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.     

De novo purine biosynthesis in the crustacean Artemia: Influence of salinity and geographical origin

Summary In vivo studies of the incoporation of [U-¹⁴C]glycine into purine nucleotides have established the de novo pathway for purine biosynthesis in Artemia sp. during the early period of larval development. This pathway can be modified by the salt concentration of the incubation media. In addition, Artemia of different geographical origins may differ with respect to the detection, functionality and variability of this metabolical pathway.Abbreviations ADP adenosine, diphosphate - ASN acid soluble nucleotides - ATP adenosine triphosphate - DNA desoxyribonucleic acid - GDP guanosine diphosphate - GP₄G pl, p4-diguanosine 5-tetraphosphate - HPLC high performance liquid chromatography - PCA perchloric acid - RNA ribonucleic acid     

The catabolism of endogenous adenine nucleotides in rat liver mitochondria

Summary The degradation of intramitochondrial adenine nucleotides to nucleosides and bases was investigated by incubating isolated rat liver mitochondria at 37°C under non-phosphorylating conditions in the presence of oligomycin and carboxyatractyloside. Within 30 min the adenine nucleotides were degraded by about 25 per cent. The main products formed were adenosine and inosine the contents of which increased five- to sevenfold.Compartmentation studies revealed that about 50 to 60 per cent of the adenosine formed remained inside the organelles whereas inosine was almost completely released into the surrounding medium. Outside the mitochondria only very small amounts of adenine nucleotides were detected. Similar incubations in the presence of [¹⁴C]-adenosine yielded no [¹⁴C]-inosine ruling out extramitochondrial adenosine deamination.It is concluded that endogenous adenine nucleotides can be degraded in mitochondria via AMP dephosphorylation and subsequent adenosine deamination. A purine nucleoside transport system mediating at least the efflux of inosine from the mitochondria is suggested.     

Uric acid synthesis by rat liver supernatants from purine bases,nucleosides and nucleotides. Effect of allopurinol

The synthesis of uric acid from purine bases, nucleosides and nucleotides has been measured in reaction mixtures containing rat liver supernatant and each one of the following compounds at 1 mM concentration (except xanthine, 0·5 mM and guanosine and guanine, 0·1 mM). The rates of the reaction, expressed as nanomoles of uric acid synthesized g^?1 of wet liver min^?1 were: ATP, 10; ADP, 37; AMP, 62; adenosine, 108; adenine 6; adenylo-succinate, 9; IMP 32; inosine, 112; hypoxanthine, 50; GTP, 19; GDP, 19; GMP, 27; guanosine, 34; guanine, 72; XMP, 10; xanthosine, 24; xanthine, 144. These figures divided by 55 correspond to nanomoles of uric acid synthesized min^?1 per mg^?1 of protein. The rate of synthesis of uric acid obtained with each one of those compounds at 0·1 and 0·05 mM concentrations was also determined. ATP (1 nM) strongly inhibited uric acid synthesis from 0·05 mM AMP (91 per cent) and from 0·05 mM ADP (88 per cent), but not from adenosine. CTP or UTP (1 mM ) also inhibited (by more than 90 per cent) the synthesis of uric acid from 0·05 mM AMP. Xanthine oxidase was inhibited by concentrations of hypoxanthine higher than 0·012 mM. The results favour the view that the level of uric acid in plasma may be an index of the energetic state of the organism. Allopurinol, besides inhibiting uric acid synthesis, reduced the rate of degradation of AMP. The ability of crude extracts to catabolize purine nucleotides to uric acid is an important factor to be considered when some enzymes related to purine nucleotide metabolism, particularly CTP synthase, are measured in crude liver extracts.     

Colorimetric Determination of α-Amino Nitrogen in Urine and Plasma with Ninhydrin Reaction

Bacillus stearothermophilus TH 6–2 has two kinds of purine nucleoside phosphorylases (Pu-NPase I and Pu-NPase II). The Pu-NPase I is a functional homolog of eukaryotic purine nucleoside phosphorylases that can catalyze the phosphorolysis of inosine and guanosine, but not adenosine, the primary substrate of Pu-NPase II. The Pu-NPase I gene of TH 6–2 has been cloned, sequenced, and expressed in E. coli. The gene corresponded to an open reading frame of 822 nucleotides that translates into a putative 274-amino acid protein with a molecular weight of 29,637. The deduced amino terminus sequence completely coincided with that found for the purified enzyme. The cloned gene was overexpressed in E. coli by using the trc promoter to produce an active enzyme in large quantities. The amino acid sequence of Pu-NPase I shared 50% similarity with those of human and mouse purine nucleoside phosphorylases.     

Sugar-free growth of mammalian cells on some ribonucleosides but not on others

It was shown earlier that a variety of vertebrate cells could grow indefinitely in sugar-free medium supplemented with either uridine or cytidine at greater than or equal to 1 mM. In contrast, most purine nucleosides do not support sugar-free growth for one of the following reasons. The generation of ribose-1-P from nucleoside phosphorylase activity is necessary to provide all essential functions of sugar metabolism. Some nucleosides, e.g. xanthosine, did not support growth because they are poor substrates for this enzyme. De novo pyrimidine synthesis was inhibited greater than 80% by adenosine or high concentrations of inosine, e.g. 10 mM, which prevented growth on these nucleosides; in contrast, pyrimidine synthesis was inhibited only marginally on 1 mM inosine or guanosine, but normal growth was only seen on 1 mM inosine, not on guanosine. The inhibition of de novo adenine nucleotide synthesis prevented growth on guanosine, since guanine nucleotides could not be converted to adenine nucleotides. Guanine nucleotides were necessary for this inhibition of purine synthesis, since a mutant blocked in their synthesis grew normally on guanosine. De novo purine synthesis was severely inhibited by adenosine, inosine, or guanosine, but in contrast to guanosine, adenosine and inosine could provide all purine requirements by direct nucleotide conversions.     

Metabolism of Exogenous Purine Bases and Nucleosides by Salmonella typhimurium

Purine-requiring mutants of Salmonella typhimurium LT2 containing additional mutations in either adenosine deaminase or purine nucleoside phosphorylase have been constructed. From studies of the ability of these mutants to utilize different purine compounds as the sole source of purines, the following conclusions may be drawn. (i) S. typhimurium does not contain physiologically significant amounts of adenine deaminase and adenosine kinase activities. (ii) The presence of inosine and guanosine kinase activities in vivo was established, although the former activity appears to be of minor significance for inosine metabolism. (iii) The utilization of exogenous purine deoxyribonucleosides is entirely dependent on a functional purine nucleoside phosphorylase. (iv) The pathway by which exogenous adenine is converted to guanine nucleotides in the presence of histidine requires a functional purine nucleoside phosphorylase. Evidence is presented that this pathway involves the conversion of adenine to adenosine, followed by deamination to inosine and subsequent phosphorolysis to hypoxanthine. Hypoxanthine is then converted to inosine monophosphate by inosine monophosphate pyrophosphorylase. The rate-limiting step in this pathway is the synthesis of adenosine from adenine due to lack of endogenous ribose-l-phosphate.     

Biosynthesis of coenzyme F420 and methanopterin in Methanobacterium thermoautotrophicum

Growing cultures of Methanobacterium thermoautotrophicum were supplemented with [U-¹⁴C]adenosine or [1-¹⁴C]adenosine. 7,8-Didemethyl-8-hydroxy-5-deazariboflavin (factor F₀) and 7-methylpterin were isolated from the culture medium. Hydrolysis of cellular RNA yielded purine and pyrimidine nucleotides. The ribose side chain of proffered adenosine is efficiently incorporated into cellular adenosine and guanosine nucleotide pools but not into pyrimidine nucleotides. Thus, M. thermoautotrophicum can utilize exogenous adenosine by direct phosphorylation without hydrolysis of the glycosidic bond, and AMP can be efficiently converted to GMP. Factor F₀ and 7-methylpterin had approximately the same specific activities as the purine nucleotides. It follows that the ribityl side chain of factor F₀ is derived from the ribose side chain of a nucleotide precursor by reduction. The pyrazine ring of methanopterin is formed by ring expansion involving the ribose side chain of the precursor, GTP.Abbreviations Factor F₀ 8-hydroxy-6,7-didemethyl-5-deazariboflavin - APRT adenine phosphoribosyltransferase - GPRT guanine phosphoribosyltransferase - PRPP phosphoribosylpyrophosphate - HPLC high performance liquid chromatography     

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.     

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 interconversions in the Australian termite,Nasutitermes walkeri hill

Purine interconversions leading to urate synthesis were studied over 60 min in isolated fat bodies from freshly collected Nasutitermes walkeri using ¹⁴C-hypoxanthine, ¹⁴C-inosine and ¹⁴C-guanosine. All were taken up, inosine the most efficiently at an initial rate of 0.06 ± 0.009 nmol/min/mg protein. The major purines, nucleosides and nucleotides were separated and examined for radioactivity. Based on uptake data, 1.3% of ¹⁴C-hypoxanthine, 0.3% of ¹⁴C-inosine and 37% of ¹⁴C-guanosine were converted to urate while 3% of the ¹⁴C-guanosine taken up was also salvaged to nucleotides. Feeding experiments with allopurinol showed that there was no significant production of urate via guanine, guanosine and adenosine. Incorporation data indicated the presence of the enzymes purine nucleoside phosphorylase, xanthine dehydrogenase, guanase and that neither inosine nor hypoxanthine could be salvaged.     

Purine salvage in Entamoeba histolytica

Pulse-labeling of the nucleotide pool in Entamoeba histolytica with radioactive precursors, and subsequent high performance liquid chromatographic (HPLC) analysis of the radiolabeled nucleotides, indicate that E. histolytica is incapable of de novo synthesis of purine nucleotides. Hypoxanthine, inosine and xanthine could not be converted to nucleotides in E. histolytica, which suggests the absence of interconversion between adenine nucleotides and guanine nucleotides through formation of IMP. Adenosine was actively incorporated into nucleotides at an initial rate of 130 pmoles per minute per 10(6) trophozoites. Adenine, guanosine and guanine were also incorporated at much lower rates. The rate of adenine incorporation was enhanced by the presence of guanosine; the rate of guanine incorporation was significantly increased by adenosine. These stimulatory effects suggest that the ribose moiety of adenosine or guanosine can be transferred to another purine base to form a new nucleoside, and that the purine nucleosides are the immediate precursors of E. histolytica nucleotides. HPLC results showed that the radiolabel in adenine was exclusively incorporated into adenine nucleotides and that guanine was found only among guanine nucleotides, whereas the radioactivity associated with the ribose moiety of adenosine or guanosine was distributed among both adenine and guanine nucleotides.     

Purine Uptake and Utilization by the Avian Malaria Parasite Plasmodium lophurae*

SYNOPSIS. Plasmodium lophurae cannot carry out extensive de novo purine biosynthesis, and depends upon the host erythrocyte for a supply of preformed purines. Exogenous purines taken up by the parasitized erythrocyte may constitute a major source of preformed purines for parasite nucleotide biosynthesis. The uptake of exogenous radioactive purine compounds and their incorporation into nucleic acids by duck erythrocytes parasitized with P. lophurae, uninfected erythrocytes, and erythrocyte-free parasites were studied. P. lophurae was found to have a remarkable ability, both intracellularly and extracellularly, to take up and utilize certain exogenous purines such as adenosine, inosine, and hypoxanthine. Incorporation studies indicated that this species has a functional purine salvage pathway by which inosine, hypoxanthine, and adenosine can be converted to both adenine and guanine nucleotides.     

Purine metabolism in Leishmania donovani and Leishmania braziliensis

We have studied purine metabolism in the culture forms of Leishmania donovani and Leishmania braziliensis. These organisms are incapable of synthesizing purines de novo from glycine, serine, or formate and require an exogenous purine for growth. This requirement is better satisfied by adenosine or hypoxanthine than by guanosine. Bothe adenine and inosine are converted to a common intermediate, hypoxanthine, before transformation to nucleotides. This is due to the activity of an adenine aminohydrolase (EC 3.5.4.2), a rather unusual finding in a eukaryotic cell. There is a preferential synthesis of adenine nucleotides, even when guanine or xanthine are used as precursors.The pathways of purine nucleotide interconversions in these Leishmania resemble those found in mammalian cells except for the absence of de novo purine biosynthesis and the presence of an adenine-deaminating activity.     

Some kinetic studies on ribonucleosides degradation by extracts of penicillium citrinum

Cell-free extracts of 3–4 days old mats of nitrate-grown Penicillium citrinum catalyze the hydrolytic cleavage of the N-glycosidic bonds of inosine, guanosine and adenosine optimally at pH 4, 0.1 M citrate buffer. The same extracts catalyze the hydrolytic deamination of cytidine at a maximum rate in 0.08 M Tris-acetate buffer pH 6.5, 40°C and 50°C were the most suitable degrees for purine nucleoside hydrolysis and cytidine deamination, respectively. The incubation of the extracts at 60°C, in the absence of cytidine caused a loss in the deaminating activity, while freezing and thawing had no effect on both activities. The deaminating activity seems to be cytidine specific as neither cytosine, adenine, adenosine nor guanosine could be deaminated. Uridine competively inhibited this activity, while ammonia had no effect. The apparent K_m value of this enzyme for cytidine was 1.57×10^?3M and its K_i value for uridine was 7.8×10^?3M. The apparent K_m values of the N-glycosidic bond cleaving enzyme for inosine, guanosine and adenosine were 13.3, 14.2 and 20×10^?3 M, respectively.     

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.     

Metabolism of purine bases, nucleosides and alkaloids in theobromine-forming Theobroma cacao leaves

We examined the purine alkaloid content and purine metabolism in cacao (Theobroma cacao L.) plant leaves at various ages: young small leaves (stage I), developing intermediate size leaves (stage II), fully developed leaves (stage III) from flush shoots, and aged leaves (stage IV) from 1-year-old shoots. The major purine alkaloid in stage I leaves was theobromine (4.5 μmol g^–1 fresh weight), followed by caffeine (0.75 μmol g^–1 fresh weight). More than 75% of purine alkaloids disappeared with subsequent leaf development (stages II–IV). In stage I leaves, ¹⁴C-labelled adenine, adenosine, guanine, guanosine, hypoxanthine and inosine were converted to salvage products (nucleotides and nucleic acids), to degradation products (ureides and CO₂) and to purine alkaloids (3- and 7-methylxanthine, 7-methylxanthosine and theobromine). In contrast, ¹⁴C-labelled xanthine and xanthosine were not used for nucleotide synthesis. They were completely degraded, but nearly 20% of [8-¹⁴C]Xanthosine was converted in stage I leaves to purine alkaloids. These observations are consistent with the following biosynthetic pathways for theobromine: (a) AMP → IMP → 5′-xanthosine monophosphate → xanthosine → 7-methylxanthosine → 7-methylxanthine → theobromine; (b) GMP → guanosine → xanthosine → 7-methylxanthosine → 7-methylxanthine → theobromine; (c) xanthine → 3-methylxanthine → theobromine. Although no caffeine biosynthesis from ¹⁴C-labelled purine bases and nucleosides was observed during 18 h incubations, exogenously supplied [8-¹⁴C]Theobromine was converted to caffeine in young leaves. Conversion of theobromine to caffeine may, therefore, be slow in cacao leaves. No purine alkaloid synthesis was observed in the subsequent growth stages (stages II–IV). Significant degradation of purine alkaloids was found in leaves of stages II and III, in which [8-¹⁴C]Theobromine was degraded to CO₂ via 3-methylxanthine, xanthine and allantoic acid. [8-¹⁴C]Caffeine was catabolised to CO₂ via theophylline (1,3-dimethylxanthine) or theobromine.     

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.     

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.     

Simultaneous quantification by HPLC of purines in umami soup stock and evaluation of their effects on extracellular and intracellular purine metabolism

Ribonucleotide flavor enhancers such as inosine monophosphate (IMP) and guanosine monophosphate (GMP) provide umami taste, similarly to glutamine. Japanese cuisine frequently uses soup stocks containing these nucleotides to enhance umami. We quantified 18 types of purines (nucleotides, nucleosides, and purine bases) in three soup stocks (chicken, consommé, and dried bonito soup). IMP was the most abundant purine in all umami soup stocks, followed by hypoxanthine, inosine, and GMP. The IMP content of dried bonito soup was the highest of the three soup stocks. We also evaluated the effects of these purines on extracellular and intracellular purine metabolism in HepG2 cells after adding each umami soup stock to the cells. An increase in inosine and hypoxanthine was evident 1 h and 4 h after soup stock addition, and a low amount of xanthine and guanosine was observed in the extracellular medium. The addition of chicken soup stock resulted in increased intracellular and extracellular levels of uric acid and guanosine. Purine metabolism may be affected by ingredients present in soups.