Measurement of Glucose Utilization by Pseudomonas fluorescens That Are Free-Living and That Are Attached to Surfaces

The assimilation and respiration of glucose by attached and free-living Pseudomonas fluorescens were compared. The attachment surfaces were polyvinylidene fluoride, polyethylene, and glass. Specific uptake of [¹⁴C]glucose was determined after bacterial biomass was measured by (i) microscopic counts or (ii) prelabeling of cells by providing [³H]leucine as substrate, followed by dual-labeling scintillation counting. The glucose concentration was 1.4, 3.5, 5.5, 7.6, or 9.7 μM. Glucose assimilation by cells which became detached from the surfaces during incubation with glucose was also measured after the detached cells were collected by filtration. The composition of the substratum had no effect on the amount of glucose assimilated by attached cells. Glucose assimilation by attached cells exceeded that by free-living cells by a factor of between 2 and 5 or more, and respiration of glucose by surface-associated cells was greater than that by free-living bacteria. Glucose assimilation by detached cells was greater than that by attached bacteria. Measurements of biomass by microscopic counts gave more consistent results that those obtained with dual-labeling, but in general, results obtained by both methods were corroborative.     

Changes in glycine and leucine transport during red cell maturation in the rat

Both glycine and leucine transport in rat red blood cells have been studied. The glycine uptake showed two different components, one sodium-dependent and another diffusion-like process. In contrast, leucine uptake was sodium independent. Both, Na⁺-dependent glycine and the overall leucine uptake in red blood cells showed a saturable pattern. Kinetic parameters in reticulocytes were: i) glycine: apparent Km 0.16 mM; V_max 100.2 nmol/ml ICW/min; ii) leucine: apparent Km 2.11 mM; V_max 3.88 mol/ml ICW/min. The erythrocytes kinetic parameters were: i) glycine: apparent Km 0.17 mM; V_max 9.47 nmol/ml ICW/min; leucine; apparent Km 4.77 mM; V_max 7.42 mol/ml ICW/min. The Kd values (sodium independent glycine uptake) were similar in both kind of cells, but the importance of this component in total glycine uptake in erythrocytes was much higher than in reticulocytes. Our results confirm that rat red blood cells have both saturable leucine and Na⁺-dependent glycine uptake, but some important changes occur during cell maturation.     

Amino Acid Assimilation and Electron Transport System Activity in Attached and Free-Living Marine Bacteria

Amino acid assimilation and electron transport system activity of a marine Pseudomonas sp. was evaluated to determine whether the activity of bacteria attached to solid surfaces differed from that of free-living bacteria or bacteria which had been attached but subsequently desorbed from the substratum (detached bacteria). Bacteria were allowed to attach to glass and to a range of plastic surfaces (Thermanox, polyvinylidene fluoride, polyethylene, polytetrafluoroethylene). Microautoradiography and staining with a tetrazolium salt to demonstrate electron transport system activity were used to compare the activity of these organisms with that of free-living or detached cells. The water-wettability of the surfaces was evaluated by measuring the advancing contact angle (θ_A) of water on each surface, to determine whether there was a relationship between activity and substratum hydrophilicity. There was an increase in the proportion of leucine-assimilating attached bacteria and in the proportion of attached cells demonstrating electron transport system activity with an increase in substratum θ_A, but the relationship between activity of attached and free-living cells depended on the substratum. Activity appeared to promote firm attachment, and detached bacteria assimilated fewer amino acids than did attached cells. There was no general effect of surfaces on attached bacterial activity, and attached cells may be more, or less, active than free-living cells, depending on the amino acid, its concentration, and substratum properties.     

Uptake of leucine by a marine Gram-negative heterotrophic bacterium during exposure to starvation conditions

The uptake of leucine by S14, an unidentified marine Gram-negative bacterium, was studied during a starvation period of 96 h. The S14 cells displayed two separate uptake systems with different affinities for leucine. The K_m values of these systems were 0.76 μM and 20 μM, respectively. The time of exposure to starvation had a marked effect on both uptake systems, not by changing the affinity for leucine, but rather by altering the velocity of uptake (V_max). A marked increase in the uptake capacity was noted for the high-affinity system, whereas the uptake velocity decreased for the low-affinity system. An osmotic shock treatment resulted in an almost complete loss of substrate binding activity. A gradual recovery of the leucine uptake subsequent to the osmotic shock was observed during a 72-h period of starvation. Separation of the osmotic shock supernatant by gel filtration revealed two proteins, 37 and 44 kDa in size, with leucine binding activity.     

Iaa oxidase and polyphenol oxidase activities of peanut peroxidase isozymes

Four anionic isozymes (A₁, A₂, A₄ and A₅) from peanut cells in suspension medium possessed IAA oxidase and polyphenol oxidase activities. The specific activities of each of the enzymes differed among the 4 isozymes. The pH optima established in these assays for peroxidase was acidic, for IAA oxidase neutral and for polyphenol oxidase alkaline. All 4 isozymes had different K_m and V_max for the enzyme activities of peroxidase and polyphenol oxidase. The sigmoid kinetics from the IAA oxidase assays for the isozymes probably indicates an allosteric nature.     

SHORT‐TERM TEMPORAL VARIABILITY OF AMMONIUM AND UREA UPTAKE BY ALEXANDRIUM CATENELLA (DINOPHYTA) IN CULTURES1

In batch cultures of four Mediterranean strains (from France, Italy, and Spain) of Alexandrium catenella (Whedon et Kof.) Balech growing on a daily light cycle, ammonium and urea uptake were estimated by the ¹⁵N tracer technique. Ammonium uptake could be described by Michaelis–Menten kinetics along a substrate gradient of 0.1–10 μgat N · L^?1 for the four strains, while two different patterns were observed for urea uptake with Michaelis–Menten kinetics for one strain and linear kinetics for the others. In all cases, an increase in uptake rates with time was noted over the daylight period. This trend led to a net increase in the maximum uptake rate (V_max; for saturable kinetics) and in the initial slope α. For ammonium, V_max increased by a factor of 2–10 depending on the strain, and, for urea, the maximal uptake rates measured increased by a factor of 2–18. Temporal variations of half‐saturation constants (K_s) for both nutrients did not show a clear trend. Increases in V_max and α showed an acclimation of the cells’ uptake system over time to a N pulse, which may be explained by the light periodicity. For two strains, extensive ammonium release was observed during urea assimilation. This mechanism removes urea from the medium, so it is no longer available to other potential competitors, but supplies N back to the medium in the form of ammonium. From a methodological point of view, the phenomenon leads to considerable underestimates of the contribution of urea to phytoplankton growth.     

Activation of a Na+-dependent amino acid transport system in preimplantation mouse embryos

The uptake of l-methionine-methyl-³H and l-leucine-³H from completely defined medium into acid-soluble fractions of preimplantation mouse embryos has been studied. Late four-cell embryos and early blastocysts raised in vitro can concentrate both amino acids by processes which exhibit saturable, Michaelis-Menten type kinetics, characteristic of carrier-mediated active transport systems. This uptake is temperature-sensitive and inhibited by certain amino acids which compete for the same uptake sites. Methionine uptake seems to be mediated by a single transport system (K_m = 6.25 × 10^?5M) at the four-cell stage. Complex kinetics suggest that two distinct transport systems exist at the early blastocyst stage (K_m = 6.25 × 10^?5M; 8.9 × 10^?4M). V_max values (mg/embryo/15 min) for methionine and leucine transport increase significantly from the late four-cell stage to the blastocyst stage, suggesting that additional carriers are produced or activated during development.Most importantly, leucine and methionine transport is Na⁺-independent at the four-cell stage, methionine transport is partially dependent at the morula stage, and both amino acids are completely Na⁺-dependent at the blastocyst stage. The cumulative results suggest that preimplantation embryos accumulate leucine and methionine by specific, chemically mediated, active transport systems. The qualitative and quantitative developmental changes in cell membrane function may represent preparatory steps for subsequent growth of embryonic and/or trophoblastic cells.     

Potassium activation of Na+-dependent leucine transport in brush-border membrane vesicles from rat jejunum

Na⁺-dependent leucine uptake was greater in potassium loaded brush-border membrane vesicles compared with controls. This effect was not mediated by an electrical potential difference, since it was still present in voltage-clamped conditions. Inhibition experiments indicate the same Na⁺-dependent leucine transport activity in the presence or in the absence of potassium. The affinity of sodium for the cotransporter was identical at 10 or 100 mM potassium. Leucine kinetics at different potassium concentrations showed a maximum 2.4-fold increase in V_max, while K_m was unaffected. The secondary plots of the kinetic results were not linear. This kinetic behaviour suggests that K⁺ acts as a non-essential activator of Na⁺-dependent leucine cotransport. A charge compensation of sodium-leucine influx is most probably a component of the potassium effect in the presence of valinomycin.     

The use and misuse of V c,max in Earth System Models

Earth System Models (ESMs) aim to project global change. Central to this aim is the need to accurately model global carbon fluxes. Photosynthetic carbon dioxide assimilation by the terrestrial biosphere is the largest of these fluxes, and in many ESMs is represented by the Farquhar, von Caemmerer and Berry (FvCB) model of photosynthesis. The maximum rate of carboxylation by the enzyme Rubisco, commonly termed V _c,max, is a key parameter in the FvCB model. This study investigated the derivation of the values of V _c,max used to represent different plant functional types (PFTs) in ESMs. Four methods for estimating V _c,max were identified; (1) an empirical or (2) mechanistic relationship was used to relate V _c,max to leaf N content, (3) V _c,max was estimated using an approach based on the optimization of photosynthesis and respiration or (4) calibration of a user-defined V _c,max to obtain a target model output. Despite representing the same PFTs, the land model components of ESMs were parameterized with a wide range of values for V _c,max (?46 to +77 % of the PFT mean). In many cases, parameterization was based on limited data sets and poorly defined coefficients that were used to adjust model parameters and set PFT-specific values for V _c,max. Examination of the models that linked leaf N mechanistically to V _c,max identified potential changes to fixed parameters that collectively would decrease V _c,max by 31 % in C₃ plants and 11 % in C₄ plants. Plant trait data bases are now available that offer an excellent opportunity for models to update PFT-specific parameters used to estimate V _c,max. However, data for parameterizing some PFTs, particularly those in the Tropics and the Arctic are either highly variable or largely absent.     

Trypanosoma lewisi: alterations in membrane function in the rat.

DEAE-cellulose-purified Trypanosoma lewisi from 4-day (dividing trypanosomes) and 7-day (non-dividing trypanosomes) infections in rats were compared for initial uptake of glucose, leucine, and potassium. Glucose entered the parasitic cells by mediated (saturable) processes, whereas leucine and K⁺ entered by mediated processes and diffusion. Glucose entry was significantly elevated in 4-day cells (V_max 4.00 ± 1.02 nmoles/ 1 × 10⁸ cells/min) with respect to 7-day cells (V_max 1.83 ± 0.62 nmoles 1 × 10⁸ cells/min). Likewise, the affinity of the glucose carrier was significantly greater in 4-day cells (K_m = 0.30 ± 0.02 mM) than in 7-day cells (K_m = 0.59 ± 0.11 mM). When leucine and K⁺ transport were compared in 4- and 7-day populations, significant elevations in the rate of entry (V_max) of both substrates were observed for 4-day cells; K_m values for leucine and K⁺ were not altered by the stage of infection. For leucine, the V_max and K_m for 4-day cells were 2.40 ± 0.50 nmoles/1 × 10⁸ cells/30 sec and 78 ± 7 μM, respectively; corresponding values in 7-day cells were 1.06 ± 0.02 nmoles/1 × 10⁸ cells/30 sec and 66 ± 11 μM. For K⁺, the V_max and K_m for 4-day cells were 15.97 ± 0.38 nmoles/1 × 10⁸ cells/min and 1.2 mM, respectively; corresponding values in 7-day cells were 4.76 ± 1.82 nmoles/1 × 10⁸ cells/min and 1.05 mM. The observed increase in the rate of K⁺ entry into 4-day cells was attributable to enhanced influx; no significant difference in the rate of K⁺ efflux was noted when 4- and 7-day cells were compared ($\text{t}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ of K⁺ leak for 4- and 7-day cells were 68.1 ± 9.3 and 67.9 ± 15.2 min, respectively). Potassium influx was ouabain insensitive. Membrane function in 7-day cells was not uniformly inhibited. No significant difference in the activity of the membrane-bound enzyme, 5′-nucleotidase, was observed when 4- and 7-day cells were compared.     

Bacterial Colonization and Ectoenzymatic Activity in Phytoplankton-Derived Model Particles: Cleavage of Peptides and Uptake of Amino Acids

Abstract Phytoplankton-derived model particles were created in laboratory from a mixture of autoclaved diatom cultures. These particles were colonized by a marine bacterial community and incubated in rolling tanks in order to examine the relationship between aminopeptidase activity and leucine uptake. Bacteria inhabiting particles and ambient water were characterized for abundance, biovolume, aminopeptidase activity, leucine uptake, and growth rate. Particles were a less favorable habitat than ambient water for bacterial growth since growth rates of particle-attached bacteria were similar or even lower than those of free-living bacteria. During the first ∼100 h of the particle decomposition process, there were not statistically significant differences in the aminopeptidase activity:leucine uptake ratio between attached and free-living bacteria. From ∼100 h to ∼200 h, this ratio was higher for attached bacteria than for free-living bacteria. This indicates an uncoupling of aminopeptidase activity and leucine uptake. During this period, attached and free-living bacteria showed similar hydrolytic activities on a cell-specific basis. In the free-living bacterial community, variations in aminopeptidase activity per cell were associated with variations in leucine uptake per cell and growth rates. However, in the attached bacterial community, when leucine uptake and growth rates decreased, aminopeptidase activity remained constant. Thus, after ∼100 h, particle-attached bacteria were not taking advantage of their high aminopeptidase activity; consequently the hydrolysed amino acids were released into the ambient water, supporting the growth of free-living bacteria. These results demonstrate that over the particle decomposition process, the relationship between hydrolysis and uptake of the protein fraction shows different patterns of variation for attached and free-living bacterial communities. However, in our experiments, this uncoupling was not based on a hyperproduction of enzymes by attached bacteria, but on lower uptake rates when compared to the free-living bacteria. Received: 4 February 1997; Accepted: 9 May 1997     

Effect of Temperature on Heterotrophic Glucose Uptake, Mineralization, and Turnover Rates in Lake Sediments

The V_max and turnover rates (TR) of [U-¹⁴C]glucose uptake and mineralization of Lake Kinneret (Israel) sediment are temperature dependent. The following activation energies were determined: glucose uptake, ~15,000 cal (62,760 J); TR of glucose uptake, ~10,000 cal (41,840 J); glucose mineralization, 7,500 to 15,000 cal (31,380 to 62,760 J); and TR of glucose mineralization, ~15,000 cal. Q₁₀ values varied as follows: glucose uptake, ~2.3; TR of glucose uptake, ~1.8; and glucose mineralization, ~2.5. K + S_n values increased slightly with temperature and might reflect an increased K with increased temperatures. Glucose respiration/uptake ratios were low (9.5 to 12%) and were apparently not greatly influenced by the presence or absence of oxygen or by different assay temperatures. Aerobic or anaerobic sediments assayed under either aerobic or anaerobic conditions did not exhibit greatly different V_max, TR, or K + S_n values.     

Photosynthetic responses of Monarch birch seedlings to differing timings of free air ozone fumigation

To study the effects of different periods of ozone (O₃) fumigation on photosynthesis in leaves of the Monarch birch (Betula maximowicziana), we undertook free air O₃ fumigation to Monarch birch seedlings at a concentration of 60 nmol mol^?1 during daytime. Plants were exposed to O₃ at early, late or both periods in the growing season. The light-saturated net photosynthetic rate (A _sat) in July and August was reduced by O₃ exposure through a reduction in the maximum rate of carboxylation (V _c,max). In early September, on the other hand, despite a reduction in V _c,max, A _sat was not reduced by O₃ due to a counteracting increase in the stomatal conductance. Through the experiment, there was no difference in sensitivity to O₃ between maturing and matured leaves. We analyzed the relationship between A _sat, V _c,max and accumulated stomatal O₃ flux (AF_st). Whereas V _c,max decreased with increasing AF_st, the correlation between A _sat and AF_st was weak because the response of stomatal conductance to O₃ was affected by season. We conclude photosynthetic response of Monarch birch to O₃ exposure changes with season. This is due to the inconstant stomatal response to O₃ but not due to the respose of biochemical assimilation capacity in chloroplasts.     

Nutrient uptake rates by the alien alga Undaria pinnatifida (Phaeophyta) (Nuevo Gulf, Patagonia, Argentina) when exposed to diluted sewage effluent

In the early nineties, Undaria pinnatifida has been accidentally introduced to Nuevo Gulf (Patagonia, Argentina) where the environmental conditions would have favored its expansion. The effect of the secondary treated sewage discharge from Puerto Madryn city into Nueva Bay (located in the western extreme of Nuevo Gulf) is one of the probable factors to be taken into account. Laboratory cultures of this macroalgae were conducted in seawater enriched with the effluent. The nutrients (ammonium, nitrate and phosphate) uptake kinetics was studied at constant temperature and radiation (16?°C and 50 μE m^?2 s^?1 respectively). Uptake kinetics of both inorganic forms of nitrogen were described by the Michaelis–Menten model during the surge phase (ammonium: V _max ^sur: 218.1 μmol h^?1 g^?1, K _s ^sur: 476.5 μM and nitrate V _max ^sur: 10.7 μmol h^?1 g^?1, K _s ^sur: 6.1 μM) and during the assimilation phase (ammonium: V _max ^ass: 135.6 μmol h^?1 g^?1, K _s ^ass: 407.2 μM and nitrate V _max ^ass: 1.9 μmol h^?1 g^?1, K _s ^ass: 2.2 μM), with ammonium rates always higher than those of nitrate. Even though a net phosphate disappearance was observed in all treatments, uptake kinetics of this ion could not be properly estimated by the employed methodology.     

The “one‐point method” for estimating maximum carboxylation capacity of photosynthesis: A cautionary tale

The maximum carboxylation capacity of Rubisco, V_c,max, is an important photosynthetic parameter that is key to accurate estimation of carbon assimilation. The gold‐standard technique for determining V_c,max is to derive V_c,max from the initial slope of an A–C_i curve (the response of photosynthesis, A, to intercellular CO₂ concentration, C_i). Accurate estimates of V_c,max derived from an alternative and rapid “one‐point” measurement of photosynthesis could greatly accelerate data collection and model parameterization. We evaluated the practical application of the one‐point method in six species measured under standard conditions (saturating irradiance and 400 μmol CO₂ mol^?1) and under conditions that would increase the likelihood for successful estimation of V_c,max: (a) ensuring Rubisco‐limited A by measuring at 300 μmol CO₂ mol^?1 and (b) allowing time for acclimation to saturating irradiance prior to measurement. The one‐point method significantly underestimated V_c,max in four of the six species, providing estimates 21%–32% below fitted values. We identified ribulose‐1,5‐bisphosphate‐limited A, light acclimation, and the use of an assumed respiration rate as factors that limited the effective use of the one‐point method to accurately estimate V_c,max. We conclude that the one‐point method requires a species‐specific understanding of its application, is often unsuccessful, and must be used with caution.     

Growth and nitrate uptake properties of plants grown at different relative rates of nitrogen supply. II. Activity and affinity of the nitrate uptake system in Pisum and Lemna in relation to nitrogen availability and nitrogen demand

Abstract Net nitrate uptake rates were measured and the kinetics calculated in non-nodulated Pisum sativum L. cv. Marma and Lemna gibba L. adapted to constant relative rates of nitrate-N additions (R_A), ranging from 0.03 to 0.27 d^?1 for Pisum and from 0.05 to 0.40 d^?1 for Lemna, V_max of net nitrate uptake (measured in the range 10 to 100 mmol m^?3 nitrate, i.e. ‘system I’) increased with R_A in the growth limiting range but decreased when R_A exceeded the relative growth rate (RGR), K_m was not significantly related to changes in R_A. On the basis of previous ¹³N-flux experiments, it is concluded that the differences in V_max at growth limiting R_A are attributable to differences in influx rates. Linear relationships between V_max and tissue nitrogen concentrations were obtained in the growth limiting range for both species, and extrapolated intercepts relate well with the previously defined minimal nitrogen concentrations for plant growth (Oscarson, Ingemarsson & Larsson, 1989). Analysis of V_max for net nitrate uptake on intact plant basis in relation to nitrogen demand during stable, nitrogen limited, growth shows an increased overcapacity at lower R_A values in both species, which is largely explained by the increased relative root size at low R_A. A balancing nitrate concentration, defined as the steady state concentration needed to sustain the relative rate of increase in plant nitrogen (R_N), predicted by R_A, was calculated for both species. In the growth limiting range, this value ranges from 3.5 mmol m^?3 (R_A 0.03 d^?1) to 44 mmol m^?3 (R_A 0.21 d^?1) for Pisum and from 0.2 mmol m^?3 (R_A 0.05 d^?1) to 5.4 mmol m^?3 (R_A 0.03 d^?1) for Lemna. It is suggested that this value can be used as a unifying measure of the affinity for nitrate, integrating the performance of the nitrate uptake system with nitrate flux and long term growth and demand for nitrogen.     

SIZE-DEPENDENT PHOSPHORUS UPTAKE KINETICS AND CELL QUOTA IN PHYTOPLANKTON1

The allometric equation, y = aX^b, described the interspecific variation of phosphate uptake kinetics and cell quota with phytoplankton cell size and showed that smaller cells are superior in uptake rate to large. Species-specific measurements, made by track autoradiography in phosphorus deficient cultures of communities from a phosphorus-limited lake, revealed that eight different species did not differ significantly in the Michaelis-Menten half-saturation constant, K_m. However, both saturated uptake rates (V_max) and the initial slope of the uptake curve (V_max:K_m) decreased per unit biomass with increasing cell size. Biomass-specific cell phosphorus quotas also decreased with increasing cell volume, but less rapidly than did V_max or V_max: K_m. Comparable data from the literature showed that marine species were superior in phosphorus uptake to freshwater species of similar size, but allometric variation of kinetics appeared to exist within both groups. Together with a variable internal stores model of phosphorus-limited growth, the allometric relationships of uptake kinetics and quotas predicted competition to favor smaller cells, with a differential in growth rate diminishing as competitive intensity increased.     

Cotugnia digonopora: Transport of leucine

The uptake of leucine through the tegument of Cotugnia digonopora, a cestode found in the fowl intestine, occurs by a process of active transport. The K_t of transport is 0.87 mM and the V_max is 0.223 μmol/min/g. Uptake of the amino acid is competitively inhibited by valine (K_t = 1.30 mM). Potassium cyanide and 2,4-dinitrophenol do not completely block the entry of leucine into the parasite.     

Transport of thiamine and 4-methyl-5-hydroxyethylthiazole by Salmonella typhimurium

The transport of thiamine and 4-methyl-5-hydroxyethylthiazole (MHET), its thiazole moiety, was studied using whole cells of Salmonella typhimurium. It was found that the bacteria possessed an active transport system for thiamine that had K_m 0.21 μM and V_max 33 nmol·min^?1·(mg dry wt. cells)^?1. Transport of thiamine was glucose dependent, whereas MHET uptake was dependent on both glucose and 2-methyl-4-amino-5-hydroxymethylpyrimidine (MAHMP), the pyrimidine moiety of thiamine. Uptake of both thiamine and MHET was severely curtailed by cyanide, azide, N-ethylmaleimide and carbonyl cyanide m-chlorophenylhydrazone. Oxythiamine inhibited thiamine, but not MHET, uptake and thiamine slightly inhibited MHET uptake. 2-Methyl-4-amino-5-methoxymethylpyrimidine and 4-amino-5-hydroxymethylpyrimidine were unable to replace MAHMP as stimulators of MHET uptake, but 2-methyl-4-amino-5-aminomethylpyrimidine was marginally effective in this regard. Similar results were obtained with attempts to replace MAHMP as a growth requirement for a purD mutant of Salmonella typhimurium. MHET uptake showed saturation kinetics only in the presence of MAHMP, and is not otherwise actively transported.     

Characterization of uptake and release processes ford- andl-aspartate in primary cultures of astrocytes and cerebellar granule cells

The uptake ofl-andd-aspartate was studied in astrocytes cultured from prefrontal cortex and in granule cells cultured from cerebellum. A high affinity uptake system forl- andd-aspartate was found in both cell types, and the two stereoisomers exhibited essentially the sameK _m - andV _max -values in bouth astrocytes (l-aspartate:K _m 77 μM;V _max 11.8 nmol×min^?1×mg^?1;d-aspartate:K _m 83 μM;V _max 14.0 nmol×min^?1×mg^?1) and granule cells (l-aspartate:K _m 32 μM;V _max 2.8 nmol ×min^?1×mg^?1;d-aspartate:K _m 26 μM;V _max 3.0 nmol×min^?1×mg^?1). To investigate whetherl-glutamate,l-aspartate andd-aspartate use the same uptake system a detailed kenetic analysis was performed. The uptake kinetics of each one of the three amino acids was studied in the presence of the two other amino acids, and no essential differences between the uptake characteristics of the amino acids were found. In addition to the uptake studies the release ofD-aspartate from cerebellar granule cells was investigated and compared withl-glutamate release. A Ca²⁺-dependent, K⁺-induced release was found for both amino acids.