Storage and remobilisation of sulphur in beech trees (Fagus sylvatica)

Three-year-old beech trees were fed ³⁵S-sulphate in August 1993 via a flap in a mature leaf of an upper branch. Harvest of beech trees was performed 24 h after feeding ³⁵S-sulphate, before leaf senescence, after leaf abscission, in early winter (January 1994). in late winter (March 1994). before bud break and after bud break. Twenty-four h after feeding ³⁵S-sulphate, 0.7 ± 0.5% of the ³⁵S-radioactivity taken up was exported out of the fed leaf. When trees were analysed 2 months later, i.e., before leaf senescence, this value had increased to 22 ± 7%. The exported ³⁵S-radioactivity was located in the branch containing the fed leaf (2.8 ± 13%). in basipetal parts of the trunk (41 ± 77%) and in the main rool (21 ± 6%). Leaves and apical parts of the trunk were no sink organs for the exported sulphur. Along the tree axis the main proportion of the radiolabel was located in the wood, predominantly in the acid soluble fraction. In the bark the greater portion of the radiolabel was found in the acid insoluble fraction. In both tissues the bulk of the ³⁵S of the soluble fraction was sulphate together with small amounts of glutathione. This pattern did not change until bud break. After bud break, basipetal parts of the trunk lost part of its ³⁵S-radioactivity. Of the ³⁵S-radioactivity which had been exported out of the fed leaf during the previous autumn, 16 ± 2% remained in the trunk, whereas 47 ± 7% of the ³⁵S was found in branches, mainly in the newly developed leaves. The present results show that sulphur, mainly in the form of sulphate, is stored along the tree axis in both bark and wood of beech trees and is re-mobilised during leaf development in spring.     

Translocation and cycling through roots of recently absorbed nitrogen and sulphur in wheat (Triticum aestivum) during vegetative and generative growth

¹⁵N-Nitrate and ³⁵S-sulphate labelling experiments were performed with spring wheat ( Triticum aestivum L. cv. Timmo) 44. 64, 79, 95 and 115 days after sowing (growth stages arbitrarily denoted I to V). Label was fed to the plants via a fraction of the root system, termed "donor root", whereas the rest of the root ("receiver root") was fed non-labelled nutrient solution. Net uptake rates for both nitrate and sulphate per unit root weight changed little from growth stage I to IV, but were considerably lower at stage V. On a whole-plant weight basis, uptake declined from stage I to IV, because root contribution to total plant weight declined. Between 80 and 95% of absorbed label was translocated to the shoot at all growth stages. At stage V, up to 30% of absorbed label was recovered in the ears. Labelling of the receiver root indicated that, at all growth stages, 10 to 17% of N and 12 to 32% of S translocated to the shoot was retranslocated to the root. This corresponds to between 35 and 85% of the label actually recovered in the roots. Analysis of ¹⁵N-labelling of xylem sap collected from receiver roots at growth stages I to IV indicated that about half of the reduced N in the sap is derived from cycling through roots of recently assimilated N. Evidence of cycling was also obtained at stage V. Labelled sulphate was the only form of S cycled in the plant, but it accounted for only 1 to 7% of the sulphate in the xylem sap.     

Rate of 59Fe Uptake into Brain and Cerebrospinal Fluid and the Influence Thereon of Antibodies Against the Transferrin Receptor

Abstract: Uptake of ⁵⁹Fe from blood into brains of anaesthetized rats and mice has been studied by intravenous infusion of [⁵⁹Fe]ferrous ascorbate or of ⁵⁹Fe-transferrin, the results not being significantly different. Uptakes in the rat were linear with time, but increased at longer times in the mouse. Transfer constants, K _in (in ml/g/h × 10³), for cerebral hemispheres were 5.2 in the adult rat and 5.6 in the mouse. These K _in values corresponded to ⁵⁹Fe influxes of 145 and 322 pmol/g/h, respectively. ⁵⁹Fe uptake into the mouse brain occurred in the following order: cerebellum > brainstem > frontal cerebral cortex > parietal cortex > occipital cortex > hippocampus > caudate nucleus. In genetically hypotransferrinaemic mice, ⁵⁹Fe uptake into brain was 80–95 times greater than in To strain mice. Pretreatment of young rats and mice with monoclonal antibodies to transferrin receptors, i.e., the anti-rat immunoglobulin G OX 26 and the anti-mouse immunoglobulin M RI7 208, inhibited ⁵⁹Fe uptake into spleen by 94% and 98%, respectively, indicating saturation of receptors. The antibodies reduced ⁵⁹Fe uptake into rat brain by 35–60% and that into mouse brain by 65–85%. Although a major portion of iron transport across the blood-brain barrier is normally transferrin-mediated, non-transferrin-bound iron readily crosses it at low serum transferrin levels.     

Selenium uptake by Butyrivibrio fibrisolvens and Bacteroides ruminicola

Abstract The uptake and incorporation of ⁷⁵[Se]selenite by Butyrivibrio fibrisolvens and Bacteroides ruminicola were by constitutive systems. Rates of uptake were higher in chemostat culture than in batch culture and there may be some inducible component. Uptake of [⁷⁵Se]selenite was distinct from sulphate or selenate transport, since sulphate and selenate did not inhibit selenite uptake, nor could sulphate or selenate uptake be demonstrated in these organisms. Selenite uptake in B. fibrisolvens had and apparent K _m of 1.74 mM and a V _max of 109 ng Se · min⁻¹· (mg protein)⁻¹. An apparent K _m of 1.76 mM and V _max of 1.5 μg Se · min⁻¹· (mg protein)⁻¹ was obtained for B. ruminicola . [⁷⁵Se]Selenite uptake by both organisms was partially sensitive to inhibition by 2,4-DNP. Uptake by B. fibrisolvens was also partially inhibited by azide and arsenate and in B. ruminicola it was partially inhibited by fluoride. CCCP, CPZ, DCCD or quinine did not inhibit uptake in either B. fibrisolvens or B. ruminicola . Selenite transport by both organisms was sensitive to IAA and NEM and was strongly inhibited by sulphite and nitrite. [⁷⁵Se]Selenite was converted to selenocystine, selenohomocystine and selenomethionine by B. fibrisolvens. B. ruminicola did not incorporate [⁷⁵Se]selenite into organic compounds, but did reduce it to red elemental selenium.     

Inhibition of sulphur redistribution into new leaves of vegetative soybean by excision of the maturing leaf

When soybean plants are pulsed with [³⁵S]sulphate, label is subsequently redistributed from the roots to the leaves. This confounds studies to measure the redistribution of label from leaves. Accordingly, soybean plants ( Glycine max [L.] Merr. cv. Stephens) were grown in 20 μ M sulphate and a small portion of the root system (donor root) was pulsed with [³⁵S]sulphate for 24 h. After removing the donor root, the plants were transferred into unlabelled solution, either without sulphate (S20→SO) or with 20 μ M sulphate (S20→20) (intact plants). Also at this time, the expanding leaf (L3) was excised from half of the plants in each treatment (excised plants). Immediately after the pulse, only ca 15% of the label occurred in the roots and ca 40% in the expanding leaf, L3, mostly in the soluble fraction. In intact S20→20 plants, ³⁵S-label was exported from the soluble fraction of L3, mostly as sulphate, whilst L4 and L5 imported label. Similar responses occurred in S20→SO plants except that export of label from L3 was more rapid. Excision of L3 from S20→S20 plants inhibited labelling of leaves L4-L6 but not total sulphur, whereas in S20→SO plants, excision of L3 inhibited the import of both total sulphur and ³⁵S-label in leaves L4, L5 and L6. The data suggest that the soluble fraction of almost fully expanded leaves is an important reserve of sulphur for redistribution to growing leaves. The ³⁵S-label in the root system exhibited fluctuations consistent with its proposed role in the recycling of soluble sulphur from the leaves.     

High Doses of Vitamin B6 in the Rat Are Associated with Inhibition of Hepatic Tryptophan Metabolism and Increased Uptake of Tryptophan into the Brain

Abstract: Acute administration of vitamin B₆ to rats (10 mg/kg body weight) led to reduced urinary excretion of N ¹-methyl nicotinamide and methyl pyridone carboxamide, indicating inhibition of the oxidative metabolism of tryptophan. There was a considerable reduction in the production of ¹⁴CO₂ from [ ring -2-¹⁴C]tryptophan, and a significant inhibition of hepatic tryptophan oxygenase when measured in liver homogenates, together with an increase in the concentration of tryptophan in plasma. There was an increase in both the concentration of tryptophan in the brain and the uptake into the brain of peripherally administered [³H]tryptophan, accompanied by a small increase in the rate of synthesis of 5-hydroxy-tryptamine in the brain. It is suggested that this increase in the uptake of tryptophan into the brain following a relatively large dose of vitamin B₆ may explain the beneficial action of the vitamin in some cases of depressive illness.     

Lindane Administration to the Rat Induces Modifications in the Regional Cerebral Binding of [3H]Muscimol, [3H]-Flunitrazepam, and t-[35S]Butylbicyclophosphorothionate: An Autoradiographic Study

Abstract: The effect of lindane administration on the specific binding of ligands to different sites on the GABA_A receptor-ionophore complex was studied in the rat brain by receptor mapping autoradiography. [³H]Muscimol (Mus), [³H]flunitrazepam (Flu), and t -[³⁵S]butylbicyclophosphorothionate (TBPS) were used as specific ligands of GABA, benzodiazepine, and picrotoxinin binding sites, respectively. Rats received a single oral dose of 30 mg/kg lindane and they were classified into two groups according to the absence or presence of convulsions. Vehicle-treated groups acted as controls. The effect of the xenobiotic on ligand binding was measured in different brain areas and nuclei 12 min or 5 h after its administration. Lindane induced a generalized decrease in [³⁵S]TBPS binding, which was present shortly after dosing. In addition, [³H]Flu binding was increased in lindane-treated animals, this modification also appearing shortly after administration but diminishing during the studied time. Finally, lindane induced a decrease in [³H]Mus binding, which became more evident over time. These modifications were observed both in the presence and in the absence of convulsions. However, an increase in [³H]-Mus binding was detected shortly after lindane-induced convulsions. The observed decrease in [³⁵S]TBPS binding is in agreement with the postulated action of lindane at the picrotoxinin binding site of the GABA_A receptor chloride channel. The effects observed on the binding of [³H]Flu and [³H]Mus may be secondary to the action of lindane as an allosteric antagonist of the GABA_A receptor.     

Quantitative aspects of triacylglycerol metabolism and sterol synthesis from 14C-acetate in etiolated seedlings of Euphorbia lambii

Triacylglycerols occur in both the endosperm and embryo of Euphorbia lambii seeds. Upon germination, the amount of these neutral lipids in the endosperm decreased with 1.06 mg fatty acid day^-1. The embryo contained 1.4 mg fatty acids in the triacylglycerols and this value declined slowly to 0.4 mg seedling^-1 during the 8 day period of endosperm depletion. Radioactive acetate was rapidly taken up by the cotyledons of intact seedlings, translocated throughout the entire seedling, and up to 10.5% of the ¹⁴C proceeded to the sterols and latex triterpenols. Maximum uptake values of 1.4 μmol seedling^-1 day^-1 of acetate were measured. Acetate uptake and subsequent incorporation into sterols and triterpenols decreased substantially in the presence of increasing amounts of sucrose (up to 0.3 M). Traces of acetate did not effect [¹⁴C]-sucrose uptake and corresponding synthesis of [¹⁴C]-sterols and triterpenols, but increased concentrations of acetate (0.05 M and up) reduced both uptake of sucrose and its conversion into unsaponifiable lipids.
The uptake capacity of the cotyledons for [¹⁴C]-glycerol exceeded the daily production in the endosperm, but only a small amount of label proceeded to the sterols and triterpenols. [¹⁴C]-Triacylglycerols were never detected in the seedling, regardless of the labeled substrate used. Although acetate is an efficient precursor in triterpenol and sterol synthesis, the uptake capacity of the cotyledons for this metabolite is too small in relation to the daily production of water soluble substrates in the endosperm. If acetate is released by the endosperm, only a marginal contribution towards triterpenol and sterol synthesis in the seedling is to be anticipated from this substrate.     

Melatonin Effect on [3H]Glutamate Uptake and Release in the Golden Hamster Retina

Abstract: The effect of melatonin on [³H]glutamate uptake and release in the golden hamster retina was studied. In retinas excised in the middle of the dark phase, i.e., at 2400 h, melatonin (0.1 and 10 n M ) significantly increased [³H]glutamate uptake, and this effect persisted in a Ca²⁺-free medium. On the other hand, melatonin significantly increased [³H]glutamate release in retinas excised at 2400 h, but this effect was Ca²⁺ sensitive. Melatonin significantly increased ⁴⁵Ca²⁺ uptake by a crude synaptosomal fraction from retinas of hamsters killed at 2400 h. In retinas excised at 1200 h, melatonin had no effect on [³H]glutamate uptake, [³H]glutamate release, or ⁴⁵Ca²⁺ uptake at any concentration tested. Cyclic GMP analogues, i.e., 8-bromoguanosine 3',5'-cyclic monophosphate and 2'- O -dibutyrylguanosine 3',5'-cyclic monophosphate, significantly increased [³H]glutamate uptake, [³H]glutamate release, and ⁴⁵Ca²⁺ uptake by tissue removed at 1200 and 2400 h, suggesting that the effects of melatonin could correlate with a previously described effect of melatonin on cyclic GMP levels in the golden hamster retina. Taking into account the key role of glutamate in visual mechanisms, the results suggest the participation of melatonin in retinal physiology.     

Actual escape area and lateral escape from the xylem of the alkali ions Na+, K+, Rb+ and Cs+ in tomato

Approximation of the total escape area of the xylem in an inbred line of tomato (Ly-copersicon escutentum Mill. cv. Tiny Tim) with help of the frequency distribution of xylem vessel radii provides the possibility to calculate realistic escape constant values from uptake experiments of several elements into tomato stem segments. Comparison of the lateral escape rates of ²⁴Na⁺, ⁴²K⁺, ⁸⁶Rb⁺ and ¹³⁴Cs⁺ indicate that Na⁺ escape is rate-limited by its uptake into a rather constant number of surrounding cells, regardless of changes in the total escape area of the xylem vessels. The escape of K⁺, Rb⁺ and Cs⁺ seems to be proportional to the surface area of the xylem vessels and their escape is apparently controlled by their transport across the cell walls of the transport channels. The calculated small values for the escape rate constants (apparent permeability of the xylem cell walls, ca 2–3 · 10−⁹ m s−⁷) are probably due to the presence of lignin in the xylem cell walls, the discrimination between ions as a result of differing affinities and selectivities and the presence of other solutes in the applied solution.     

Translocation of carbon and nitrogen from the turbellarian host Amphiscolops langerhansi (Turbellaria: Acoela) to its algal endosymbiont Amphidinium klebsii (Dinophyceae)

The movement of carbon and nitrogen from Amphiscolops langerhansi , fed on ¹⁴C- and ¹⁵N-labelled Tigriopus japonica , to its algal endosymbiont Amphidinium klebsii is examined in a series of experiments designed to detect both sources in the host and utilization by the symbiont. Data for carbon indicate that.1. klebsii utilizes host carbon primarily as respired CO₂ taken up in photosynthesis. Data for nitrogen suggest that translocation takes place primarily through the excretion of ammonia, and that uptake and incorporation by the alga is light-dependent.     

Mechanisms of cobalt uptake in plants: 60CO uptake and distribution in Chara

The mechanism of cobalt uptake was investigated using cells of the giant alga Chara corallina in which it is possible to resolve separately uptake by the cell wall and actual influx across the cell membrane. The absorption of ⁶⁰Co by Chara cells appeared to saturate within 2 h, but this was mainly due to rapid uptake into the cell wall which accounted for 87–92% of the total activity. Even after prolonged desorption most of the cell‐associated ⁶⁰Co was found on the cell wall. The intracellular distribution of absorbed ⁶⁰Co was investigated by fractionating the cell into cytoplasm and vacuole. It was shown that ⁶⁰Co influx to the vacuole occurs simultaneously with influx to the cytoplasm. The transported species appears to be Co²⁺ rather than the less charged Co(OH)⁺ or Co(OH)₂. ⁶⁰Co influx is pH dependent (optimum pH 7–9), and is sensitive to some other divalent metals. Influx from solutions containing 1 µ M ⁶⁰Co was inhibited by 5 µ M Cd²⁺, Cu²⁺, and Zn²⁺, but Mn²⁺ and Ni²⁺ had no significant effect. The sensitivity of Co uptake to N ‐ethyl maleimide (NEM) and cysteine suggests that the transport system involves direct binding of CO²⁺ to ‐SH groups.     

Formation and Utilization of the Active Sulfate Donor [35S]3'-Phosphoadenosine 5'-Phosphosulfate in Brain Slices: Effects of Depolarizing Agents

Abstract: The accumulation and utilization of [³⁵S]3'-phos-phoadenosine 5'-phosphosulfate (PAPS) were studied in slices from rat cerebral cortex incubated in the presence of inorganic [³⁵S]sulfate. [³⁵S]PAPS levels were directly evaluated after either isolation by ion-exchange chromatography or quantitative enzymatic transfer of its active [³⁵S]sulfate group to an acceptor phenol under the action of added phenolsulfotransferase activity. [³⁵S]PAPS formation was also indirectly followed by incubating slices in the presence of β-naphthol and measuring the levels of [³⁵S]β-naphthyl sulfate ([³⁵S]β-NS). Whereas [³⁵S]PAPS levels rapidly reached a plateau, [³⁵S]β-NS formation proceeded linearly with time for at least 1h, an observation indicating that the nucleotide was continuously synthesized and utilized for endogenous sulfation reactions. [³⁵S]PAPS formation in ices was completely and rather potently blocked by 2,6-dichloro-4-nitrophenol (IC₅₀= .10 μM), an inhibitor of the PAPS-synthesizing enzyme system in a cytosolic preparation. [³⁵S]PAPS accumulation and [³⁵S]β-NS'formation were strongly reduced by depolarizing agents such as potassium or veratridine. At millimolar concentrations, various excitatory amino acids (glutamate, aspartate, cysteate, quisqualate, and homocysteate) also elicited similar effects, whereas kainate and N -methyl-D-aspartate were inactive. This suggests that PAPS synthesis is turned off when cerebral cells are strongly depolarized.     

Comparison of (R)-[3H] Tomoxetine and (R/S)-[3H] Nisoxetine Binding in Rat Brain

Abstract: ( R )-[³H]Tomoxetine is a radioligand that binds to the norepinephrine (NE) uptake site with high affinity but also binds to a second, lower-affinity site. The goal of the present study was to identify the nature of this low-affinity site by comparing the binding properties of ( R )-[³H]tomoxetine with those of ( R/S )-[³H]nisoxetine, a highly selective ligand for the NE uptake site. In homogenate binding studies, both radioligands bound to the NE uptake site with high affinity, whereas ( R )-[³H]tomoxetine also bound to a second, lower-affinity site. The autoradiographic distribution of binding sites for both radioligands is consistent with the known distribution of NE-containing neurons. However, low levels of ( R )-[³H]-tomoxetine binding were seen in the caudate-putamen, globus pallidus, olfactory tubercle, and zona reticulata of the substantia nigra, where ( R/S )-[³H]nisoxetine binding was almost absent. In homogenates of the caudate-putamen, the NE uptake inhibitors desipramine and ( R )-nisoxetine and the serotonin (5-HT) uptake inhibitor citalopram produced biphasic displacement curves. Autoradiographic studies using 10 n M ( R )-nisoxetine to mask the binding of ( R )-[³H]tomoxetine to the NE uptake site produced autoradiograms that were similar to those produced by [³H]citalopram. Therefore, ( R )-[³H]tomoxetine binds to the NE uptake site with high affinity and the 5-HT uptake site with somewhat lower affinity.     

Effect of cytokinin on the distribution of 14C-glyphosate in Xanthium pennsylvanicum

Cytokinin N⁶-(δ²-isopentenyl) adenosine (i⁶A) was applied five times to runoff on the first leaf of Xanthium pennsylvanicum Wallr. After a 24 h application period the second leaf was treated with the herbicide methyl-¹⁴C-glyphosate, [N-(phosphonomethyl)glycine]. The distribution of radioactivity was determined after 12 and 24 h. At no time after treatment did ¹⁴C-glyphosate move preferentially into the i⁶A treated leaf. Radioactivity accumulated in all plant parts, but in i⁶A-treated plants, there were more counts in the actively growing areas after 12 h. The primary effect of i⁶A after 24 h was enhanced uptake of ¹⁴C-glyphosate into the plant. To determine if the increased uptake and altered movement of ¹⁴C-glyphosate was due to a cytokinin-induced transpiration increase, leaf diffusive resistance was measured. Leaf diffusive resistances were unaffected by i⁶A during the 48 h after application. By 12 h after the application of glyphosate, the herbicide was found not to affect the cytokinin content in the Xanthium root tips.     

Passive uptake of K+(86Rb+) in sunflower roots at low external K+ concentrations

Passive fluxes of K⁺ (⁸⁶Rb) into roots of sunflower ( Helianthus annuus L. cv. Uniflorus) were determined at low K⁺ concentration (0.1 and 1.0 mM K⁺) in the ambient solution. Metabolic uptake of K⁺ was inhibited by 10⁻⁴M 2,4-dinitrophenol (DNP). K⁺ (⁸⁶Rb) fluxes were studied both continuously and by time differentiation of uptake. In high K⁺ roots passive uptake was directly proportional to the K⁺ concentration of the uptake solution, indicating free diffusion. This assumption was supported by the fact that passive Rb⁺ uptake was not affected by high K⁺ concentrations. In low K⁺ roots the passive uptake of K⁺ was higher than in high K⁺ roots. The increase was possibly due to carrier-mediated K⁺ transport. As K⁺ effluxes were quantitatively similar to influxes, it is suggested that passive K⁺ fluxes represent exchange diffusion without relation to net K⁺ transport.     

UPTAKE AND RELEASE OF TAURINE FROM RAT BRAIN SLICES

Abstract— Rapid efflux of [³⁵S]taurine from rat brain slices was observed on electrical stimulation. Slower release resulted when the Ca²⁺ content of the perfusion medium was replaced with Mg²⁺. Uptake of [³⁵S]taurine into rat cortical slices was unaffected by GABA, glutamic acid, glycine and leucine but was inhibited by alanine, ouabain, KCN and 2,4-dinitrophenol. Of a number of analogues of taurine, 2-aminoethylsulphinic acid was the most potent in inhibiting the uptake of [³⁵S]taurine. The rate of uptake was found to be decreased by lowering the incubation temperature. The possibility that taurine may be a neurotransmitter is discussed.     

Uptake of 36Cl and 22Na by the Choroid Plexus-Cerebrospinal Fluid System: Evidence for Active Chloride Transport by the Choroidal Epithelium

Abstract: Cl and Na transport by the lateral ventricle (LVCP) and fourth ventricle (4VCP) choroid plexuses were examined by kinetic analysis of ³⁶Cl and ²²Na uptake into the choroid plexus-CSF system of the adult rat. Both radioisotopes required more than 5 h to reach steady-state distribution in the in vivo choroid plexuses and CSF after intraperitoneal injection. Whereas the LVCP and 4VCP ³⁶Cl steady-state spaces were comparable (55–56%), the 4VCP ²²Na space (39%) tended to be greater than the LVCP ²²Na space (36%). No evidence for inexchangeable Cl or Na was found for the choroid plexuses; the radioisotopic and chemical spaces were not significantly different. Choroid plexus ³⁶Cl and ²²Na uptake curves were resolved into two components, a fast component ( t _1/2 0.02–0.05 h) and a slow component ( t _1/2 0.85–1.93 h). By analysis of the distribution of [³H]inulin, [³H]mannitol, and ⁵¹Cr-tagged erythrocytes within the choroid plexuses, the fast component of ³⁶Cl and ²²Na uptake was found to represent extracellular and erythrocyte contributions to the tissue radioactivity, whereas the slow component represented isotope movement into the epithelial cell compartment. The calculated cell [Cl] of LVCP and 4VCP, 67 mmol/kg cell water, was 3.9 times greater than that predicted by the membrane potential for passive distribution. It is postulated that Cl is actively transported into the choroid epithelial cell across the basolateral membrane; the energy source for active Cl transport may be the Na electrochemical potential gradient (˜90 mV), which is twice that of the Cl electrochemical potential gradient (˜45 mV).     

Characterization, Distribution, and Protein Kinase C-Mediated Regulation of [35S]Glutathione Binding Sites in Mouse and Human Spinal Cord

Abstract: We have characterized a high-affinity [³⁵S]-glutathione ([³⁵S]GSH) binding site in mouse and human spinal cord. [³⁵S]GSH binding sites in mouse and human spinal cord were observed largely within the gray matter in both the dorsal and ventral horns of spinal cord at cervical, thoracic, and lumbosacral segments. High-affinity [³⁵S]GSH binding was saturable, showing a B _max of 72 fmol/mg of protein and a K _D of 3.0 n M for mouse spinal cord and a B _max of 52 fmol/mg of protein and a K _D of 1.6 n M for human spinal cord. [³⁵S]GSH binding was displaceable by GSH, l -cysteine, and S -hexyl-GSH, but not by glutamate, glycine, or NMDA. These [³⁵S]GSH binding sites exhibited kinetic and saturation characteristics similar to GSH binding sites in rat brain astrocytes. To determine whether [³⁵S]GSH binding sites could be regulated by protein kinase C, we exposed human spinal cord sections to phorbol 12,13-diacetate for 1 h before ligand binding. Phorbol ester treatment increased [³⁵S]GSH binding by ∼60%, an effect that could be blocked by exposure of spinal cord sections to 1-(5-isoquinolinylsulfonyl)-2-methylpiperazine, a general protein kinase inhibitor. [³⁵S]GSH binding sites in the spinal cord of both species exhibited many of the characteristics of a receptor including saturable binding, high affinity, ligand specificity, and modulation by kinase activity. These data suggest that GSH is a neurotransmitter in the CNS.     

COMPARATIVE STUDIES ON THE DEGRADATION OF GABA and TAURINE IN THE BRAIN

Abstract— The degradation of taurine and GABA in mammalian brain was studied in vivo and in vitro. Small amounts of [³⁵S]isethionate (10–20 pmol/g brain wet weight) and [³⁵S]sulphate (about 2 pmol/g) were detected in mouse brain after intramuscular injection of [³⁵S]taurine. Taurine also produced isethionate in rat brain homogenates (about 20 nmol/h/g protein) and subcellular fractions (about 40 nmol/h/g protein in synaptosomes and about 300 nmol/h/g in mitochondria), but the reaction was not stimulated either by external electrical pulses or by the addition of various cofactors (NAD and NADP in both oxidized and reduced forms, riboflavin, glutathione. pyridoxal-5'-phosphate, ATP) to the incubation medium. [¹⁴C]GABA was readily metabolized to [¹⁴C]succinate both in vivo and in vitro. Isethionate formation activity was concentrated in the mitochondrial fraction, as was also GABA-T activity. Partially purified GABA-T from calf brain also slightly catalysed the formation of [³⁵S]isethionate (about 1.3 μmol/min/g protein) from [³⁵S]taurine. It appears that the slight formation of isethionate from taurine is coupled to GABA-T activity. The formation of isethionate from taurine is so small, that it apparently has no role in the control of the brain taurine pool.