Biosynthesis of Polyamines in Mouse Brain: Effects of Methionine Sulfoximine and Adenosylhomocysteine

Abstract: This study examines the consequences on cerebral polyamine biosynthesis of increases and decreases in cerebral methylation. Increases were elicited by administering the convulsant agent methionine sulfoximine (MSO) and decreases by elevating in vivo the cerebral levels of the methylation inhibitor S -adenosyl-homocysteine. Following the intraventricular (i.vt.) administration of one of the two possible polyamine precursors, [1,4-¹⁴C]putrescine, the specific radioactivity (sra) of the newly formed [¹⁴C]spermidine remained unchanged. Conversely, after i.vt. l -[3,4-¹⁴C]methionine, the other polyamine precursor, significantly higher sra values for [¹⁴C]spermidine and [¹⁴C]spermine were recorded in the brains of the MSO-treated animals. [¹⁴C] S - adenosylmethionine in the brain of the MSO-treated animals was also more highly labeled following [1-¹⁴C]-methionine, indicating its accelerated formation relative to controls. We also investigated the effect of the administration of adenosine + homocysteine, a treatment that results in elevated brain adenosylhomocysteine levels, on polyamine biosynthesis from [3,4-¹⁴C]-methionine. The results of these experiments show both significantly lower sra values for [¹⁴C]spermidine and [¹⁴C]spermine and significantly higher than control endogenous methionine levels, a clear sign of the existence of a retardation in the conversion of methionine to polyamines under these conditions. In conclusion, the present study demonstrates that while interference with cerebral methylation results in significant alterations of the rate of formation of the methionine moiety of spermidine and spermine, it has no effect on the entry of the putrescine moiety into the two polyamine molecules.     

Localization of ornithine decarboxylase and changes in polyamine content in root meristems of Zea mays

Polyamine content and the activity of arginine decarboxylase (EC 4.1.1.19) and ornithine decarboxylase (EC 4.1.1.17) were studied with respect to meristematic activity in primary roots and in developing lateral roots of Zea mays L. (cv. Neve Ya'ar 170) seedlings. Comparative localization of active ornithine decarboxylase and of meristematic activity were determined by labelling roots either with α-[5-¹⁴C]-difluoromethyl ornithine or with [³H]-thymidine, respectively.
Lateral roots were formed during the 72 h post-decapitation period, accompanied by an initial decline in putrescine content and by a significant increase in spennidine con-tent at 48–72 h. High levels of spermidine and lower levels of putrescine were found in the primary root apex as well. A marked increase in ornithine and arginine decarboxylase activity, as measured by ¹⁴CO₂ release, was found during the 72 h post-decapitation period of lateral root development. This increase in ornithine decarboxylase activity was confirmed also by a parallel rise in the incorporation of α-[5-¹⁴C]-difluoromethyl ornithine into trichloroacetic acid-insoluble fractions. Microautoradiographs of longitudinal and cross sections of roots, labelled with α-[5-¹⁴C]-difluoromethyl ornithine, showed that ornithine decarboxylase is localized mainly in the meristematic zones, as evidenced by [³H]-thymidine incorporation. A close correlation between meristematic activity and polyamines was demonstrated in situ , suggesting that polyamine content and biosynthesis may have a role in meristematic activity in corn roots.     

Effects of Osmotic Pressure on the Oxidative Metabolism of Leishmania major Promastigotes

Leishmania major promastigotes were washed and resuspended in an iso-osmotic buffer. The rate of oxidation of ¹⁴C-labeled substrates was then measured as a function of osmolality. An acute decrease in osmolality (achieved by adding H₂O to the cell suspension) caused an increase in the rates of ¹⁴CO₂ production from [6-¹⁴C]glucose and, to a lesser extent, from [1, (3)-¹⁴C]glycerol. An acute increase in osmolality (achieved by adding NaCl, KCl, or mannitol) strongly inhibited the rates of ¹⁴CO₂ production from [1-: ¹⁴C]alanine, [1-¹⁴C]glutamate, and [1, (3)-¹⁴C]glycerol. The rates of ¹⁴CO₂ formation from [1-¹⁴C]laurate, [1-¹⁴C]acetate, and [2-¹⁴C]glucose (all of which form [1-¹⁴C]acetyl CoA prior to oxidation) were also inhibited, but less strongly, by increasing osmolality. These data suggest that with increasing osmolality there is an inhibition of mitochondrial oxidative capacity, which could facilitate the increase in alanine pool size that occurs in response to hyper-osmotic stress. Similarly, an increase in oxidative capacity would help prevent a rebuild up of the alanine pool after its rapid loss to the medium in response to hypo-osmotic stress.     

The Synthesis, Storage, and Release of Propionylcholine by the Electric Organ of Torpedo marmorata

Abstract: Little is known about the specificity of the mechanisms involved in the synthesis and release of acetylcholine for the acetyl moiety. To test this, blocks of tissue from the electric organ of Torpedo were incubated with either [1-¹⁴C]acetate or [1-¹⁴C]propionate, and the synthesis, storage, and release of [1-¹⁴C]acetylcholine and [¹⁴C]propionylcholine were compared. To obtain equivalent amounts of the two labeled choline esters, a 50-fold higher concentration of propionate than of acetate was needed. Following subcellular fractionation, similar proportions of [¹⁴C]acetylcholine and [¹⁴C]propionylcholine were recovered with synaptosomes and with synaptic vesicles. Furthermore, both labeled choline esters were protected to a similar extent from degradation during homogenization of tissue in physiological medium, indicating that the two choline esters were equally well incorporated into synaptic vesicles. Yet depolarization of tissue blocks by 50 m M KCI released much less [¹⁴C]propionylcholinc than [¹⁴C]acetylcholine. During field stimulation of the tissue blocks, the difference between the releasibility of the two choline esters was less marked, but acetylcholine was still released in preference to propionylcholine. Evidence for specificity of the release mechanism was also obtained when the release of the two choline esters in response to field stimulation was compared in tissue blocks preincubated with both [³H]choline and [¹⁴C]propionate.     

GAMMA-HYDROXYBUTYRATE DEGRADATION IN THE BRAIN IN VIVO: NEGLIGIBLE DIRECT CONVERSION TO GABA

Abstract— The metabolism of γ-hydroxybutyrate (GHB) was studied by following the fate of [1-¹⁴C]GHB in mouse brain after an intravenous injection. Cerebral uptake of GHB was rapid and this substance disappeared from brain tissue with a half-life of approx 5 min. Degradation of [1-¹⁴C]GHB took place in the brain since ¹⁴C was incorporated in amino acids associated with the tricarboxylic acid cycle: the labelling pattern was consistent with the oxidation of GHB via succinate through the cycle, rather than with β-oxidation of GHB. Conversion of [¹⁴C]GHB into [¹⁴C]GABA prior to oxidation was negligible, thus it is unlikely that the pharmacological action of GHB would be mediated through GABA formation. [¹⁴C]GHB oxidation also elicited the signs of metabolic compartmentation of the tricarboxylic acid cycle in the brain (glutamine/glutamate specific radioactivity ratio was about 4).     

Incorporation of label from root-applied N6[8-14C]furfiuryIadenine into the guanine nucleotide fraction of pea bud ribonucleic acid

The pattern of incorporation of label into the nucleotides of axillary bud ribonucleic acid was investigated in Pisum sativum L. cv. Meteor following the application of N ⁶[8-^I4C]furfuryladenine or of [8-¹⁴C]adenine to the root system of decapitated plants and to cultured excised buds. When N ⁶[8-¹⁴C]furifaryladenine was applied to the root system label was confined to the guanine nucleotide moiety of the axillary bud ribonucleic acid; label from [8-¹⁴C]adenine was incorporated preferentially into adenine nucleotide in the molar ratio adenine nucleotide/guanine nucleotide = 3.23. When isolated buds were incubated in media containing [8-¹⁴C]adenine or N ⁶[8-¹⁴C]furfuryladenine, label was incorporated into both purine moieties of the ribonucleic acid. However, the relative incorporation into the guanine nucleotide fraction was considerably greater for N ⁶[8-^I4C]furfuryladenine (adenine nucleotide/guanine nucleotide = 2.23) than for [8-¹⁴C]adenine (ratio = 4.67).
It was concluded that the pattern of metabolism of adenine to guanine and its incorporation into the guanine nucleotide moiety of pea axillary bud ribonucleic acid, is influenced by the presence of a substitution in the N ⁶ position of the adenine base.     

Oligodendroglial Glycerophospholipid Synthesis: Incorporation of Radioactive Precursors into Ethanolamine Glycerophospholipids by Calf Oligodendroglia Prepared by a Percoll Procedure and Maintained in Suspension Culture

Abstract: Oligodendroglia prepared from minced calf cerebral white matter by trypsinization at pH 7.4, screening, and isosmotic Percoll (polyvinylpyr-rolidone-coated silica gel) density gradient centrifugation survived in culture on polylysine-coated glass, extending processes and maintaining phenotypic characteristics of oligodendroglia. In the present study, ethanolamine glycerophospholipid (EGP) metabolism of the freshly isolated cells was examined during short-term suspension culture by dual label time course and substrate concentration dependence experiments with [2-³H]glycerol and either [1,2-¹⁴C]ethanolamine or L-[U-¹⁴C]serine. Rates of incorporation of ³H from the glycerol and of ¹⁴C from the ethanolamine into EGP were constant for 14 h. In medium containing 3 mM-[1,2-¹⁴C]ethanolamine and 4.8 mM-[2-³H]glycerol, rates of incorporation of ¹⁴C and ³H into diacyl glycerophosphoethanolamine (diacyl GPE) were similar. Under the same conditions, ³H specific activities of alkylacyl GPE and alkenylacyl GPE were much lower than ¹⁴C specific activities, likely as a result of the loss of tritium during synthesis of these forms of EGP via dihydroxyacetone phosphate. L-[U-¹⁴C]serine was incorporated into serine glycerophospholipid (SGP) by base exchange rather than de novo synthesis. ¹⁴C from L-[U-¹⁴C]serine also appeared in EGP after an initial lag period of several hours. Methylation of oligodendroglial EGP to choline glycerophospholipid (CGP) was not detected.     

Metabolism of Cysteine in Astroglial Cells: Synthesis of Hypotaurine and Taurine

Abstract: The synthesis of hypotaurine and taurine was investigated in astroglia-rich primary cultures obtained from brains of neonatal Wistar rats using ¹H and ¹³C nuclear magnetic resonance (NMR) spectroscopy. Cell extracts of astroglial cultures analyzed by ¹H NMR spectroscopy show prominent signals of hypotaurine. To identify cysteine as precursor for hypotaurine and taurine synthesis in astroglial cells, primary cultures were incubated with [3-¹³C]cysteine for 24 or 72 h. Cell extracts and incubation media were then analyzed with ¹³C NMR spectroscopy. Labeled hypotaurine, taurine, glutathione, and lactate were identified in the cell extracts. Within 72 h, 35.0% of the total intracellular hypotaurine and 22.5% of taurine were newly synthesized from [3-¹³C]cysteine. The presence of [1-¹³C]hypotaurine and [1-¹³C]taurine in the incubation medium proves the release of those products of cysteine metabolism into the medium. Minor amounts of the [3-¹³C]cysteine were used for the synthesis of glutathione in astroglial cells or metabolized to [3-¹³C]lactate, which was found in cell extracts and media. These results indicate that the formation of hypotaurine and taurine is a major pathway of cysteine metabolism in astroglial cells.     

Feruloyl- and caffeoylputrescine in stem explants from photoperiodically determined Nicotiana species

The distribution and formation of hydroxycinnamoyl amides (HCA) in in vitro grown stem explants from day-neutral Nicotiana tabacum L. var. Xanthi no were investigated using [¹⁴C]-labelled precursors. Feruloylputrescine (FP) (labelled from [¹⁴C]-putrescine) was continually formed with a decreasing formation rate during a culture period of 35 days. The specific radioactivity, however, remained constant. In contrast, caffeoylputrescine (CP) was labelled in the second part of culture only. [UC]-Label from putrescine, spermidine, spermine, and cinnamic acid was incorporated into FP and p -coumaroylspermidine. CP was labelled from [¹⁴C]-putreseine and [¹⁴C]-spermidine only, while diferuloylputrescine (diFP) failed to be labelled by any of the precursors used. Tissue investigations showed that FP is dominant in cortex during formation of cortical callus, while CP showed a sequential gradient from pith to cortical callus to floral buds. Experiments with photoperiodical Nicotiana species were used to investigate the pattern of HCAs in induced and non-induced plants. The origin of explants had no influence on the pattern of FP and CP but resulted in different growth responses. FP is a marker of cortical callus formation in all explants. CP does not trigger the growth processes observed in the expants since CP also increased in photoperiodically non-induced explants of tobacco, which stopped growing after the formation of cortical callus.     

Mode of action of glyphosate in relation to metabolism of indole-3-acetic acid

Studies were conducted with radio-labeled indole-3-acetic acid ([2-¹⁴C] IAA) and tobacco callus culture ( Nicotiana tabacum L. cv. White Gold) to investigate the mode of action of the herbicide glyphosate (N-phosphonomethylglycine). The tissue was first grown with or without glyphosate for 1 to 14 days and then incubated with [2-¹⁴C] IAA for 4 h. Metabolism of [2-¹⁴C] IAA in the tissue was studies by solvent fractionation, high performance liquid chromatography and liquid scintillation counting. The tissue grown with 0.2 m M glyphosate had low level of free [2-¹⁴C] IAA and high levels of other fractions containing metabolites and conjugates of the labeled IAA. After 1 day of glyphosate treatment the free [2-¹⁴C] IAA level in the tissue was reduced by 77% compared to that of the control; after 10 days of treatment the decrease was 96%. The decrease in the free [2-¹⁴C] IAA level was not due to inhibition of IAA uptake, but due to enhanced rates of oxidation and conjugate formation of IAA. The increased oxidation of IAA in the treated tissue was not due to a direct effect of glyphosate on IAA-oxidase since glyphosate was inactive on IAA oxidation in a cell-free system in vitro. The glyphosate-induced growth inhibition was partially overcome by addition of 1 μ M 2,4-dichlorophenoxyacetic acid to the medium. The results lead to the conclusion that glyphosate inhibits growth by depletion of free IAA through rapid acceleration of both conjugate formation and oxidative degradation of IAA.     

Metabolism of Glycine in Primary Astroglial Cells: Synthesis of Creatine, Serine, and Glutathione

Abstract: The metabolism of [2-¹³C]glycine in astrogliarich primary cultures obtained from brains of neonatal Wistar rats was investigated using ¹³C NMR spectroscopy. After a 24-h incubation of the cells in a medium containing glucose, glutamate, cysteine, and [2-¹³C]glycine, cell extracts and incubation media were analyzed for ¹³C-labeled compounds. Labeled creatine, serine, and glutathione were identified in the cell extracts. If arginine and methionine were present during the incubation with [2-¹³C]glycine, the amount of de novo synthesized [2-¹³C]creatine was two-fold increased, and in addition, ¹³C-labeled guanidinoacetate was found in cell extracts and in the media after 24 h of incubation. A major part of the [2-¹³C]glycine was utilized for the synthesis of glutathione in astroglial cells. ¹³C-labeled glutathione was found in the cell extracts as well as in the incubation medium. The presence of newly synthesized [2-¹³C]serine, [3-¹³C]serine, and [2,3-¹³C]serine in the cell extracts and the incubation medium proves the capability of astroglial cells to synthesize serine out of glycine and to release serine. Therefore, astroglial cells are able to utilize glycine as a precursor for the synthesis of creatine and serine. This proves that at least one cell type of the brain is able to synthesize creatine. In addition, guanidinoacetate, the intermediate of creatine synthesis, is released by astrocytes and may be used for creatine synthesis by other cells, i.e., neurons.     

Altered cerebral glucose and acetate metabolism in succinic semialdehyde dehydrogenase-deficient mice: evidence for glial dysfunction and reduced glutamate/glutamine cycling

Succinic semialdehyde dehydrogenase (SSADH) catalyzes the NADP-dependent oxidation of succinic semialdehyde to succinate, the final step of the GABA shunt pathway. SSADH deficiency in humans is associated with excessive elevation of GABA and γ-hydroxybutyrate (GHB). Recent studies of SSADH-null mice show that elevated GABA and GHB are accompanied by reduced glutamine, a known precursor of the neurotransmitters glutamate and GABA. In this study, cerebral metabolism was investigated in urethane-anesthetized SSADH-null and wild-type 17-day-old mice by intraperitoneal infusion of [1,6-¹³C₂]glucose or [2-¹³C]acetate for different periods. Cortical extracts were prepared and measured using high-resolution ¹H-[¹³C] NMR spectroscopy. Compared with wild-type, levels of GABA, GHB, aspartate, and alanine were significantly higher in SSADH-null cortex, whereas glutamate, glutamine, and taurine were lower. ¹³C Labeling from [1,6-¹³C₂]glucose, which is metabolized in neurons and glia, was significantly lower (expressed as μmol of ¹³C incorporated per gram of brain tissue) for glutamate-(C4,C3), glutamine-C4, succinate-(C3/2), and aspartate-C3 in SSADH-null cortex, whereas Ala-C3 was higher and GABA-C2 unchanged. ¹³C Labeling from [2-¹³C]acetate, a glial substrate, was lower mainly in glutamine-C4 and glutamate-(C4,C3). GHB was labeled by both substrates in SSADH-null mice consistent with GABA as precursor. Our findings indicate that SSADH deficiency is associated with major alterations in glutamate and glutamine metabolism in glia and neurons with surprisingly lesser effects on GABA synthesis.     

Effect of Chronic Lead Ingestion by Rats on Glucose Metabolism and Acetylcholine Synthesis in Cerebral Cortex Slices

Abstract: The effect of chronic low-level lead (Pb²⁺) ingestion on the metabolic pathways leading to the acetyl moiety of acetylcholine (ACh) was examined. Cerebral cortex slices, prepared from untreated or Pb²⁺-exposed rats (600 ppm lead acetate in the drinking water for 20 days), were incubated in Krebs-Ringer bicarbonate buffer with 10 m M glucose and tracer amounts of [6-³H]glucose and either [6-¹⁴C]glucose or [3-¹⁴C] β -hydroxybutyrate. Altering the concentration of Pb²⁺ in the drinking water produced a dose-related increase in blood and brain lead levels. When tissue from Pb²⁺-exposed rats was incubated with mixed-labeled glucose, incorporation into lacate, citrate, and ACh was considerably decreased, although no changes occurred in the ³H/¹⁴C ratios. Similar effects of Pb²⁺ were found when ¹⁴C-labeled β -hydroxy-butyrate was substituted for the [¹⁴C]glucose. It appears from these data that Pb²⁺ exerts a generalized effect on energy metabolism and not on a specific step in glucose metabolism. The impairment of glucose metabolism may explain partially the Pb²⁺-induced changes observed in cholinergic function.     

Partitioning of CO2 Production Between Glucose and Lactate in Excised Sympathetic Ganglia, with Implications for Brain

Abstract: Chains of lumbar sympathetic ganglia from 15-day-old chicken embryos were incubated for 4 h at 36°C in a bicarbonate-buffered salt solution equilibrated with 5% CO₂-95% O₂. Glucose (1–10 m M ), lactate (1–10 m M ), [U-¹⁴C]glucose, [1-¹⁴C]glucose, [6-¹⁴C]glucose, and [U-¹⁴C]lactate were added as needed. ¹⁴CO₂ output was measured continuously by counting the radioactivity in gas that had passed through the incubation chamber. Lactate reduced the output of CO₂ from [U-¹⁴C]glucose, and glucose reduced that from [U-¹⁴C]lactate. When using uniformly labeled substrates in the presence of 5.5 m M glucose, the output of CO₂ from lactate exceeded that from glucose when the lactate concentration was >2 m M . The combined outputs at each concentration tested were greater than those from either substrate alone. The ¹⁴CO₂ output from [1-¹⁴C]glucose always exceeded that from [6-¹⁴C]glucose, indicating activity of the hexose monophosphate shunt. Lactate reduced both of these outputs, with the maximum difference between them during incubation remaining constant as the lactate concentration was increased, suggesting that lactate may not affect the shunt. Modeling revealed many details of lactate metabolism as a function of its concentration. Addition of a blood-brain barrier to the model suggested that lactate can be a significant metabolite for brain during hyperlactemia, especially at the high levels reached physiologically during exercise.     

On the Role of Long-Chain Aldehydes in Mammalian Plasmalogen Biosynthesis

Abstract: [1-³H, 1-¹⁴C]Palmitaldehyde(³H:¹⁴C= 15) was injected intracerebrally to 18-day-old rats and incorporation of radioactivity into brain lipids was followed over a 24-h period. The substrate was metabolized primarily by oxidation to palmitic acid with loss of tritium and, to a lesser extent, by reduction to hexadecanol. The alkyl moieties of the ethanolamine phospholipids showed considerably lower ³H:¹⁴C ratios than the substrate, indicating a substantial participation in ether lipid synthesis by tritium-free alcohols derived from ¹⁴C-labeled fatty acids. Virtually no ³H radioactivity was found in alkenyl moieties, indicating stereospecificity of both reduction of aldehyde and dehydrogenation of alkyl to alkenyl glycerolipid. The data are consistent with the general concept that plasmalogen biosynthesis proceeds exclusively through fatty alcohols and alkyl glycerolipids and that fatty aldehydes cannot be utilized directly.     

METABOLISM OF γ-HYDROXY-[1-14C] BUTYRATE BY RAT BRAIN: RELATIONSHIP TO THE KREBS CYCLE AND METABOLIC COMPARTMENTATION OF AMINO ACIDS

Abstract— Ninhydrin decarboxylation experiments were carried out on the labelled amino acids produced following intraventricular injection of either γ-hydroxy-[1-¹⁴C]butyric acid (GHB) or [1-¹⁴C] succinate. The loss of isotope (as ¹⁴CO₂) was similar for both substances. The [1-¹⁴C]GHB metabolites lost 75% of the label and the [1-¹⁴C] succinate metabolites lost 68%. This observation gives support to the hypothesis that the rat brain has the enzymatic capacity to metabolize [1-¹⁴C]GHB to succinate and to amino acids that have the isotope in the carboxylic acid group adjacent to the a-amino group. These results also indicate that the label from [1-¹⁴C]GHB does not enter the Krebs cycle as acetate. The specific activity ratio of radiolabelled glutamine to glutamic acid was determined in order to evaluate which of the two major metabolic compartments preferentially metabolize GHB. It was found that for [1-¹⁴C]GHB this ratio was 4.20 ± 0.18 (S.E. for n = 7) and for [l-¹⁴C]succinate this ratio was 7.71 (average of two trials, 7.74 and 7.69). These results suggest that the compartment thought to be associated with glial cells and synaptosomal structures is largely responsible for the metabolism of GHB. Metabolism as it might relate to the neuropharmacological action of GHB is discussed.     

IN VIVO FORMATION AND CATABOLISM OF [14C]HISTAMINE IN MOUSE BRAIN

Abstract— The formation of histamine in brain was studied in mice injected with l -[¹⁴C]-histidine (ring 2-¹⁴C) intravenously (i.v.) or intracerebrally; [¹⁴C]histamine appeared rapidly and exhibited a rapid rate of turnover. Drugs known to block various pathways of histamine catabolism were tested for effects on brain–[¹⁴C]histamine and [¹⁴C]-methyl-histamine in mice given (1) [¹⁴C]histamine i.v., (2) [¹⁴C]histamine intracerebrally, and (3) l -[¹⁴C]histidine i.v. Blood-borne histamine did not enter brain; brain histamine was formed locally by decarboxylation of histidine Methylhistamine did cross the blood-brain barrier. Methylation was the major route of histamine catabolism in mouse brain and some of the methylhistamine formed was destroyed by monoamine oxidase. No evidence for catabolism by the action of diamine oxidase was found.     

14C-LABELLED AMINO ACIDS AND GLUCOSE IN RAT BRAIN AND LIVER AFTER INJECTION OF [2-14C]PROPIONATE

Abstract— [2-¹⁴C]Propionate injected into rats was metabolized into [¹⁴C]glucose and ¹⁴C-labelled aspartate, glutamate, glutamine and alanine. The results are consistent with the conversion of propionate into succinate and the oxidation of succinate into oxaloacetate, the precursor of labelled amino acids and the substrate for gluconeogenesis.
The ratio of the specific radioactivity of glutamine to glutamate was greater than 1 during the 30 min period in the brain, indicating that propionate taken up by the brain was metabolized mainly in the 'small glutamate compartment' in the brain. The results, therefore, support the previous conclusion (G aitonde , 1975) that the labelling of amino acids by [¹⁴C]propionate formed from [U-¹⁴C>]-threonine in thiamin-deficient rats was metabolized in the 'large glutamate compartment' of the brain.
The specific radioactivity ratio of glutamine to glutamate in the liver was less than 1 during the 10 min period but greater than 1 at 30min. These findings which gave evidence against metabolic compartments of glutamate in the liver, were interpreted as indicative of the entry of blood-borne [¹⁴C]glutamine synthesized in other tissues, e.g. brain. The labelling of amino acids when compared to that after injection of [U-¹⁴C]glucose showed that [2-¹⁴C]propionate was quantitatively a better source of amino acids in the liver. The concentration of some amino acids in the brain and liver was less in the adult than in the young rats, except for alanine and glutathione, where the liver content was more than double that in the adult.     

Oxidation of Leucine by Leishmania donovani

ABSTRACT. The metabolism of leucine by Leishmania donovani was investigated. Washed promastigotes were incubated with (1-¹⁴C]-or [U-¹⁴C]leucine or [1 -¹⁴C]α-ketoisocaproate (KIC) and ¹⁴C0₂ release was measured. The amount of KIC-derived acetyl-CoA oxidized in the citric acid cycle was computed. Promastigotes from mid-stationary phase cultures oxidized each of these labeled substrates less rapidly than cells from late log phase cultures, and significantly less acetyl-CoA derived from KIC oxidation was oxidized in the citric acid cycle. Glucose was a stronger inhibitor than was acetate of CO₂ formation in the citric acid cycle in log phase promastigotes, but the reverse was observed in cells from mid-stationary phase. Alanine also inhibited leucine catabolism, but glutamate had little effect. Acute hypo-osmotic stress did not affect leucine catabolism, but hyper-osmotic stress caused appreciable inhibition of leucine oxidation. Cells grown under hypo-or hyper-osmotic conditions showed no changes in the effects of hypo-or hyper-osmotic stress on leucine catabolism, i.e. L. donovani is not an osmoconformer with respect to leucine metabolism. Leucine utilization in L. donovani was insensitive to a number of drugs that affect leucine metabolism in mammalian cells, indicating that the leucine pathway in L. donovani is not regulated in the same manner as in mammalian cells.     

Exchange-mediated dilution of brain lactate specific activity: implications for the origin of glutamate dilution and the contributions of glutamine dilution and other pathways

The magnitude of metabolic activation is greatly underestimated in autoradiographic studies using [1- or 6-¹⁴C]glucose compared to parallel assays with [¹⁴C]deoxyglucose indicating that most of the label corresponding to the additional [¹⁴C]glucose consumed during activation compared to rest is quickly released from activated structures. Label could be lost by net release of [¹⁴C]lactate from brain or via lactate exchange between blood and brain. These possibilities were distinguished by comparison of glucose and lactate specific activities in arterial blood and brain before, during, and after generalized sensory stimulation and during spreading cortical depression. Over a wide range of brain lactate concentrations, lactate specific activity was close to the theoretical maximum, i.e. half that of [6-¹⁴C]glucose, indicating that exchange-mediated dilution of lactate is negligible and that efflux of [¹⁴C]lactate probably accounts for most of the label loss. Low lactate dilution also indicates that dilution of glutamate C4 fractional enrichment in [¹³C]glucose studies, currently ascribed predominantly to lactate exchange, arises from other unidentified pathways or factors. Alternative explanations for glutamate dilution (presented in Supporting Information) include poorly labeled amino acid pools and oxidative metabolism of minor substrates in astrocytes to first dilute the astrocytic glutamine pool, followed by dilution of glutamate via glutamate–glutamine cycling.