Pentatrichomonas hominis: purine salvage pathway

1. Pentatrichomonas hominis was found incapable of de novo synthesis of purines. 2. Pentatrichomonas hominis can salvage adenine, guanine, hypoxanthine, adenosine, guanosine and inosine, but not xanthine for the synthesis of nucleotides. 3. HPLC tracing of radiolabelled purines or purine nucleosides revealed that adenine, adenosine and hypoxanthine are incorporated into adenine nucleotides and IMP through a similar channel while guanine and guanosine are salvaged into guanine nucleotides via another route. There appears to be no direct interconversion between adenine and guanine nucleotides. Interconversion between AMP and IMP was observed. 4. Assays of purine salvage enzymes revealed that P. hominis possess adenosine kinase; adenosine, guanosine and inosine phosphotransferases; adenosine, guanosine and inosine phosphorylases and AMP deaminase.     

Purine requirements for the expression of Cape buffalo serum trypanocidal activity

Cape buffalo serum contains xanthine oxidase which generates trypanocidal H₂O₂ during the catabolism of hypoxanthine and xanthine. The present studies show that xanthine oxidase-dependent trypanocidal activity in Cape buffalo serum was also elicited by purine nucleotides, nucleosides, and bases even though xanthine oxidase did not catabolize those purines. The paradox was explained in part, by the presence in serum of purine nucleoside phosphorylase and adenosine deaminase, that, together with xanthine oxidase, catabolized adenosine, inosine, hypoxanthine, and xanthine to uric acid yielding trypanocidal H₂O₂. In addition, purine catabolism by trypanosomes provided substrates for serum xanthine oxidase and was implicated in the triggering of xanthine oxidase-dependent trypanocidal activity by purines that were not directly catabolized to uric acid in Cape buffalo serum, namely guanosine, guanine, adenine monophosphate, guanosine diphosphate, adenosine 3′:5-cyclic monophosphate, and 1-methylinosine. The concentrations of guanosine and guanine that elicited xanthine oxidase-dependent trypanocidal activity were 30–270-fold lower than those of other purines requiring trypanosome-processing which suggests differential processing by the parasites.     

Conversion of purines to xanthine by Methanococcus vannielii

Based on the finding that Methanococcus vannielii can employ any of several purines as the sole nitrogen source, an investigation was undertaken to elucidate the pathways of purine metabolism in this organism. Cell-free extracts of M. vannielii converted guanine, uric acid, and hypoxanthine to xanthine and also formed guanine from guanine nucleotides or guanosine. The conversions of guanine and uric acid to xanthine appear to occur by pathways similar to those described in clostridia. The conversion of hypoxanthine to xanthine, however, is different than that described for Clostridium cylindrosporum and C. acidiurici, but is similar to that of C. purinolyticum, and apparently involves the direct oxidation of hypoxanthine to xanthine.     

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

To find general metabolic profiles of purine ribo- and deoxyribonucleotides in potato (Solanum tuberosum L.) plants, we looked at the in situ metabolic fate of various ¹⁴C-labelled precursors in disks from growing potato tubers. The activities of key enzymes in potato tuber extracts were also studied. Of the precursors for the intermediates in de novo purine biosynthesis, [¹⁴C]formate, [2-¹⁴C]glycine and [2-¹⁴C]5-aminoimidazole-4-carboxyamide ribonucleoside were metabolised to purine nucleotides and were incorporated into nucleic acids. The rates of uptake of purine ribo- and deoxyribonucleosides by the disks were in the following order: deoxyadenosine > adenosine > adenine > guanine > guanosine > deoxyguanosine > inosine > hypoxanthine > xanthine > xanthosine. The purine ribonucleosides, adenosine and guanosine, were salvaged exclusively to nucleotides, by adenosine kinase (EC 2.7.1.20) and inosine/guanosine kinase (EC 2.7.1.73) and non-specific nucleoside phosphotransferase (EC 2.7.1.77). Inosine was also salvaged by inosine/guanosine kinase, but to a lesser extent. In contrast, no xanthosine was salvaged. Deoxyadenosine and deoxyguanosine, was efficiently salvaged by deoxyadenosine kinase (EC 2.7.1.76) and deoxyguanosine kinase (EC 2.7.1.113) and/or non-specific nucleoside phosphotransferase (EC 2.7.1.77). Of the purine bases, adenine, guanine and hypoxanthine but not xanthine were salvaged for nucleotide synthesis. Since purine nucleoside phosphorylase (EC 2.4.2.1) activity was not detected, adenine phosphoribosyltransferase (EC 2.4.2.7) and hypoxanthine/guanine phosphoribosyltransferase (EC 2.4.2.8) seem to play the major role in salvage of adenine, guanine and hypoxanthine. Xanthine was catabolised by the oxidative purine degradation pathway via allantoin. Activity of the purine-metabolising enzymes observed in other organisms, such as purine nucleoside phosphorylase (EC 2.4.2.1), xanthine phosphoribosyltransferase (EC 2.4.2.22), adenine deaminase (EC 3.5.4.2), adenosine deaminase (EC 3.5.4.4) and guanine deaminase (EC 3.5.4.3), were not detected in potato tuber extracts. These results suggest that the major catabolic pathways of adenine and guanine nucleotides are AMP → IMP → inosine → hypoxanthine → xanthine and GMP → guanosine → xanthosine → xanthine pathways, respectively. Catabolites before xanthosine and xanthine can be utilised in salvage pathways for nucleotide biosynthesis.     

Purine metabolism and urate biosynthesis in isolated chicken hepatocytes

The metabolism of some purine compounds to urate and their effects on de novo urate synthesis in chicken hepatocytes were investigated. The purines, listed in descending order of rates of catabolism to urate, were hypoxanthine, xanthine, inosine, guanosine, guanine, IMP, GMP, adenosine, AMP, and adenine. During a 1-h incubation period, conversion to urate accounted for more than 80% of the total quantities of guanine, guanosine, and inosine metabolized, but only 42% of the adenosine and 23% of the adenine metabolism. Adenine, adenosine, and AMP inhibited de novo urate synthesis [( 14C]formate incorporation into urate), whereas the other purines, especially guanine, guanosine, and GMP, stimulated de novo urate synthesis. When hepatocytes were incubated with glutamine and adenosine, AMP, guanine, guanosine, or GMP, the rates of de novo urate synthesis were lower than the additive effects of glutamine and the purine in separate incubations. Increasing phosphate concentrations had no effect on urate synthesis in the absence of added purines but, in combination with adenosine, AMP, guanosine, or GMP, increased urate synthesis. These results indicate that the ratio of adenine to guanine nucleotides and the interaction between substrates and purine nucleotides are involved in the regulation of urate biosynthesis in chicken liver.     

Purine metabolism in mesophyll protoplasts of tobacco (Nicotiana tabacum) leaves.

The overall metabolism of purines was studied in tobacco (Nicotiana tabacum) mesophyll protoplasts. Metabolic pathways were studied by measuring the conversion of radioactive adenine, adenosine, hypoxanthine and guanine into purine ribonucleotides, ribonucleosides, bases and nucleic acid constituents. Adenine was extensively deaminated to hypoxanthine, whereupon it was also converted into AMP and incorporated into nucleic acids. Adenosine was mainly hydrolysed to adenine. Inosinate formed from hypoxanthine was converted into AMP and GMP, which were then catabolized to adenine and guanosine respectively. Guanine was mainly deaminated to xanthine and also incorporated into nucleic acids via GTP. Increased RNA synthesis in the protoplasts resulted in enhanced incorporation of adenine and guanine, but not of hypoxanthine and adenosine, into the nucleic acid fraction. The overall pattern of purine-nucleotide metabolic pathways in protoplasts of tobacco leaf mesophyll is proposed.     

Purine metabolism in Acholeplasma laidlawii B: novel PPi-dependent nucleoside kinase activity

Acholeplasma laidlawii B-PG9 was examined for 16 cytoplasmic enzymes with activity for purine salvage and interconversion. Phosphoribosyltransferase activities for adenine, guanine, xanthine, and hypoxanthine were shown. Adenine, guanine, xanthine, and hypoxanthine were ribosylated to their nucleoside. Adenosine, inosine, xanthosine, and guanosine were converted to their base. No ATP-dependent phosphorylation of nucleosides to mononucleotides was found. However, PPi-dependent phosphorylation of adenosine, inosine, and guanosine to AMP, inosine monophosphate, and GMP, respectively, was detected. Nucleotidase activity for AMP, inosine monophosphate, xanthosine monophosphate, and GMP was also found. Interconversion of GMP to AMP was detected. Enzyme activities for the interconversion of AMP to GMP were not detected. Therefore, A. laidlawii B-PG9 cannot synthesize guanylates from adenylates or inosinates. De novo synthesis of purines was not detected. This study demonstrates that A. laidlawii B-PG9 has the enzyme activities for the salvage and limited interconversion of purines and, except for purine nucleoside kinase activity, is similar to Mycoplasma mycoides subsp. mycoides. This is the first report of a PPi-dependent nucleoside kinase activity in any organism.     

Preservation of red blood cells with purines and nucleosides. II. Uptake and utilization of purines and nucleosides by stored red blood cells

The uptake of adenine, guanine, guanosine and inosine by stored red cells was investigated in whole blood and red cell resuspensions at initial concentrations of 0.25, 0.5 and 0.75 mM for adenine and 0.5 mM for the other additives using a rapid ion-exchange chromatographic microanalysis of purines and nucleosides in plasma and whole blood. Increasing adenine concentrations from 0.25 to 0.75 mM in blood elevated the adenine uptake from 0.3 up to 0.8 mmol/l red cells during 2 hours after collecting blood. The intra-/extracellular distribution ratio changed from 1 : 1.3 to 1: 1.7. Some 2 hours after withdrawing blood into CPD--solution with purines and nucleosides the uptake of adenine and guanine resulted in 40 per cent and 70 per cent respectively and of guanosine and inosine in 80 and 90 per cent respectively. The replacement of plasma by a resuspending solution gave the same uptake rates for purines and nucleosides. The nucleosides were rapidly split to purines and R-1-P and disappeared from blood during one week. Adenine and guanine were utilized to 80 to 90 per cent only after 3 weeks. During the same period the utilization of guanine was smaller by 40 per cent than that of adenine due to the different activity of the purine nucleoside phosphorylase for these substrates. The plasma of all analyzed blood samples contained hypoxanthine and inosine, but guanine and guanosine were detected only in those samples to which one of them was added. After 3 weeks of storage the highest concentration of hypoxanthine was found in CPD-AI blood with 600 microM in plasma and the highest concentration of synthesized inosine in CPD-AG blood with a concentration of 100 microM in plasma. Three ways of utilization of purines by stored red cells were discussed : the synthesis of nucleotide monophosphates, the formation of nucleosides, and the deamination. The portions of these ways change during storage. The most effective concentrations of adenine and guanosine in stored blood seems to be 0.25 and 0.5 mM respectively. The full utilization of the nucleoside requires the addition of inorganic phosphate.     

Absorption and metabolism of purines by the small intestine of the chicken.

1. Absorption of purines and their metabolism by the small intestine were estimated by using the everted gut sacs from the duodenum, jejunum and ileum of the chicken. 2. When no purine was added to the mucosal fluid, large amounts of uric acid, much less but appreciable adenine, hypoxanthine and xanthine and no detectable guanine were released from both sides of all segments of the small intestine, and these released amounts were largest in the duodenum. 3. Similar absorption rates of adenine from the jejunum and ileum were about 1.7-3.0 times as high as those of hypoxanthine and uric acid from these intestines and those of adenine and uric acid from the duodenum (P less than 0.05). 4. Guanine was not absorbed unchanged from any segments of the intestine and a little xanthine was absorbed only from the jejunum and ileum. 5. Guanine and xanthine seem to be absorbed in uric acid form, hypoxanthine in xanthine and uric acid forms and adenine in hypoxanthine form, from the small intestine especially from the jejunum. 6. Adenine, guanine, xanthine and hypoxanthine were greatly metabolized in the mucosa of the duodenum, and the conversions of hypoxanthine to xanthine and uric acid were most active.     

Characterizing Substrate Properties of Purine-Related Compounds with Purine Metabolism Enzymes for Enzymatic Peak-Shift HPLC Method

We have extended peak-shift method for measuring purine bases to make it suitable for other purine-related compounds. We optimized the reactions of the purine metabolism enzymes 5′-nucleotidase (EC 3.1.3.5), purine nucleoside phosphorylase (PNP) (EC 2.4.2.1), xanthine oxidase (XO) (EC 1.17.3.2), urate hydroxylase (EC 1.7.3.3), adenosine deaminase (ADA) (EC 3.5.4.4), and guanine deaminase (EC 3.5.4.3) by determining their substrate specificity and reaction kinetics. These enzymes eliminate the five purine base peaks (adenine, guanine, hypoxanthine, xanthine, and uric acid) and four nucleosides (adenosine, guanosine, inosine, and xanthosine). The bases and nucleosides can be identified and accurately quantified by comparing the chromatograms before and after treatment with the enzymes. Elimination of the individual purine compound peaks was complete in a few minutes. However, when there were multiple substrates, such as for XO, and when the metabolites were purine compounds, such as for PNP and ADA, it took longer to eliminate the peaks. The optimum reaction conditions for the peak-shift assay methods were an assay mixture containing the substrate (10 μL, 0.1 mg/mL), the combined enzyme solution (10 μL each, optimum concentration), and 50 mM sodium phosphate (up to 120 μL, pH 7.4). The mixture was incubated for 60 minutes at 37°C. This method should be suitable for determining the purine content of a variety of samples, without interference from impurities.     

Purine uptake by Alcaligenes eutrophus H16

The uptake of adenine, guanine, xanthine, hypoxanthine and uric acid by whole cells was studied, using spectrophotometric techniques, ¹⁴C-labelled compounds and metabolic inhibitors. Three different non-constitutive systems were shown to maintain the uptake of adenine and that of the pairs guanine/hypoxanthine and xanthine/uric acid. —Active transport of adenine was induced by adenine only, but passive uptake was also involved. Maximum K _T values of 110–131 M were observed at the pH optimum of 8.0. —Guanine and hypoxanthine were translocated by one single mechanism as indicated by K _T and K _I values. This system was induced by both these substances but its affinity was 51/2-times higher for guanine than for hypoxanthine; it was noncompetitively stimulated by Mg²⁺. — A further system, induced by xanthine and uric acid, catalyzed the uptake of both these compounds. It exhibited two pH optima (at pH 6.6 and 7.9); inactivation by heat and stimulation or inhibition by several compounds indicated that two separate mechanisms might be involved in the uptake of xanthine and uric acid.     

Utilization of adenine and guanine as nitrogen sources by Chlamydomonas reinhardtii cells

Chlamydomonas reinhardtü Dangeard, adenine or guanine can be used as the sole nitrogen source for growth by means of an inducible system which is repressed by ammonia. Cells grown on either adenine or guanine were able to take up both purines, although the adenine uptake rate was always about 40% of the guanine uptake rate. Both adenine and guanine were taken up by an inducible system(s) exhibiting hyperbolic kinetics with identical apparent A, values of 3-2 mmol m^?3 for adenine and 3-2mmol m^?3 for guanine. Adenine and guanine utilization depended on pH, with similar optimal pH values of 7·3 and 7·4, respectively. Adenine and guanine each acted as a competitive inhibitor of the other's uptake, and their utilization was also inhibited by hypoxanthine, xanthine and urate. Inhibition of adenine uptake by guanine and hypoxanthine was competitive, with A′, values of 5·5 and 1. 6 mmol m^?3 respectively. Guanine uptake was also inhibited competitively by adenine (K₁= 1·3mmol m^?3) and hypoxanthine (K₁= 3. 3 mmol m^?3). Utilization of both adenine and guanine was inhibited by cyanide, azide, 3-(3,4-dichlorophenyl)-1,1-dimethyl urea, 2,4-dinitrophenol and carbonylcyanide m-chlorophenylhydrazone, and was also sensitive to p-hydroxymercuribenzoate and N-ethyl-maleimide. On the basis of these results, taken together, the possibility that adenine and guanine are translocated into Chlamydomonas by a common system is discussed.     

降血尿酸益生菌株的筛选和降血尿酸机理的探索

【背景】高尿酸血症是人体内血尿酸含量显著高于正常水平的代谢性疾病,利用益生菌降解食物中外源性嘌呤类成分成为治疗高尿酸血症的新方法。【目的】筛选具有降低血尿酸作用的益生菌,并探索其作用机制。【方法】利用HPLC从多株实验菌株中筛选降解核苷酸(腺苷酸、鸟苷酸)、核苷(腺苷、鸟苷)、嘌呤(黄嘌呤、次黄嘌呤、鸟嘌呤)、尿酸能力最强的益生菌。首次利用质谱定性与定量检测菌株降解核苷与核苷酸过程中代谢物的变化,结合菌株对高尿酸血症模型大鼠血尿酸水平的影响,初步探索其降低血尿酸的机理。【结果】首次筛选出具有较强降解核苷酸与核苷能力的干酪乳杆菌ZM15(CGMCC No.13980),高尿酸血症模型大鼠验证其具有降低血尿酸的作用。结果显示菌株ZM15在胞内降解核苷酸、核苷后,胞内、外均测到鸟嘌呤、黄嘌呤、次黄嘌呤,且胞内3种嘌呤含量显著高于正常菌体内含量(P0.01),尿酸和尿囊素在胞内、外均未发现。【结论】干酪乳杆菌ZM15具有较强的降解核苷酸、核苷的能力,推测其主要通过与肠道上皮细胞竞争吸收核苷酸与核苷,从而对高尿酸血症模型大鼠具有降血尿酸作用。     

Biosynthesis of Ureides from Purines in a Cell-free System from Nodule Extracts of Cowpea [Vigna unguiculata (L) Walp.

The synthesis of ¹⁴C-labeled xanthine/hypoxanthine, uric acid, allantoin, allantoic acid, and urea from [8-¹⁴C]guanine or [8-¹⁴C]hypoxanthine, but not from [8-¹⁴C]adenine, was demonstrated in a cell-free extract from N₂-fixing nodules of cowpea (Walp.). The ¹⁴C recovered in the acid/neutral fraction was present predominantly in uric acid and allantoin (88-97%), with less than 10% of the ¹⁴C in allantoic acid and urea. Time courses of labeling in the cell-free system suggested the sequence of synthesis from guanine to be uric acid, allantoin, and allantoic acid. Ureide synthesis was confined to soluble extracts from the bacteroid-containing tissue, was stimulated by pyridine nucleotides and intermediates of the pathways of aerobic oxidation of ureides, but was completely inhibited by allopurinol, a potent inhibitor of xanthine dehydrogenase (EC 1.2.1.37). The data indicated a purine-based pathway for ureide synthesis by cowpea nodules, and this suggestion is discussed.     

NUCLEOTIDE METABOLISM IN RAT BRAIN

Abstract— The uptake, the conversion to nucleotides, and their incorporation into RNA for labelled glycine, aspartate, the free bases and nucleosides of purines and pyrimidines were investigated with cortical slices of rat cerebrum. At the end of a 1-hr incubation time the slice-to-medium ratio of the radioactivities for labelled aspartate, glycine, adenine and adenosine were 34, 26, 20 and 5, respectively, while the slice-to-medium ratios for hypoxanthine, inosine, guanine, guanosine, xanthine, orotate, cytidine, cytosine, uridine, and uracil ranged from 1.3:1 to 2:1. Over 99 per cent of the total radioactivity taken up by the cortical slices was present in the TCA supernatant and 86, 82, 65, 50, 34, 23, 20 and 1.6 per cent of this radioactivity was in the form of nucleotides at the end of a 1-hr incubation with labelled adenine, adenosine, hypoxanthine, inosine, uridine, orotate, cytidine, and glycine, respectively. The incorporation of various radioactive precursors into RNA of cortical slices suggests that nucleotides originating from either de novo synthesis or preformed purine derivatives enter the same nucleotide pool utilized for RNA synthesis. The supernatant fraction from homogenized cerebrum was investigated for the presence of various anabolic and catabolic enzymes associated with nucleotide metabolism. These results were correlated with the data from the RNA incorporation studies, and a possible role for AMP: pyrophosphate phosphoribosyltransferase (adenine phosphoribosyltransferase, I.U.B. 2.4.2.7) to achieve intercellular transfer of AMP is discussed.     

Effect of purines on the oxidation of ascorbic acid induced by copper

Summary Uric acid and other purines including 1-methyl-, 7-methyl-, and 1,7-dimethyluric acid, adenine, guanine, xanthine, hypoxanthine, purine, and the structurally similar compound allopurinol protected ascorbic acid from oxidation catalyzed by copper. If the hydrogen at either the 3 or 9 nitrogen of the uric acid was replaced by a methyl group, the compound did not protect ascorbate. 3-Ribosyluric acid, xanthosine, adenosine, and guanosine also failed to protect ascorbate. It was concluded that in order for purines to complex with copper to protect ascorbate from copper-catalyzed oxidation, the nitrogens at both positions 3 and 9 of the purine must be unsubstituted.     

A comparison of purine metabolism and nucleotide pools in normal and hypoxanthine-guanine phosphoribosyltransferase-deficient neuroblastoma cells

Purine nucleotide synthesis and interconversion were examined over a range of purine base and nucleoside concentrations in intact N4 and N4TG (hypoxanthine-guanine phosphoribosyltransferase (HGPRT) deficient) neuroblastoma cells. Adenosine was a better nucleotide precursor than adenine, hypoxanthine or guanine at concentrations greater than 100 μM. With hypoxanthine or guanine, N4TG cells had less than 2% the rate of nucleotide synthesis of N4 cells. At substrate concentrations greater than 100 μM the rates for deamination of adenosine and phosphorolysis of guanosine exceeded those for any reaction of nucleotide synthesis. Labelled inosine and guanosine accumulated from hypoxanthine and guanine, respectively, in HGPRT-deficient cells and the nucleosides accumulated to a greater extent in N4 cells indicating dephosphorylation of newly synthesized IMP and GMP to be quantitatively significant. A deficiency of xanthine oxidase, guanine deaminase and guanosine kinase activities was found in neuroblastoma cells. Hypoxanthine was a source for both adenine and guanine nucleotides, whereas adenine or guanine were principally sources for adenine (>85%) or guanine (>90%) nucleotides, respectively. The rate of [¹⁴C]formate incorporation into ATP, GTP and nucleic acid purines was essentially equivalent for both N4 and N4TG cells. Purine nucleotide pools were also comparable in both cell lines, but the concentration of UDP-sugars was 1.5 times greater in N4TG than N4 cells.     

Utilization of organic compounds as the sole source of nitrogen by Thiobacillus thiooxidans

Thiobacillus thiooxidans DSM 504 was shown to grow with adenine, hypoxanthine, xanthine and uric acid as sole sources of nitrogen. Growth with these compounds was observed after lag periods of varying lengths, unless the cells had been previously grown with the same purine base. The disappearance of adenine was accompanied by a temporary accumulation of hypoxanthine in the medium. The utilization of purines was inhibited by ammonia (1 mM). Guanine, pyrimidines and some other organic compounds were not utilized.Non-standard abbreviation U-¹⁴C uniformly labeled by ¹⁴C     

The purine and pyrimidine metabolism of Tetrahymena pyriformis

The metabolism of purines and pyrimidines by the ciliated protozoan Tetrahymena was investigated with the use of enzymatic assays and radioactive tracers. A survey of enzymes involved in purine metabolism revealed that the activities of inosine and guanosine phosphorylase (purine nucleoside: orthophosphate ribosyltransferase, E.C. 2.4.2.1) were high, but adenosine phosphorylase activity could not be demonstrated. The apparent K_m for guanosine in the system catalyzing its phosphorolysis was 4.1 ± 0.6 × 10^?3 M. Pyrophosphorylase activities for IMP and GMP (GMP: pyrophosphate phosphoribosyltransferase, E.C. 2.4.2.8), AMP (AMP: pyrophosphate phosphoribosyltransferase, E.C. 2.4.2.7), and 6-mercaptopurine ribonucleotide were also found in this organism; but a number of purine and pyrimidine analogs did not function as substrates for these enzymes. The metabolism of labeled guanine and hypoxanthine by intact cells was consistent with the presence of the phosphorylases and pyrophosphorylases of purine metabolism found by enzymatic studies. Assays for adenosine kinase (ATP: adenosine 5'-phosphotransferase, E.C. 2.7.1.20) inosine kinase, guanosine kinase, xanthine oxidase (xanthine: O₂ oxidoreductase, E.C. 1.2.3.2), and GMP reductase (reduced-NADP: GMP oxidoreductase [deaminating], E.C. 1.6.6.8) were all negative. In pyrimidine metabolism, cytidine-deoxycytidine deaminase (cytidine aminohydrolase, E.C. 3.5.4.5), thymidine phosphorylase (thymidine: orthophosphate ribosyltransferase, E.C. 2.4.2.4), and uridine-deoxyuridine phosphorylase (uridine: orthophosphate ribosyltransferase, E.C. 2.4.2.3) were active; but cytidine kinase, uridine kinase (ATP: uridine 5'-phosphotransferase, E.C. 2.7.1.48), and CMP pyrophosphorylase could not be demonstrated.     

Purine metabolite concentrations in portal and peripheral blood of steers, sheep and rats.

1. The concentration of purine derivatives in portal and peripheral blood of steers, sheep and rats was measured by reverse-phase high performance liquid chromatography. 2. Nucleotides, nucleosides (apart from inosine), adenine and guanine were not found in the plasma samples. Allantoin, uric acid, hypoxanthine and xanthine accounted for virtually all purine metabolites in plasma samples. 3. Non-oxidized derivatives (hypoxanthine and xanthine) were consistently detected in sheep but not in steer or rat plasma samples showing a differential availability of reutilizable purine derivatives between species.