine phosphorylase; and uracil is metabolized to uridine 5'-monophosphate by uracil phosphoribosyltransferase. Thus, uridine 5'-monophosphate is the end-product of both de novo pyrimidine biosynthesis and pyrimidine salvage in T. gondii.     

Effect of short-term salt stress on the metabolic profiles of pyrimidine, purine and pyridine nucleotides in cultured cells of the mangrove tree, Bruguiera sexangula

To investigate the short‐term (3 h) effect of salt on the metabolism of purine, pyrimidine and pyridine nucleotides in mangrove (Bruguiera sexangula) cells, we examined the uptake and overall metabolism of radiolabelled intermediates involved in the de novo pathways and substrates of salvage pathways for nucleotide biosynthesis in the presence and absence of 100 mM NaCl. Uptake by the cells of substrates for the salvage pathways was much faster than uptake of intermediates of the de novo pathways. The activity of the de novo pyrimidine biosynthesis estimated by [2‐¹⁴C]orotate metabolism was not significantly affected by the salt. About 20–30% of [2‐¹⁴C]uridine, [2‐¹⁴C]uracil and more than 50% of [2‐¹⁴C]cytidine were salvaged for pyrimidine nucleotide biosynthesis. However, substantial quantities of these compounds were degraded to ¹⁴CO₂ via β‐ureidopropionate (β‐UP), and degradation of β‐UP was increased by the salt. The activities of the de novo pathway, estimated by [2‐¹⁴C] 5‐aminoimidazole‐4‐carboxamide ribonucleoside, and the salvage pathways from [8‐¹⁴C]adenosine and [8‐¹⁴C]guanosine for the purine nucleotide biosynthesis were not influenced by the salt. Most [8‐¹⁴C]hypoxanthine was catabolised to ¹⁴CO₂, and other purine compounds are also catabolised via xanthine. Purine catabolism was stimulated by the salt. [³H]Quinolinate, [carbonyl‐¹⁴C]nicotinamide and [carboxyl‐¹⁴C]nicotinic acid were utilised for the biosynthesis of pyridine nucleotides. The salvage pathways for pyridine nucleotides were significantly stimulated by the salt. Trigonelline was synthesised from all pyridine precursors that were examined; its synthesis was also stimulated by the salt. We discuss the physiological role of the salt‐stimulated reactions of nucleotide metabolism.     

Profiles of pyrimidine biosynthesis,salvage and degradation in disks of potato (Solanum tuberosum L.) tubers

In order to obtain general metabolic profiles of pyrimidine ribo- and deoxyribonucleotides in potato (Solanum tuberosum L.) plants, the in situ metabolic fate of various (14)C-labelled precursors in disks from growing potato tubers was investigated. The activities of key enzymes in potato tuber extracts were also studied. The following results were obtained. Of the intermediates in de novo pyrimidine biosynthesis, [(14)C]carbamoylaspartate was converted to orotic acid and [2-(14)C]orotic acid was metabolized to nucleotides and RNA. UMP synthase, a bifunctional enzyme with activities of orotate phosphoribosyltransferase (EC 2.4.2.10) and orotidine 5'-monophosphate decarboxylase (EC 4.1.1.23), exhibited high activity. The rates of uptake of pyrimidine ribo- and deoxyribonucleosides by the disks were high, in the range 2.0-2.8 nmol (g FW)(-1) h(-1). The pyrimidine ribonucleosides, uridine and cytidine, were salvaged exclusively to nucleotides, by uridine/cytidine kinase (EC 2.7.1.48) and non-specific nucleoside phosphotransferase (EC 2.7.1.77). Cytidine was also salvaged after conversion to uridine by cytidine deaminase (EC 3.5.4.5) and the presence of this enzyme was demonstrated in cell-free tuber extracts. Deoxycytidine, a deoxyribonucleoside, was efficiently salvaged. Since deoxycytidine kinase (EC 2.7.1.74) activity was extremely low, non-specific nucleoside phosphotransferase (EC 2.7.1.77) probably participates in deoxycytidine salvage. Thymidine, which is another pyrimidine deoxyribonucleoside, was degraded and was not a good precursor for nucleotide synthesis. Virtually all the thymidine 5'-monophosphate synthesis from thymidine appeared to be catalyzed by phosphotransferase activity, since little thymidine kinase (EC 2.7.1.21) activity was detected. Of the pyrimidine bases, uracil, but not cytosine, was salvaged for nucleotide synthesis. Since uridine phosphorylase (EC 2.4.2.3) activity was not detected, uracil phosphoribosyltransferase (EC 2.4.2.9) seems to play the major role in uracil salvage. Uracil was degraded by the reductive pathway via beta-ureidopropionate, but cytosine was not degraded. The activities of the cytosine-metabolizing enzymes observed in other organisms, pyrimidine nucleoside phosphorylase (EC 2.4.2.2) and cytosine deaminase (EC 3.5.4.1), were not detected in potato tuber extracts. Operation of the de novo synthesis of deoxyribonucleotides via ribonucleotide reductase and of the salvage pathway of deoxycytidine was demonstrated via the incorporation of radioactivity from both [2-(14)C]cytidine and [2-(14)C]deoxycytidine into DNA. A novel pathway converting deoxycytidine to uracil nucleotides was found and deoxycytidine deaminase (EC 3.5.4.14), an enzyme that may participate in this pathway, was detected in the tuber extracts.