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.     

Degradation and resynthesis of adenine nucleotides in adult rat heart myocytes

The degradation and short-term resynthesis of adenine nucleotides have been examined in a preparation of isolated rat heart myocytes. These myocyte preparations are essentially free of vascular and endothelial cells, contain levels of adenine nucleotides quite comparable to those of intact heart tissue, and retain these components remarkably well for up to 2 h of aerobic incubation in the presence of 1 mM Ca2+. When the cells are rapidly and synchronously de-energized by addition of uncoupler, an inhibitor of respiration and iodoacetate, cellular ATP is degraded almost quantitatively to AMP. The AMP is then converted to either intracellular adenosine, which accumulates to high concentrations before release to the cell exterior, or to IMP. The relative contribution of these two pathways depends on the metabolic state of the cells just prior to de-energization, with IMP production favored when respiring cells are de-energized and adenosine formation predominant when glycolyzing myocytes are subjected to this treatment. Cells de-energized by anaerobiosis in the absence of glucose lose ATP and adenine nucleotides with the production of IMP and adenosine. Upon reoxygenation, these cells restore a high adenylate energy charge and about 60% of control levels of GTP. There is a net resynthesis of 5-7 nmol of adenine nucleotides.mg-1 protein with a corresponding decline in IMP. Added [14C]adenosine labels the adenine nucleotide pool, but little net resynthesis of adenine nucleotides via adenosine kinase can be detected. It therefore appears that a rapid regeneration of adenine nucleotides can occur via the enzymes of the purine nucleotide cycle in heart myocytes and is limited by the size of the IMP pool retained.     

Distinct roles for recombinant cytosolic 5'-nucleotidase-I and -II in AMP and IMP catabolism in COS-7 and H9c2 rat myoblast cell lines

Catabolism of AMP during ATP breakdown produces adenosine, which restores energy balance. Catabolism of IMP may be a key step regulating purine nucleotide pools. Two, cloned cytosolic 5'-nucleotidases (cN-I and cN-II) have been implicated in AMP and IMP breakdown. To evaluate their roles directly, we expressed recombinant pigeon cN-I or human cN-II at similar activities in COS-7 or H9c2 cells. During rapid (more than 90% in 10 min) or slower (30-40% in 10 min) ATP catabolism, cN-I-transfected COS-7 and H9c2 cells produced significantly more adenosine than cN-II-transfected cells, which were similar to control-transfected cells. Inosine and hypoxanthine concentrations increased only during slower ATP catabolism. In COS-7 cells, 5'-nucleotidase activity was not rate-limiting for inosine and hypoxanthine production, which was therefore unaffected by cN-II- and actually reduced by cN-I- overexpression. In H9c2 cells, in which 5'-nucleotidase activity was rate-limiting, only cN-II overexpression accelerated inosine and hypoxanthine formation. Guanosine formation from GMP was also increased by cN-II. Our results imply distinct roles for cN-I and cN-II. Under the conditions tested in these cells, only cN-I plays a significant role in AMP breakdown to adenosine, whereas only cN-II breaks down IMP to inosine and GMP to guanosine.     

Glutamine and glucose metabolism in thymocytes from normal and spontaneously diabetic BB rats.

Metabolism of glutamine and glucose was studied in thymocytes from normal rats and BB rats with the spontaneous autoimmune diabetic syndrome to assess their potential roles as fuels. The major measured products from glucose were lactate and, to a lesser extent, CO2, and pyruvate. Glutamine had no effect on the rates of their production from glucose. Glutamine was metabolized to ammonia, aspartate, glutamate, and CO2, with aspartate being the major product of carbons from glutamine in the absence of glucose. Glucose markedly decreased the formation of ammonia, aspartate, and CO2 from glutamine, but increased that of glutamate, with an overall decrease in glutamine utilization by 55%. More glutamate than aspartate was produced from glutamine in the presence of glucose. The potential production of ATP from glucose was similar to that when glutamine was present alone. However, glucose markedly decreased production of ATP from glutamine, but not vice versa. This resulted in ATP production from glucose being 2.5 times that from glutamine when both substrates were present. The oxidation of glucose to CO2 via the Krebs cycle accounts for 75-80% of glucose-derived ATP production. Cellular ATP levels markedly decreased in the absence of exogenous substrates, but were constant throughout a 2-h incubation in the presence of glutamine, glucose, or both. There were no differences in thymocyte glucose or glutamine metabolism between normal and diabetic BB rats, in contrast to previous findings in peripheral lymphoid organs. Our results suggest that glucose is a more important fuel than glutamine for "resting" thymocytes, again in contrast to the cells of peripheral lymphoid organs in which glutamine is as important as glucose as a fuel.(ABSTRACT TRUNCATED AT 250 WORDS)     

Rate-limiting factors in urate synthesis and gluconeogenesis in avian liver

1. Urate synthesis and other metabolic characteristics of isolated chicken hepatocytes were studied. 2. The distinction is made between immediate precursors of the purine ring (glycine, glutamine, aspartate, formyltetrahydrofolate, bicarbonate) and ultimate precursors from which the immediate precursors are formed in the liver. 3. In hepatocytes from well-fed chickens the rate of urate synthesis was not greatly increased by the addition of amino acids or NH₄Cl, but in hepatocytes from 72h-starved chickens the rate was much increased when alanine or asparagine was added as the only substrate. Other amino acids, when added alone, did not affect the rate. The exceptional effect of alanine and asparagine is due to the ready formation of the immediate precursors. 4. Conditions are described under which glutamine, serine, glycine plus formate, ribose and glucose increased the rate of urate synthesis. 5. At 1mm-NH₄Cl (a concentration not much higher than that of blood plasma) the rate of urate synthesis in the presence of lactate was increased, but higher concentrations inhibited urate synthesis in the presence of lactate or alanine; with alanine even 1mm-NH₄Cl was inhibitory. 6. Glucose synthesis from lactate, alanine or dihydroxyacetone was also inhibited by 1mm-NH₄Cl. 7. NH₄Cl inhibition of urate and glucose synthesis was paralleled by an increased rate of glutamine synthesis. Thus in the presence of NH₄Cl the gluconeogenic precursors are diverted from the pathway of gluconeogenesis to that of glutamate and glutamine synthesis. This implies that the synthesis of these amino acids is the primary process in the detoxication of ammonia in the avian liver. 8. Urate synthesis, like urea synthesis, can be looked on as a cyclic process with either phosphoribosyl pyrophosphate or ribose acting as the carrier on which the purine ring is assembled. 9. The energy requirements of urate synthesis depend on whether phosphoribosyl pyrophosphate is regenerated from IMP by pyrophosphorylase or by phosphorylation and pyrophosphorylation of ribose. It is 6 or 9 pyrophosphate bonds of ATP respectively.     

Preservation of red blood cells with purines and nucleosides. III. Synthesis of adenine, guanine, and hypoxanthine nucleotides

Purine nucleotides of fresh human red cells and of red cells during storage at 4 degrees and 25 degrees C with additions of adenine, guanine, guanosine and inosine were estimated by HPLC. Six nucleotides were found in red cells: ATP, ADP, AMP, GTP, GDP, and IMP. The adenine nucleotides represented 92 per cent of the total purine nucleotides, guanine nucleotides 7 per cent and IMP less than 1 per cent. In red cells stored with adenine the total concentration of purine nucleotides increased to 125 per cent of the normal value. An adenine-free but guanine and guanine + inosine containing medium caused a decrease of the concentration of purine nucleotides by 10 to 20 per cent. When red cells were stored without adding guanine or guanosine the content of the guanine nucleotides decreased from 0.32 to 0.17 mumol/g Hb due to the decrease in the GTP content, but the GDP concentration increased slightly. In CPD-AG blood, however, the concentration of guanine nucleotides increased considerably up to 0.6 mumol/g Hb. IMP was estimated in all investigated stored red cells. In CPD-A and in CPD-AG blood 0.4 mumol/g Hb were produced during 3 weeks of storage, but twice of that in CPD-AI blood. The principles of the synthesis and the degradation of purine nucleotides in stored red cells are discussed in detail.     

Elucidation of aberrant purine metabolism: application to hypoxanthine-guanine phosphoribosylstransferase- and adenosine kinase-deficient mutants, and IMP dehydrogenase- and adenosine deaminase-inhibited human lymphoblasts

We propose that the ratio of [14C]formate-labelled purine nucleosides and bases (both intra and extracellular) to nucleic acid purines provides, in exponentially growing cultures, a sensitive index for comparative studies of purine metabolism. This ratio was 4-fold greater for an HGPRT- mutant than for the parental HGPRT+ human lymphoblast line. The major components of the labelled nucleoside and base fraction were hypoxanthine and inosine. By blocking adenosine deaminase activity with coformycin we found that approx. 90% of inosine was formed directly from IMP rather than the route IMP leads to AMP leads to adenosine leads to inosine. The ratio of labelled base + nucleosides to nucleic acids was essentially unchagned for an AK- lymphoblast line and 2-fold greater than control for an HGPRT(-)-KAK- line, demonstrating that a deficiency of adenosine kinase alone has little effect on the accumulation of purine nucleosides and bases. Although adenosine was a minor component of the nucleoside and base fraction, the adenosine fraction increased from 3 to 13% with the addition of coformycin to the HGPRT(-)-AK- line. In the parental and HGPRT- lines, adenosine was shown to be primarily phosphorylated rather than deaminated at concentrations less than 5 microM. Inhibition of IMP dehydrogenase activity by mycophenolic acid caused a 12- and 3-fold increase in the rate of production of labelled base and nucleoside in the parent and HGPRT- cells respectively. These results suggest that a mutationally induced partial deficiency in the activities converting IMP to guanine nucleotides may result in an increased catabolism of IMP.     

Lactate: A major product of glutamine metabolism by human diploid fibroblasts

Human diploid fibroblasts metabolize up to 13% of the glutamine in tissue culture medium to lactate. Four μCi of glutamine-U-¹⁴C were added to media containing 5 mM or 65 μM glucose or medium containing no added glucose, but supplemented with purine and pyrimidine nucleosides (HGTU). Aliquots of the media were taken at daily intervals and were assayed for glucose, lactate, pyruvate, malate, citrate, aspartate, glutamine, and glutamate. The label incorporation into these compounds was determined, except for glutamine and glucose. The distribution of label from glutamine-U¹⁴C in 5 mM glucose medium by day 4 was lactate (10.2%), glutamate (15.2%), citrate (1.9%), pyruvate (2.0%), malate (1.1%), and aspartate (< 0.1%). The accumulation of label in lactate and glutamate occurred continuously during the growth cycle. Malate, citrate, and aspartate accumulation occurred primarily in confluent cultures. The label in aspartate was seen only in stationary phase cells or when the glucose concentration was decreased to 65 μM or less; net aspartate accumulation was increased twofold in low glucose media. These data demonstrate an actively functioning pathway for the conversion of 4-carbon TCA-cycle intermediates to 3-carbon glycolytic intermediates in human diploid fibroblasts.     

Uptake of AMP, ADP, and ATP in Escherichia coli W

The uptake activity ratio for AMP, ADP, and ATP in mutant (T-1) cells of Escherichia coli W, deficient in de novo purine biosynthesis at a point between IMP and 5-aminoimidazole-4-carboxiamide-1-β-D-ribofuranoside (AICAR), was 1:0.43:0.19. This ratio was approximately equal to the 5'-nucleotidase activity ratio in E. coli W cells. The order of inhibitory effect on [2-3H]ADP uptake by T-1 cells was adenine > adenosine > AMP > ATP. About 2-fold more radioactive purine bases than purine nucleosides were detected in the cytoplasm after 5 min in an experiment with [8-1?C]AMP and T-1 cells. Uptake of [2-3H]adenosine in T-1 cells was inhibited by inosine, but not in mutant (Ad-3) cells of E. coli W, which lacked adenosine deaminase and adenylosuccinate lyase. These experiments suggest that AMP, ADP, and ATP are converted mainly to adenine and hypoxanthine via adenosine and inosine before uptake into the cytoplasm by E. coli W cells.     

Adenosine, Inosine, and Guanosine Protect Glial Cells During Glucose Deprivation and Mitochondrial Inhibition: Correlation Between Protection and ATP Preservation

Abstract: The purpose of this study was to determine the mechanism by which adenosine, inosine, and guanosine delay cell death in glial cells (ROC-1) that are subjected to g lucose d eprivation and m itochondrial respiratory chain inhibition with amobarbital (GDMI). ROC-1 cells are hybrid cells formed by fusion of a rat oligodendrocyte and a rat C6 glioma cell. Under GDMI, ATP was depleted rapidly from ROC-1 cells, followed on a much larger time scale by a loss of cell viability. Restoration of ATP synthesis during this interlude between ATP depletion and cell death prevented further loss of viability. Moreover, the addition of adenosine, inosine, or guanosine immediately before the amobarbital retarded the decline in ATP and preserved cell viability. The protective effects on ATP and viability were dependent on nucleoside concentration between 50 and 1,500 µ M . Furthermore, protection required nucleoside transport into the cell and the continued presence of nucleoside during GDMI. A significant positive correlation between ATP content at 16 min and cell viability at 350 min after the onset of GDMI was established ( r = 0.98). Modest increases in cellular lactate levels were observed during GDMI (1.2 nmol/mg/min lactate produced); however, incubation with 1,500 µ M inosine or guanosine increased lactate accumulation sixfold. The protective effects of inosine and guanosine on cell viability and ATP were >90% blocked after treatment with 50 µ M BCX-34, a nucleoside phosphorylase inhibitor. Accordingly, lactate levels also were lower in BCX-34-treated cells incubated with inosine or guanosine. We conclude that under GDMI, the ribose moiety of inosine and guanosine is converted to phosphorylated glycolytic intermediates via the pentose phosphate pathway, and its subsequent catabolism in glycolysis provides the ATP necessary for maintaining plasmalemmal integrity.     

Evidence for the involvement of cytosolic 5'-nucleotidase (cN-II) in the synthesis of guanine nucleotides from xanthosine

In this paper, we show that in vitro xanthosine does not enter any of the pathways known to salvage the other three main natural purine nucleosides: guanosine; inosine; and adenosine. In rat brain extracts and in intact LoVo cells, xanthosine is salvaged to XMP via the phosphotransferase activity of cytosolic 5'-nucleotidase. IMP is the preferred phosphate donor (IMP + xanthosine --> XMP + inosine). XMP is not further phosphorylated. However, in the presence of glutamine, it is readily converted to guanyl compounds. Thus, phosphorylation of xanthosine by cytosolic 5'-nucleotidase circumvents the activity of IMP dehydrogenase, a rate-limiting enzyme, catalyzing the NAD(+)-dependent conversion of IMP to XMP at the branch point of de novo nucleotide synthesis, thus leading to the generation of guanine nucleotides. Mycophenolic acid, an inhibitor of IMP dehydrogenase, inhibits the guanyl compound synthesis via the IMP dehydrogenase pathway but has no effect on the cytosolic 5'-nucleotidase pathway of guanine nucleotides synthesis. We propose that the latter pathway might contribute to the reversal of the in vitro antiproliferative effect exerted by IMP dehydrogenase inhibitors routinely seen with repletion of the guanine nucleotide pools.     

Purine uptake and release in rat C6 glioma cells: nucleoside transport and purine metabolism under ATP-depleting conditions

Adenosine, through activation of membrane-bound receptors, has been reported to have neuroprotective properties during strokes or seizures. The role of astrocytes in regulating brain interstitial adenosine levels has not been clearly defined. We have determined the nucleoside transporters present in rat C6 glioma cells. RT-PCR analysis, (3)H-nucleoside uptake experiments, and [(3)H]nitrobenzylthioinosine ([(3)H]NBMPR) binding assays indicated that the primary functional nucleoside transporter in C6 cells was rENT2, an equilibrative nucleoside transporter (ENT) that is relatively insensitive to inhibition by NBMPR. [(3)H]Formycin B, a poorly metabolized nucleoside analogue, was used to investigate nucleoside release processes, and rENT2 transporters mediated [(3)H]formycin B release from these cells. Adenosine release was investigated by first loading cells with [(3)H]adenine to label adenine nucleotide pools. Tritium release was initiated by inhibiting glycolytic and oxidative ATP generation and thus depleting ATP levels. Our results indicate that during ATP-depleting conditions, AMP catabolism progressed via the reactions AMP --> IMP --> inosine --> hypoxanthine, which accounted for >90% of the evoked tritium release. It was surprising that adenosine was not released during ATP-depleting conditions unless AMP deaminase and adenosine deaminase were inhibited. Inosine release was enhanced by inhibition of purine nucleoside phosphorylase; ENT2 transporters mediated the release of adenosine or inosine. However, inhibition of AMP deaminase/adenosine deaminase or purine nucleoside phosphorylase during ATP depletion produced release of adenosine or inosine, respectively, via the rENT2 transporter. This indicates that C6 glioma cells possess primarily rENT2 nucleoside transporters that function in adenosine uptake but that intracellular metabolism prevents the release of adenosine from these cells even during ATP-depleting conditions.     

Early events in the initiation of ammonia formation in kidney

Experiments were designed to examine the early events in the initiation of glutamate deamination in kidney. Perfused kidneys from methionine sulfoximine-treated rats formed ammonia from [15N]glutamate via the purine nucleotide cycle. The turnover of the 6-amino group of adenine nucleotides to yield ammonia occurred at the rate of 0.30 mumol/g of kidney/min. This rate is 3-4 times larger than in liver and is in agreement with published rates of the purine nucleotide cycle in kidney. The addition of 0.1 mM fluorocitrate to glutamate perfusions stimulated ammonia formation 3 1/2-fold. The turnover of the 6-amino group of adenine nucleotides increased during the first 5 min after adding fluorocitrate to form ammonia predominately from tissue glutamate and aspartate. This turnover correlates with a 3 1/2-fold increase in kidney tissue IMP levels. As the ATP/ADP ratio fell the purine nucleotide cycle was inhibited and glutamate dehydrogenase was stimulated to form ammonia stoichiometric with glutamate taken up from the perfusate. Ammonia formation via glutamate dehydrogenase occurred at a rate of 1.0 mumol/g of kidney/min. Fluorocitrate completely blocked ammonia formation from aspartate in perfusions. The perfused kidney formed ammonia from aspartate via the purine nucleotide cycle at a rate of 1.0 mumol/g of kidney/min. The results indicate a discrete role for aspartate in renal metabolism. Ammonia formation via the purine nucleotide cycle can occur at significant rates and equal to the rate of ammonia formation from glutamate via glutamate dehydrogenase.     

gsk disruption leads to guanosine accumulation in Escherichia coli.

We tried some improvement of inosine production using an inosine-producing mutant of Escherichia coli which is deficient in purF (phosphoribosylpyrophosphate (PRPP) amidotransferase gene), purA (succinyl-adenosine 5'-monophosphate (AMP) synthetase gene), deoD (purine nucleoside phosphorylase gene), purR (purine repressor gene) and add (adenosine deaminase gene), and harboring the desensitized PRPP amidotransferase gene as a plasmid. The guaB (inosine 5'-monophosphate (IMP) dehydrogenase gene) disruption brought about a slightly positive effect on the inosine productivity. Alternatively, the gsk (guanosine-inosine kinase gene) disruption caused a considerable amount of guanosine accumulation together with a slight increase in the inosine productivity. The further addition of guaC (guanosine 5'-monophosphate (GMP) reductase gene) disruption did not lead to an increased guanosine accumulation, but brought about the decrease of inosine accumulation.     

Enhancement of proliferation in cultures of Chinook salmon embryo cells by interactions between inosine and bovine sera

The influence of inosine on DNA synthesis by Chinook salmon embryo cells (CHSE-214) was investigated because previously cell number was shown to increase from six- to thirtyfold if inosine was added to the basal medium (L-15) supplemented with either dialyzed fetal bovine serum (dFBS), calf serum (CS), or dCS. Relative to L-15, ³H-thymidine incorporation was inhibited by these sera alone but elevated in nondialyzed (intact) FBS. Inosine at 10 μM stimulated ³H-thymidine incorporation from ten- to seventyfold in dFBS, CS, and dCS but was only slightly stimulatory in FBS and in L-15 alone. As well as inosine, hypoxanthine, cIMP, IMP, IDP, and ITP were just as stimulatory, but the nonsalvageable purines (xanthine, xanthosine, and XMP) were not. The stimulatory action of inosine was highest in low density cultures. Dipyridamole and S-(p-nitrobenzyl)-6-thioinosine (NBTI), inhibitors of facilitated nonconcentrative nucleoside transport, did not completely block the enhancement of cell number by inosine and by themselves increased proliferation in CS and dCS. Overall, these results suggest that exogenous inosine promoted CHSE-214 proliferation by overcoming factors in the nondialyzable fraction of sera that led to purine loss and by raising intracelular purine nucleotides to levels necessary for cells to respond to growth factors in the nondialyzable fraction of sera. © 1994 Wiley-Liss, Inc.     

Pathways of purine metabolism in human adipocytes. Further evidence against a role of adenosine as an endogenous regulator of human fat cell function

Previous results demonstrated that the adenosine that accumulates in human fat cell suspensions is derived from extracellular sources (Kather, H. (1988) J. Biol. Chem. 263, 8803-8809). To get insight into the mechanisms responsible for the lack of adenosine release, extracellular adenine nucleotide catabolism was minimized by 10 mmol/liter beta-glycerophosphate and 10 mumol/liter alpha,beta-methyleneadenosine 5'-diphosphate. Intracellular adenine nucleotide catabolism resulted in a release of inosine and hypoxanthine under these conditions that was increased markedly by isoproterenol. Experiments with inhibitors of adenosine deaminase and adenosine kinase indicated that the production of inosine and hypoxanthine proceeded via AMP deamination. Consistently, IMP levels were increased transiently in the presence of isoproterenol. In addition, the cells possessed a nucleotide phosphomonoesterase that was resistant to the inhibitory actions of ATP and alpha,beta-methyleneadenosine 5'-diphosphate and showed preference for IMP over AMP. Adenosine (approximately 1 nmol/10(6) cells/h) was also produced inside the cells. However, adenosine production was unrelated to ATP turnover via adenylate cyclase, and any adenosine formed was immediately reconverted to adenine nucleotides in the absence and presence of isoproterenol. It was concluded that adenosine is not released by intact human adipocytes, because the alternative routes of intracellular AMP catabolism are compartmentalized (at least in functional terms), and adenosine kinase is not saturated with substrate in the absence and presence of isoproterenol.     

Is adenosine a second metabolic substrate for human red blood cells?

Adenosine is present in the micromolar range in human plasma. In this study, metabolism of adenosine, which was maintained between 0.62 +/- 0.03 and 2.92 +/- 0.43 microM by means of a continuous infusion using a Harvard infusion pump, was investigated in human red blood cells. It was found that lactate production increases linearly as the adenosine concentration was raised. Cells infused with an average adenosine concentration of 2 microM produced lactate comparable to that produced by 5 mM glucose. The extent to which ATP concentration is maintained by adenosine also depends on its concentration. After a 4 h infusion with an average adenosine concentration of 0.7 microM, ATP content amounts to 75% of the glucose control. Raising the adenosine infusion concentration to 1.5 microM results in a full maintenance of ATP levels and at concentrations higher than 1.5 microM, adenosine produces a net synthesis of ATP. A net synthesis of ATP also occurs with adenosine concentration below 1.5 microM, if supplemented with glucose. In contrast, inosine infusion provides only a partial support of ATP and fails to produce a net synthesis of ATP in the presence of glucose. In addition, the presence of purine nucleoside and glucose together influence the metabolism of each other, depending on inorganic phosphate content (Pi). At a Pi concentration of 1 mM, the glucose consumption rate is reduced by approx. 25% by purine nucleoside infusion and vice versa. In sharp contrast, glucose consumption at 16 mM Pi is potentiated by adenosine. These findings suggest that plasma adenosine contributes significantly to human red cell energetics, even though it is present at a concentration several orders of magnitude lower than glucose.     

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.     

Rates of utilization and fates of glucose, glutamine, pyruvate, fatty acids and ketone bodies by mouse macrophages.

The concentrations of ATP and the ATP/AMP concentration ratios were maintained in thioglycollate-elicited mouse peritoneal macrophages incubated in vitro for 90 min in the presence or absence of added substrate: rates of glycolysis, lactate formation and glutamine utilization were approximately linear with time for at least 60 min of incubation. The rate of oxygen consumption by macrophages was only increased above the basal rate (i.e. that in the absence of added substrate) by addition of succinate or pyruvate, or by addition of the uncoupling agent carboxyl cyanide m-chlorophenylhydrazone ('CCCP'); it was decreased by 75% by the addition of KCN. These findings suggest that metabolism of endogenous substrate can provide most, if not all, of the energy requirement of these cells, at least for a short period. The rates of glucose and glutamine utilization by incubated macrophages were approx. 300 and 100 nmol/min per mg of protein respectively. A large proportion of the glutamine that is utilized is converted into glutamate and aspartate, and very little (perhaps less than 10%) is oxidized. Similarly almost all of the glucose that is utilized is converted into lactate and very little is oxidized. This characteristic is similar to that of resting lymphocytes and rapidly dividing cells; in non-proliferating macrophages it may be a mechanism to provide precision in control of the rate of biosynthetic processes that utilize intermediates of these pathways, e.g. purines and pyrimidines for mRNA for the synthesis of secretory proteins and glycerol 3-phosphate for phospholipid synthesis for membrane recycling. No utilization of acetoacetate or 3-hydroxybutyrate by macrophages was detected. In contrast, both butyrate and oleate were oxidized. The rate of [14C]oleate conversion into 14CO2 (1.3 nmol/h per mg of protein) could account for most of the oxygen consumption by incubated macrophages, suggesting that long-chain fatty acids might provide an important fuel in situ. This may be one explanation for the secretion of lipoprotein lipase by these cells, to provide fatty acids for oxidation from the degradation of local triacylglycerol.     

Photosynthesis in phosphoenolpyruvate carboxykinase-type C4 plants: pathways of C4 acid decarboxylation in bundle sheath cells of Urochloa panicoides

The mechanism of C4 acid decarboxylation was studied in bundle sheath cell strands from Urochloa panicoides, a phosphoenolpyruvate carboxykinase (PCK)-type C4 plant. Added malate was decarboxylated to give pyruvate and this activity was often increased by adding ADP. Added oxaloacetate or aspartate plus 2-oxoglutarate (which produce oxaloacetate via aspartate aminotransferase) gave little metabolic decarboxylation alone but with added ATP there was a rapid production of PEP. For this activity ADP could replace ATP but only when added in combination with malate. In addition, the inclusion of aspartate plus 2-oxoglutarate with malate plus ADP often increased the rate of pyruvate production from malate by more than twofold. Experiments with respiratory chain inhibitors showed that the malate-dependent stimulation of oxaloacetate decarboxylation (PEP production) was probably due to ATP generated during the oxidation of malate in mitochondria. We could provide no evidence that photophosphorylation could serve as an alternative source of ATP for the PEP carboxykinase reaction. We concluded that both PEP carboxykinase and mitochondrial NAD-malic enzyme contribute to C4 acid decarboxylation in these cells, with the required ATP being derived from oxidation-linked phosphorylation in mitochondria.