Accumulation Of Glycine Research Articles

Metabolic network analysis during fed-batch cultivation of Corynebacteriumglutamicum for pantothenic acid production: first quantitative data and analysis of by-product formation

A first generation genetically modified strain of Corynebacterium glutamicum has been assessed for its potential to synthesise and accumulate the vitamin pantothenic acid in the medium using fed-batch cultivation technology, with biomass concentration controlled by isoleucine limitation. Kinetic analysis of specific rates throughout the process has been used to model carbon flux through both central metabolism and the specific pathways involved in product formation. Flux towards pantothenic acid is potentially high but much of this flux is dissipated as by-products within associated pathways, notably linked to amino acid synthesis. The major limitation of vitamin production in this strain is linked to the tenfold higher flux of keto-isovalerate towards valine rather than pantothenic acid. Attempts to modify this ratio by imposing nitrogen limitation provoked carbon overflow as unidentified non-nitrogenous compounds. The observed accumulation of glycine suggests that the flux towards pantothenate production may by limited by the rate of the pathway intermediate (5,10-methylene-tetrahydrofolate) regeneration.

An assessment of the physiological properties of the so-called compatible solutes using in vitro experiments with leaf discs

Despite the considerable efforts directed over the past four decades to investigate, via correlative physiology and genetic engineering, the contribution of osmolytes to osmotolerance of higher plants, their physiological function(s) still remain a matter of debate. Here, we summarize the functions of proline and glycine betaine, recently documented in our group by using canola ( Brassica napus L. var. oleifera), leaf discs treated in vitro with medium of low or high osmolarities. These discs accumulate proline when subjected to osmotic upshifts, whereas proline is rapidly consumed when the discs are further treated under non-stressing conditions. The period of time needed to osmoregulate the amount of proline is assumed to be too long to assign to the proline response a relevant function in turgor regulation which can be achieved with carbohydrates. In addition, proline accumulation is suppressed by supplying the osmotically stressing medium with polyamines, phyto-oxylipins and glycine betaine with no detrimental effect on the capacity to cope with osmotic stress. In opposition to proline, glycine betaine does not occur in canola. We take advantage of this situation to test the compatibility of exogenously supplied betaine in this plant material. Comparisons are done with spinach ( Spinacia oleracea L.) leaf discs, which contain an endogenous amount of betaine and also exhibit the capacity to absorb a large amount of it. The betaine absorbed is not degraded and thus cannot be considered in this situation as a true osmoregulator. In turgid tissues of plants that do not produce betaine, this compound induces an elevated level of glutamine, a decreased level of protein, and photo-dependent glycine accumulation. This can rely on senescent-like effects and on a blockage in the glycolate cycle. Such damages to nitrogen metabolism are not observed in leaf explants treated simultaneously with betaine and the stressing agent. We also find that the treatments of turgid canola leaf discs with proline or betaine induce important changes in polyamine contents, mimicking, but with a higher order of magnitude, the effects of osmotic stress. Interestingly, this inducing activity is not observed when osmolytes are supplied under osmotic upshift conditions. Antagonistic effects between osmolytes and osmotic stress are described for the first time in higher plants. They suggest that proline and betaine can act as signal molecules in counteracting destabilizing effects to polyamine metabolism and lead us to decipher other counter-stress effects of nitrogenous osmolytes which, obviously, do not behave just as inert osmobalancers.

The enigma of transmitter-selective receptor accumulation at developing inhibitory synapses.

The control of synaptic inhibition is crucial for normal brain function. More than 20 years ago, glycine and gamma-aminobutyric acid (GABA) were shown to be the two major inhibitory neurotransmitters. They can be released independently from different terminals or co-released from the same terminal to activate postsynaptic glycine and GABA(A) receptors. The anchoring protein gephyrin is involved in the postsynaptic accumulation of both glycine and GABA(A) receptors. In lower brain regions, both receptors can be concentrated in synapses, whereas in higher brain regions, glycine receptors are mostly excluded from postsynaptic sites. The activation of glycine and/or GABA(A) receptors determines the strength and precise timing of inhibition. Therefore, tight regulation of postsynaptic glycine versus GABA(A) receptor localization is crucial for optimizing synaptic inhibition in neurons. This review focuses on recent findings and discusses questions concerning the specificity of postsynaptic inhibitory neurotransmitter receptor accumulation during inhibitory synapse formation and development.

Interaction between exogenous glycine betaine and the photorespiratory pathway in canola leaf discs

The uptake and accumulation of exogenously supplied glycine betaine (GB) by canola (which never accumulates GB in response to stress) leaf discs has been found to induce damage to some of their structural and functional components. As a consequence some free amino acids were accumulated, particularly glutamine and glycine. Similar results were obtained with leaf discs of Arabidopsis thaliana i.e. another cruciferous plant that does not naturally produce significant amounts of GB. In contrast no changes in glutamine and glycine contents were observed in response to the GB treatment in leaf discs of spinach, a natural producer of GB. The change in glutamine content might be related to the senescing effects caused by the GB treatment. Glycine accumulation in response to GB has been more thoroughly studied with canola leaf discs. It only occurred under light conditions and was suppressed under non-photorespiratory conditions. The accumulation of glycine in canola leaf discs in response to GB was either restricted when GB was added in the presence of aminooxyacetate (an inhibitor of transaminases) or enhanced when added in the presence of aminoacetonitrile (an inhibitor of glycine decarboxylation by mitochondria). Both compounds are known to block the glycolate pathway. Glycine accumulation was not found in leaf discs of Zea mays treated in the light in the presence of GB. These results suggest that the absorbed GB could exert destabilizing effects on the photorespiration of the C 3 cruciferous plants canola and Arabidopsis via competitive effects between GB and glycine at the mitochondrial step of the glycolate pathway. The mechanism of the GB effect remains to be elucidated as well as that of its apparent compatibility in spinach, the well known natural producer of GB.

Steps towards an integrated view of nitrogen metabolism.

This article discusses how nitrate assimilation is integrated with nitrate uptake, with ammonium assimilation and amino acid synthesis, with pH regulation, and with the sugar supply in tobacco leaves. During the first part of the light period, nitrate assimilation exceeds nitrate uptake by 2-fold and ammonium assimilation by 50%, leading to rapid depletion of nitrate and accumulation of ammonium, glutamine, glycine and serine. NIA, NII and PPC expression show a shared maximum early in the diurnal cycle to direct carbon towards malate synthesis for pH regulation. Later in the diurnal cycle an orchestrated increase of GLN2, PKc, CS, and ICDH-1 expression re-establishes a balance between nitrate assimilation and ammonium metabolism. Nitrate uptake continues throughout the night, replenishing the leaf nitrate pool. These diurnal changes are attenuated or abolished in mutants with low NIA activity, and modified in wild-type plants growing on different nitrogen sources or elevated [CO(2)]. Comparison across genotypes and conditions reveals that NIA transcript levels are always closely related to the balance between nitrate influx and assimilation, but are unrelated to changes of glutamine or 2-oxoglutarate. In a systematic search for other downstream regulators, a wide range of downstream metabolites was fed to detached leaves and glutamate, cysteine, asparagine, and malate identified as candidates. Low sugars totally inhibit nitrate assimilation, overriding signals from nitrogen metabolism. Moderate changes act post-transcriptionally, and larger changes lead to a collapse of the NIA transcript. Low sugars also lead to a collapse of minor amino acids and a dramatic decrease of phenylpropanoids and nicotine. Consequently, wild-type plants growing in unfavourable light regimes and antisense RBCS transformants are simultaneously carbon- and nitrogen-limited.

Structure and expression of the glycine cleavage system in rat central nervous system

The glycine cleavage system (GCS) is a mitochondrial multienzyme system consisting of four individual proteins, three specific components (P-, T-, and H-proteins) and one house-keeping enzyme, dihydrolipoamide dehydrogenase. Inherited deficiency of the GCS causes nonketotic hyperglycinemia (NKH), an inborn error of glycine metabolism. NKH is characterized by massive accumulation of glycine in serum and cerebrospinal fluids and severe neuronal dysfunction in neonates. To elucidate the neuropathogenesis of NKH, we cloned cDNAs encoding three specific components of the GCS and studied the gene expression in rat central nervous system. P-, T-, and H-protein cDNAs encoded 1024, 403, and 170 amino acids, respectively. In situ hybridization analysis revealed that P-protein mRNA was expressed mainly in glial-like cells, including Bergmann glias in the cerebellum, while T- and H-protein mRNAs were detected in both glial-like cells and neurons. T- and H-protein mRNAs, but not P-protein mRNA, were expressed in the spinal cord. Primary astrocyte cultures established from cerebral cortex had higher GCS activities than hepatocytes whereas those from spinal cord expressed only H-protein mRNA and had no enzymatic activity. An important role of glycine as inhibitory neurotransmitter has been established in the brainstem and spinal cord and another role of glycine as an excitation modulator of N-methyl- d-aspartate receptor is suggested in the hippocampus, cerebral cortex, olfactory bulbus, and cerebellum. Our results suggest that the GCS plays a major role in the forebrain and cerebellum rather than in the spinal cord, and that N-methyl- d-aspartate receptor may participate in neuropathogenesis of NKH.

17 beta-estradiol protects neurons from ischemic damage and attenuates accumulation of extracellular excitatory amino acids.

We studied the effect of 17 beta-estradiol on ischemia-induced increases in extracellular amino acids (aspartate, glutamate, glycine, and taurine) in the gerbil hippocampal CA1 region by a microdialysis-high-performance liquid chromatography system in vivo. Transient forebrain ischemia for 3 min produced a marked accumulation of aspartate, glutamate, taurine, and glycine (498%, 410%, 266%, and 178%, respectively). 17 beta-estradiol reduced the increase of aspartate and glutamate by 57% and 53% of the peak values. This finding suggests that 17 beta-estradiol may protect neurons against ischemic damage by attenuating the ischemia-induced increase in extracellular excitatory amino acids.

The dendritic architecture of the cholinergic plexus in the rabbit retina: Selective labeling by glycine accumulation in the presence of sarcosine

The dendritic architecture of the cholinergic plexus in the rabbit retina: Selective labeling by glycine accumulation in the presence of sarcosine

The dendritic architecture of the cholinergic plexus in the rabbit retina: Selective labeling by glycine accumulation in the presence of sarcosine

The cholinergic amacrine cells in the rabbit retina slowly accumulate glycine to very high levels when the tissue is incubated with excess sarcosine (methylglycine), even though these cells do not normally contain elevated levels of glycine and do not express high-affinity glycine transporters. Because the sarcosine also depletes the endogenous glycine in the glycine-containing amacrine cells and bipolar cells, the cholinergic amacrine cells can be selectively labeled by glycine immunocytochemistry under these conditions. Incubation experiments indicated that the effect of sarcosine on the cholinergic amacrine cells is indirect: sarcosine raises the extracellular concentration of glycine by blocking its re-uptake by the glycinergic amacrine cells, and the excess glycine is probably taken-up by an unidentified low-affinity transporter on the cholinergic amacrine cells. Neurobiotin injection of the On-Off direction-selective (DS) ganglion cells in sarcosine-incubated rabbit retina was combined with glycine immunocytochemistry to examine the dendritic relationships between the DS ganglion cells and the cholinergic amacrine cells. These double-labeled preparations showed that the dendrites of the DS ganglion cells closely follow the fasciculated dendrites of the cholinergic amacrine cells. Each ganglion cell dendrite located within the cholinergic strata is associated with a cholinergic fascicle and, conversely, there are few cholinergic fascicles that do not contain at least one dendrite from an On-Off DS cell. It is not known how the dendritic co-fasciculation develops, but the cholinergic dendritic plexus may provide the initial scaffold, because the dendrites of the On-Off DS cells commonly run along the outside of the cholinergic fascicles.

Neuronal and Glial Glycine Transporters Have Different Stoichiometries

A neurotransmitter transporter can potentially mediate uptake or release of substrate, and its stoichiometry is a key factor that controls the driving force and thus the neurotransmitter flux direction. We have used a combination of electrophysiology and radio-tracing techniques to evaluate the stoichiometries of two glycine transporters involved in glycinergic or glutamatergic transmission. We show that GlyT2a, a transporter present in glycinergic boutons, has a stoichiometry of 3 Na +/Cl −/glycine, which predicts effective glycine accumulation in all physiological conditions. GlyT1b, a glial transporter, has a stoichiometry of 2 Na +/Cl −/glycine, which predicts that glycine can be exported or imported, depending on physiological conditions. GlyT1b may thus modulate glutamatergic synapses by increasing or decreasing the glycine concentration around N-methyl-D-aspartate receptors (NMDARs).

Susceptibility to β-Amyloid-Induced Toxicity Is Decreased in Goto-Kakizaki Diabetic Rats: Involvement of Oxidative Stress

The response of synaptosomes isolated from Wistar non-diabetic rats and Goto–Kakizaki (GK) diabetic rats to the β-amyloid fragment Aβ25–35 was compared. The synaptosomal redox activity, evaluated by the MTT [3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide] assay, was shown to be decreased in GK rats (72.8±7.45% of MTT reduction). However, the reduction of MTT was decreased in synaptosomes of Wistar rats upon Aβ25–35 treatment (84.47±3.73%), while in GK rats it was not affected. Aβ25–35 induced lipid peroxidation in synaptosomes of Wistar rats, but not in that of GK rats, leading to an 1.5-fold increase in thiobarbituric acid reactive substances (TBARS) levels. In the absence of Aβ25–35, basal TBARS levels were 1.6-fold higher in GK rats. In the former preparations, the content in vitamin E was also higher (2-fold). A decrease in ATP levels, of about 2-fold, was observed in synaptosomes of Wistar rats treated with Aβ25–35, while no significant changes were observed in synaptosomes of GK rats. No significant differences between the two groups were detected in the basal ATP levels. The extrasynaptosomal accumulation of aspartate and glutamate increased upon Aβ25–35 treatment, only in synaptosomes of Wistar rats (aspartate and glutamate accumulation increased by 1.1-fold and 1.5-fold, respectively), while the accumulation of glycine increased in both Wistar (by 1.8-fold) and GK (by 2.2-fold) rats. No statistical differences in the basal accumulation of aminoacids were observed. These results show that synaptosomes of GK diabetic rats have a lower redox activity, but are less susceptible to the Aβ25–35-induced toxicity. Data also suggest that oxidative stress occurs in this hyperglycemia animal model and that an increase in the antioxidant defense systems may exert protection against toxic insults. This mechanism, occuring in the early phases of diabetes, may correspond to an adaptive response.

Comparative Study of Glycine, Alanine or Casein as Inert Nitrogen Sources in Endotoxemic Rats

Pharmacological effects of dietary amino acids (AA) and peptides must be compared to an isonitrogenous control that is as inert as possible. To establish a rationale for the choice of such a control, potential metabolic and nutritional effects of three currently used nitrogenous controls (glycine, alanine, and casein) were evaluated in an endotoxemic rat model that has well-defined alterations in AA and protein metabolism. Five-week-old male Sprague-Dawley rats (113 ± 1 g) were randomly assigned to four groups and received at d 0 an intraperitoneal injection of endotoxin (3 mg/kg). After withdrawal of food for 24 h, the rats were enterally refed for 48 h with a liquid diet (Osmolite®) supplemented with 0.19 g N · kg-1 · d-1 in the form of glycine [lipopolysaccharide (LPS)-GLY group], alanine (LPS-ALA group) or casein (LPS-CAS group). One group (LPS group) received only Osmolite®. Plasma, two skeletal muscles, the liver and the intestine were then removed. Body and tissue weights and tissue protein contents did not differ among the four groups. Intestine histomorphometry showed no significant difference among groups. Jejunal hydrolase activities were significantly affected by the nitrogenous supplementations, but no effect was observed in the ileum. Only limited significant effects were observed on plasma and tissue-free AA concentrations, except for an accumulation of glycine in the plasma and tissues from the LPS-GLY group, compared to other groups. Overall, whereas glycine as a nitrogenous control should be used with care, either alanine or casein may be used as the “placebo,” with the choice depending on the study to be performed.

The role of photorespiration during drought stress: an analysis utilizing barley mutants with reduced activities of photorespiratory enzymes

C i, intercellular CO2 concentration Fv/Fm, quantum efficiency of excitation capture by open photosystem II centres FBPase, fructose‐1,6‐bisphosphatase GAPDH, glyceraldehyde‐3‐phosphate dehydrogenase GDC, glycine decarboxylase GS‐2, chloroplastic glutamine synthetase HPR, hydroxypyruvate reductase PFD, photon flux density ΦCO2, quantum efficiency of CO2 assimilation ΦPSII, quantum efficiency of photosystem II electron transport ψ, water potential qN, non‐photochemical chlorophyll a fluorescence quenching qP, photochemical chlorophyll a fluorescence quenching RuBP, ribulose‐1,5‐bisphosphate Rubisco, ribulose‐1,5‐bisphosphate carboxylase‐oxygenase SBPase, sedoheptulose‐1,7‐bisphosphatase SGAT, serine : glyoxylate aminotransferase The significance of photorespiration in drought‐stressed plants was studied by withholding water from wild‐type barley (Hordeum vulgare L.) and from heterozygous mutants with reduced activities of chloroplastic glutamine synthetase (GS‐2), glycine decarboxylase (GDC) or serine : glyoxylate aminotransferase (SGAT). Well‐watered plants of all four genotypes had identical rates of photosynthesis. Under moderate drought stress (leaf water potentials between –1 and –2 MPa), photosynthesis was lower in the mutants than in the wild type, indicating that photorespiration was increased under these conditions. Analysis of chlorophyll a fluorescence revealed that, in the GDC and SGAT mutants, the lower rates of photosynthesis coincided with a decreased quantum efficiency of photosystem II and increased non‐photochemical dissipation of excitation energy. Correspondingly, the de‐epoxidation state of xanthophyll‐cycle carotenoids was increased several‐fold in the drought‐stressed GDC and SGAT mutants compared with the wild type. Accumulation of glycine in the GDC mutant was further evidence for increased photorespiration in drought‐stressed barley. The effect of drought on the photorespiratory enzymes was determined by immunological detection of protein abundance. While the contents of GS‐2 and P‐ and H‐protein of the GDC complex remained unchanged as drought stress developed, the content of NADH‐dependent hydroxypyruvate reductase increased. Enzymes of the Benson–Calvin cycle, on the other hand, were either not affected (ribulose‐1,5‐bisphosphate carboxylase‐oxygenase and plastidic fructose‐1,6‐bisphosphatase) or declined (sedoheptulose‐ 1,7‐bisphosphatase and NADP‐dependent glyceraldehyde‐3‐phosphate dehydrogenase). These data demonstrate that photorespiration was enhanced during drought stress in barley and that the control exerted by photorespiratory enzymes on the rate of photosynthetic electron transport and CO2 fixation was increased.

Osmolarity-dependent glycine accumulation indicates a role for glycine as an organic osmolyte in early preimplantation mouse embryos.

Mouse zygotes and early cleavage-stage embryos are sensitive to increased osmolarity. However, development can occur at higher osmolarities if any of a number of organic compounds are present. One of the most effective of these is glycine. We have found that the amount of glycine accumulated by embryos during in vitro culture from the zygote to two-cell stage depends on the osmolarity of the medium, with significantly more glycine accumulated at 310 or 340 mOsM than at 250 mOsM. The accumulated glycine is largely retained in a freely diffusible form, as it can be released via a swelling-activated pathway in two-cell embryos. Increased glycine accumulation does not seem to depend on an increase in its rate of transport. The transport rate is not higher in two-cell embryos that have been cultured from zygotes in hypertonic vs. normal medium, and hypertonicity only slightly stimulates transport in zygotes. Our results indicate that glycine functions as an organic osmolyte in early mouse embryos.

Adult carboxypeptidase E-deficient fat/fat mice have a near-total depletion of brain CCK 8 accompanied by a massive accumulation of glycine and arginine extended CCK: identification of CCK 8 Gly as the immediate precursor of CCK 8 in rodent brain.

Cholecystokinin (CCK) amide concentrations were reduced over 85% in all the major brain regions of carboxypeptidase E (Cpe)(fat)/Cpe(fat) mice in comparison to control mice. Using an radioimmunoassay (RIA) specific for glycine-extended CCK (CCK Gly), low levels of CCK Gly were detected in control (0.65 ng/g tissue) and were even lower in Cpe(fat)/Cpe(fat) (0.246 ng/g) mice brain extracts. After treatment with carboxypeptidase B, the level of CCK Gly in Cpe(fat)/Cpe(fat) in these brain extracts was elevated to 33.5 ng/g, about 51-fold higher than in control. On gel-filtration chromatography and high-performance liquid chromatography (HPLC), this material coeluted with CCK 8 Gly. These results demonstrate that CPE is required for the correct processing of arginine- and glycine-extended CCK in all major regions of the mouse brain. These results support the hypothesis that CCK 8 Gly is the immediate precursor of CCK 8 amide in mouse brain, not larger amidated forms like CCK 22 or CCK 33.

Control of photosynthesis in barley plants with reduced activities of glycine decarboxylase

A mutant (LaPr 87/30) of barley (Hordeum vulgare L.) deficient in glycine decarboxylase (GDC; EC 2.1.2.10) was crossed with wild-type plants to generate heterozygous plants with reduced GDC activities. Plants of the F2 generation were grown in air and analysed for reductions in GDC proteins and GDC activity. The leaves of heterozygous plants contained reduced amounts of H-protein, and when the content of H-protein was lower than 60% of the wild-type, the P-protein was also reduced. The contents of the other two proteins of the GDC complex, T-protein and L-protein were not affected. Glycine decarboxylase activities, measured as the decarboxylation of [1-14C]glycine by intact mitochondria released from protoplasts, were between 47% and 63% of the wild-type activity in heterozygous plants and between 86% and 100% in plants with normal contents of H-protein. The enzyme activity was linearly correlated with the relative content of H-protein. Plants with reduced GDC activities developed normally and did not show major pleiotropic effects. In air, the reduction in GDC activity had no effect on the leaf metabolite content or photosynthesis, but under conditions of enhanced photorespiration (low CO2 and high light), glycine accumulated and the rates of photosynthesis decreased compared to the wild-type. The accumulation of glycine did not lead to a depletion of amino donors or to the accumulation of glyoxylate. The lower rates of photosynthesis were probably caused by an impaired recycling of carbon in the photorespiratory pathway. It is concluded that GDC has no control over CO2 assimilation under normal growth conditions, but appreciable control by GDC becomes apparent under conditions leading to higher rates of photorespiration.

Dietary Protein and Amino Acid Levels Alter Threonine Dehydrogenase Activity in Hepatic Mitochondria of Gallus domesticus

Experiments were conducted to determine if hepatic threonine dehydrogenase (TDH) activity is influenced by dietary protein or specific amino acid concentrations. In an initial experiment, young chicks were deprived of feed for 60 h or had access for 72 h to a 22% protein basal diet, a protein-free diet or a 51% high protein diet. TDH activity was determined as aminoacetone and glycine accumulation during incubation of liver mitochondria. TDH activity was significantly (P < 0.01) lower in chicks fed the protein-free diet and significantly greater in chicks fed the high protein diet compared with chicks fed the basal diet. Food deprivation had no effect on TDH activity. A second experiment was conducted using the 22 and 51% protein diets, the 22% protein diet plus 1.14 g/100 g diet threonine (equivalent to the free plus protein-bound threonine content of the high protein diet), and the 51% protein diet containing 0.15 g/100 g diet less threonine. TDH was increased in chicks fed either high protein diet (P < 0.05). There were no significant differences in TDH activity, however, between chicks fed the basal diet and the threonine-supplemented diet or between chicks fed the two high protein diets. In two other experiments, the activity of TDH was investigated in chicks fed for 9 d dietary supplements of either serine or glycine (5.5 or 4 g/100 g basal diet, respectively). The supplements were added to the basal diet or the basal diet imbalanced by the addition of 6% branched-chain amino acids. Neither the serine nor the glycine supplement significantly altered TDH activity or the increased activity associated with a branched-chain amino acid–induced threonine imbalance. The results suggest that hepatic TDH activity is influenced by protein level or other amino acids more than by threonine itself.

Utilization of phytoplankton in seston aggregates from the Elbe estuary, Germany, during early degradation processes

A laboratory device was used to simulate the environmental conditions in a seston layer of about 1 mm thickness, similar to those in aggregates of the open water of the Elbe estuary, Germany. During 98 h of oxic, suboxic and anoxic incubation, microbial degradation was investigated by measuring both changes in the chemlcal compos~tlon of the particulate organic carbon (POC) and heterotrophic microbial activities. Mineralization of the POC at 15°C occurred at mean rates of 16 under oxic, 26 under suboxic, and 0.24 pm01 cm-3 d-' under anoxic conditions. Different respiration processes accounted for ratios of carbon degradation equal to 1 7 for oxygen:nitrate, 430 for nitrate:manganese, and 3.5 for Mn(IV):Fe(lII) Dunng o x ~ c and suboxic incubation, particulate combined amino acids (PCAA) were preferred as an organic substrate, and PCAA:C ratios decreased from about 0.12 to 0.02. A total of 85% of the PCAA wds degraded within 56 h including PCAA and dissolution of silica from the cell wall material of diatoms; the remaining amlno acids were assumed to be those from active bacteria. Chlorophyll a (chl a ) was more resistant to degradation than total POC below a threshold value of 60 p M 02, which resulted in an increase of chl a.C ratlos up to 37. A decrease in the availability of particulate substrates for heterotrophic decomposition was expressed as an accumulation of glycine and serine and a depletion in aspartic and glutamatic a c ~ d in the PCAA. During anoxic incubation PCAA decreased by only 18% and remained unchanged in composition. Along the longitudinal profile of the freshwater section of the Elbe estuary, gradual decomposition of the seston material during summer downstream transport was followed by changes in composition similar to those observed in the oxic laboratory experiments. The results indicate that phytoplankton debris that is not available as a substrate for microorganisms in sediments and small suspended particles is rapidly degraded within seston aggregates.

On the usefulness of glycylglycine in hemodialysis and peritoneal dialysis solutions.

This article reviews current knowledge on the usefulness of glycylglycine in preparing single stable hemodialysis (HD) and peritoneal dialysis (PD) solutions containing bicarbonate. The coexistence of bicarbonate and glycylglycine in a dialysis solution renders it a potent, stable buffer in which the well known reactions between bicarbonate and calcium and magnesium, and the subsequent formation of insoluble neutral calcium and magnesium carbonate salts, are avoided. Single stable bicarbonate-glycylglycine (BiGG) solutions for HD and PD have been successfully prepared and studied, both experimentally and clinically. These studies have demonstrated the advantages of BiGG solutions in terms of simplicity, stability, convenience, tolerance, biocompatibility, protection of the peritoneum, and higher ultrafiltration, compared to standard acetate or lactate solutions, and on-line prepared bicarbonate solutions. Progressive accumulation of glycylglycine or glycine, even after prolonged use, side effects, or signs of toxicity were not observed. In conclusion, BiGG solutions ensure a physiologic dialysis both from the theoretic and practical points of view.

Mechanism of clearance and transfer of dipeptides by perfused human placenta.

Glycylglutamine (Gly-Gln) is stable source of glutamine for parenteral nutrition. In the present study we have investigated whether this dipeptide is transferred intact across the human placenta. Although after 90 min of placental perfusion there was almost complete disappearance of Gly-Gln (100 microM) from the maternal compartment, only a small concentration of this dipeptide (< 6 microM) appeared in the fetal compartment. To investigate whether this transfer was due to transcellular transport, brush-border membrane vesicles of the human placenta were probed with [3H]Gly-Gln, which showed no uptake. To investigate whether hydrolysis was the mechanism of disappearance of Gly-Gln, the perfusion study was repeated with glycylsarcosine (Gly-Sar), which is resistant to hydrolysis. In sharp contrast to Gly-Gln, after 90 min of perfusion nearly 80% of Gly-Sar remained in the perfusate (half-life of 24 vs. 235 min). The rest of the Gly-Sar was recovered intact in the fetal compartment. The addition of Gly-Gln to the maternal compartment increased the accumulation of glycine, but not glutamine, in both the maternal and fetal compartments. In conclusion, our data suggest that 1) the mechanism of clearance of Gly-Gln by perfused human placenta is largely hydrolysis, whereas that of Gly-Sar is largely passive diffusion, and 2) the placenta has a greater preference for glutamine than for glycine.