Mechanism of l-histidine-induced stimulation of amino acid transport in slices of rat cerebral cortex

Effects of l-histidine on the transport of other amino acids were studied with slices of rat cerebral cortex. Histidine (0.5 mM) significantly increased the 60-min accumulation of large neutral and basic amino acids (0.5 mM). The effect was dependent on sodium ions and could be demonstrated in slices from both adult and newborn rats. Other amino acids tested were either ineffective or inhibitory; in particular, l-phenylalanine was strongly inhibitory. The 5-min influx of amino acids into slices was also enhanced by preincubation with histidine. This effect was stereospecific for l-histidine, sodium-independent and not produced by histidine metabolites or activation of histamine H₁ and H₂ receptors. Kinetic analysis of leucine influx showed that the maximal velocity of transport (V) increased relatively more than the other transport parameters. The results could be explained by stimulation of amino acid exchange by intracellular l-histidine. The opposite effects of histidine and phenylalanine on the accumulation of other amino acids are in keeping with the generally less severe impairment of cerebral functions in clinical histidinemia as compared to that in phenylketonuria.     

Tryptophan and phenylalanine transport in rat cerebral cortex slices as influenced by sodium ions

Tryptophan and phenylalanine transport in rat cerebral cortex slices was studied in sodium-free media and during influx and efflux of sodium ions. Choline as a substitute for sodium in incubation media increased efflux and decreased influx of tryptophan and phenylalanine. Exchange of intracellular [³H]tryptophan and [³H]phenylalanine with extracellular unlabeled histidine, phenylalanine, and tryptophan was sodium-independent. Efflux of sodium ions from the slices had no immediate effects on phenylalanine and tryptophan efflux, but influx decreased. Influx of sodium into the sodium-depleted slices provoked a transient increase in tryptophan and phenylalanine efflux and also enhanced influx. The results are interpreted to indicate that sodium ions may possibly affect the function of the primary transport sites for aromatic amino acids at cerebral membranes by controlling the orientation of their reactive sites towards the intracellular and extracellular sides, rather than by being directly involved in the binding of amino acids to the carriers.     

KINETIC ANALYSIS OF PHENYLALANINE-INDUCED INHIBITION IN THE SATURABLE INFLUX OF TYROSINE, TRYPTOPHAN, LEUCINE AND HISTIDINE INTO BRAIN CORTEX SLICES FROM ADULT AND 7-DAY-OLD RATS

—An attempt was made to isolate the saturable uptake from the unidirectional influx of amino acids into tissue slices and to estimate the transport constants and maximal velocities of saturable transport. The method was applied to studies on the inhibition of phenylalanine in the saturable influx of tyrosine, tryptophan, histidine and leucine into brain cortex slices from adult and 7-day-old rats. In both age groups phenylalanine inhibited the influx of the other amino acids, and vice versa. The apparent transport constants of the other amino acids increased in the presence of phenylalanine more noticeably in the slices from 7-day-old rats than in those from adult rats, whereas the concomitant influx of phenylalanine was inhibited less in the slices from 7-day-old rats. In immature animals in vivo competition between amino acids may play a more marked role in the supply of amino acids from plasma to brain, as the transport systems in brain slices from 7-day-old rats become saturated with extracellular amino acids more readily than do the transport systems in brain slices from adult rats.     

Effects of Branched-Chain L-Amino Acids, L-Phenylalanine, and L-Methionine on the Transport of L-Glutamine in Rat Brain Cortex In Vitro. Influence of Cations

Abstract: Uptake of L-glutamine (2 mM) by rat brain cortex slices against a concentration gradient is markedly inhibited (40%) by branched-chain Lamino acids (1 mM), L-phenylalanine (1 mM), or L-methionine (1 mM); that of L-asparagine (2 mM) is much less affected by these amino acids. Other amino acids investigated have little or no effect on cerebral L-glutamine uptake. The suppressions of L-glutamine uptake by the inhibitory amino acids are apparently blocked by high [K⁺], which itself has little or no effect on glutamine uptake. This abolition of suppression is partly explained by high [K⁺] retention of endogenous glutamine; in the absence of Ca²⁺ such retention disappears. The inhibitory amino acids (1 mM) also enhance the release of endogenous glutamine, exogenous glutamine with which slices have been loaded, or glutamine synthesized in the slices from exogenous glutamate. The enhanced release of endogenous glutamine is diminished by high [K⁺]. The suppression of glutamine uptake by the branched-chain amino acids is independent of the concentration of glutamine at low concentrations (0.25–0.5 mM), indicating non-competition, but is reduced with high concentration of glutamine. The inhibition by L-phenylalanine is noncompetitive. L-Glutamine (2 mM) exerts no inhibition of the cerebral uptakes of the branched-chain L-amino acids or Lphenylalanine (0.25–2 mM). The inhibitory amino acids are as active in suppressing L-glutamine uptake with immature rat brain slices as with adult, although the uptake, against a gradient, of L-glutamine in the infant rat brain is about one-half that in the adult. They are also just as inhibitory on the concentrative uptake of L-glutamine by a crude synaptosomal preparation derived from rat brain cortex. Such a nerve ending preparation takes up L-glutamine (0.25 mM), against a gradient, at about ninefold the rate at which it is taken up by cortex slices (for equal amounts of protein), and the uptake process is markedly suppressed by high [K⁺] in contrast to the effects of high [K⁺] with slices. The possible physiological and pathological consequences of the suppression of glutamine uptake are discussed.     

Transport of threonine and tryptophan by rat brain slices: relation to other amino acids at concentrations found in plasma

Threonine content of brain decreases in young rats fed a threonine-limiting, low protein diet containing a supplement of small neutral amino acids (serine, glycine and alanine), which are competitors of threonine transport in other systems (Tews et al., 1977). Threonine transport by brain slices was inhibited more by a complex amino acid mixture resembling plasma from rats fed the small neutral amino acid supplement than by mixtures resembling plasma from control rats or from rats fed a supplement of large neutral amino acids. Greater inhibition was seen with mixtures containing only the small neutral amino acids than with mixtures containing only large neutral amino acids. On an equimolar basis, serine and alanine were the most inhibitory; large neutrals were moderately so; and glycine and lysine were without effect. Threonine transport was also strongly inhibited by α-amino-n-butyric acid and homoserine, less so by α-aminoisobutyric acid, and not at all by GABA. The complex amino acid mixtures strongly inhibited α-aminoisobutyric acid transport by brain or liver slices but, in contrast to effects in brain, the extent of the inhibition in liver was not much affected by altering the composition of the mixture. Tryptophan accumulation by brain slices was effectively inhibited by other large neutral amino acids in physiologically occurring concentrations. Threonine, or a mixture of serine, glycine and alanine only slightly inhibited tryptophan uptake; basic amino acids were without effect and histidine stimulated tryptophan transport slightly. These results support the conclusion that a diet-induced decrease in the concentration in brain of a specific amino acid may be related to increased inhibition of its transport into brain by increases in the concentrations of transport-related, plasma amino acids.     

Membrane transport in Schistosoma mansoni: transport of amino acids by adult males.

The mechanisms by which adult male Schistosoma mansoni transport amino acids have been investigated using radioactive amino acids during 2-min incubation times. The transport constants (K_t) for mediated uptake of glycine, proline, methionine, arginine, glutamate, and tryptophan were calculated to be 0.60-1.05, 1.67-1.98, 2.0, 0.10-0.35, 0.30-0.50, and 0.5-1.0 mM, respectively. Maximal velocities (V_max) were 5.5–7.5, 25, 6.4, 1.5-2.0, 2.5, and 3.0–6.0 μmoles absorbed/g worm protein/2 min, respectively. Cysteine is taken up solely by diffusion. Proline uptake is unique in that no significant diffusion component was found. The other amino acids studied were absorbed by diffusion as well as by specific transport systems. In the 2-min incubation periods employed glycine, proline, glutamate, and methionine were not significantly metabolized indicating that the uptake studies using these substrates reflect transport. Metabolism of the other amino acids used in these studies was not examined. The specificity of the transport systems was studied by testing the inhibitory effects of various amino acids on the uptake of each of the amino acids studied. The results suggest the presence of at least five transport systems. There is a highly specific transport locus for proline, and one for acidic amino acids. There are probably at least two transport systems, each of broad and overlapping specificity, for most of the neutral amino acids. Basic amino acids also appear to be taken up by complex transport systems, at least one of which overlaps with the neutral sites. The results are discussed with respect to the nutrition of the parasite and the host-parasite relationship.     

Temperature dependence of amino acid transport in brain slices

A decrease in amino acid influx and exit in incubated slices when the temperature was lowered from 37 to 20°C was observed with all 16 amino acids examined at two concentrations (1 mM and 10 M). The temperature dependence of cellular amino acid influx observed in slices in vitro contrasts with the absence of temperature dependence of capillary amino acid influx in the brain in vivo that we recently reported. The temperature effects in slices varied some-what among the various amino acid transport classes. With some amino acids (Phe, Val, Tyr, Asp, Glu), especially at lower concentrations, the greater inhibition of exit than of uptake resulted in an actual increase in tissue amino acid uptake at lower temperatures in long-term experiments. The results indicate heterogeneity in the effect of temperature on the various transport classes and show a difference between capillary and cellular amino acid transport in the brain.     

HIGH AFFINITY AMINO ACID TRANSPORT BY FROG SPINAL CORD SLICES

The characteristics of amino acid uptake by frog spinal cord slices was studied by in vitro incubations in appropriate media. The uptake mechanisms exhibited saturation; kinetic analysis demonstrated 2 distinct systems for the influx of the possible neurotransmitters: GABA, glycine, L-glutamic acid and L-aspartic acid. One system showed a comparatively high substrate affinity (K_m values, 10-26 μM) while the other system had a lower affinity (K_m, 0.4-1.6 mM).-Leucine, an amino acid presumably not a transmitter, was accumulated only by a low affinity mechanism (K_m 1.6 mM). The process responsible for high affinity uptake had many of the properties of an active transport mechanism. These included temperature sensitivity, energy dependence, requirement for Na⁺ ions and inhibition by ouabain. GABA and glycine uptake was inhibited only by closely related amino acids or structural analogues. The influx of L-glutamic acid was competitively inhibited by the presence of L-aspartic acid in the medium; the converse was also demonstrated. Thus, the high affinity uptake system for possible transmitter amino acids in the frog spinal cord closely resembles that described for mammalian CNS tissue. These results are compatible with the assumption that GABA, glycine, L-glutamic acid and L-aspartic acid are neurotransmitters in the amphibian spinal cord.     

Properties of the uptake and release of neurotransmitter glutamate in cerebral cortical tissue of guinea pigs

In order to investigate the dynamics of glutamate as a neurotransmitter and to avoid a complication by its metabolism, we studied the uptake and release of labeled non-metabolizabled-isomers of aspartate and glutamate in cerebral cortical slices and synaptosome preparation from guinea-pigs. The rate of uptake ofd-aspartate and glutamate was mutually inhibited in a non-competitive fashion, indicating that their uptake mechanisms are not exactly the same. By ouabain (0.05 mM), the uptake ofd-aspartate and glutamate into synaptosome preparation was less inhibited than that into cerebral slices. In synaptosome preparation most of the preloadedd-aspartate and glutamate was released by high-potassium (50 mM) stimulation, whereas in cerebral slices only a slight release was observed. However, when the slices were superfused with a medium free of sodium ions, which are absolutely necessary for the uptake, after preloaded with the labeled amino acids in the standard medium, a distinct release of radioactivity was induced by high-potassium stimulation. This potassium-induced release corresponded to only about 20% of the radioactivity accumulated in the slices. The accumulation ofd-aspartate and glutamate into cerebral slices was much larger on the basis of their protein content than that into synaptosome preparation, when a high concentration (1 mM) of the amino acids was added to the medium. These observations suggest that the uptake system ofd-aspartate and glutamate in cerebral slices is quite different from that in synaptosome preparation, and that the accumulation into cerebral slices is mainly localized in glial cells. In vivo the glial cell uptake is probably more important in removing the released neurotransmitter glutamate.Dedicated to Professor Yasuzo Tsukada.     

Transport of neutral, cationic and anionic amino acids by systems B, b, XAG, and ASC in swine small intestine

Amino acid influx across the brush border membrane of the intact pig ileal epithelium was studied. It was examine whether in addition to system B, systems ASC and b^o,+ were involved in transport of bipolar amino acids. The kinetics of interactions between lysine and leucine demonstrates that system b^o,+ is present and accessible also to -glutamine. -aspartate (K_1/2 0.3 mM) and -glutamate (K_i 0.5 mM) share a high affinity transporter with a maximum rate of 1.3 μmol cm⁻² h⁻¹, while only -glutamate with a K_1/2 of 14.4 mM uses a low affinity transporter with a maximum rate of 2.7 μmol cm⁻² h⁻¹, system ASC, against which serine has a K_i of 1.6 mM. In the presence of 100 mM lysine, -glutamine (A), leucine (B), and methionine (C) fulfilled the criteria of the ABC test for transport by one and the same transporter. However, serine inhibits not only transport of -glutamate but also of glutamine (K_i 0.5 mM), and -glutamate inhibits part of the transport of glutamine. The test does, therefore, only indicate that the three bipolar amino acids have similar affinities for transport by systems B and ASC. Further study of the function of system B must be carried out under full inhibition by lysine and glutamate.     

DEVELOPMENTAL CHANGES IN Na+, K+ AND ATP AND IN THE LEVELS AND TRANSPORT OF AMINO ACIDS IN INCUBATED SLICES OF RAT BRAIN

Abstract—

• 1 Upon incubation, slices of brain tissue took up fluid; the degree of swelling increased with increasing age. No sweiling occurred in slices from foetal brain. Since this swelling was associated with increases in the inulin space, the percentage of inulin space in slices at the end of incubation increased during brain development.
• 2 Most of the capacity for ion transport seemed to be absent from foetal brain. In vivo and in slices, Na⁺ was very high and K⁺ was very low in comparison to levels at other ages. There was a rapid change around birth, but no significant change at later ages. Upon incubation, Na⁺ levels increased in other slices, but not in slices of foetal brain.
• 3 Upon incubation of the slices, ATP levels were restored to levels close to those in the living brain; there were no significant alterations in available energy during development to explain changes in amino acid transport.
• 4 The composition of the free pool of cerebral amino acids in vivo changed with development, with some compounds (glutamic acid and related compounds) increasing, others (mostly‘essential’amino acids) decreasing, with age. These changes were not linear with time, and the level of a compound might exhibit several peaks during development.
• 5 The uptake (influx) of taurine, glutamate and glycine into brain slices increased rapidly during the foetal and early neonatal periods, reached a maximum between 2 and 3 weeks of postnatal age and then declined to adult levels. The levels of steady-state uptake with glycine also exhibited a maximal peak at 2-3 weeks of postnatal age. Steady-state uptake of taurine and glutamate reached adult levels by about 3 weeks of age.
• 6 The pattern of inhibition of amino acid transport by two specific amino acid analogues changed during development for some amino acids (GABA, glycine and glutamate), indicating an alteration in substrate specificity.
• 7 The results demonstrate complex changes in cerebral amino acid transport during development, with several maxima or minima and with changes in specificity for at least some compounds.

     

Studies on the production and assessment of experimental histidinemia in the rat

Intraperitoneal administration to rats of D- or DL-α-hydrazunoimidazolylpropionic acid was found to produce a substantial inactivation of hepatic histidine ammonia-lysase (EC 4.3.1.3) in vivo. Proportional to this loss in enzyme activity was an impairment of the ability of treated rats to oxidize l-[ring-2-¹⁴C] histidine to ¹⁴CO₂. Rats in which hepatic histadine ammonia-lyase activity was either depressed by dl-hydrazunoimidazolylproprionic acid injection or elevated by feeding a high protein diet displayed proportionately altered rates of ³H₂O release into plasma water following l-[3-H]histidine administration. Plasma l-histidine clearance following loading with this amino acid was similarly affected by these treatments. Administration of dl-α-hydrazinoimisazolyl-proprionic acid to rats was also found to inactivate non-specifically pyridoxal 5-phosphate enzymes in vivo; pyridoxine injection was found to reverse the dl-α-hydrazinoimidazolylproprionic acid-induced inactivation of hepatic aspartate aminotransferase (EC 2.6.1.1) in vivo, but not that of hepatic histidine ammonia-lyase. These findings demonstrate that histidine ammonia-lyase is the rate-limiting factor in l-histidine degradation in the rat. The potential usefulness of dl-hydrazinoimidazolylproprionic acid in the production of an animal model for histidinemia (hereditary histidine ammonia-lyase deficiency) is discussed.     

Na+-dependent transport of glycine in renal brush border membrane vesicles: Evidence for a single specific transport system

The uptake of glycine in rabbit renal brush border membrane vesicles was shown to consist of glycine transport into an intravesicular space. An Na⁺ electrochemical gradient (extravesicular>intravesicular) stimulated the initial rate of glycine uptake and effected a transient accumulation of intravesicular glycine above the steady-state value. This stimulation could not be induced by the imposition of a K⁺, Li⁺ or choline⁺ gradient and was enhanced as extravesicular Na⁺ was increased from 10 mM to 100 mM. Dissipation of the Na⁺ gradient by the ionophore gramicidin D resulted in diminished Na⁺-stimulated glycine uptake. Na⁺-stimulated uptake of glycine was electrogenic. Substrate-velocity analysis of Na⁺-dependent glycine uptake over the range of amino acid concentrations from 25 μM to 10 mM demonstrated a single saturable transport system with apparent K_m = 996 μM and V_max = 348 pmol glycine/mg protein per min. Inhibition observed when the Na⁺-dependent uptake of 25 μM glycine was inhibited by 5 mM extravesicular test amino acid segregated dibasic amino acids, which did not inhibit glycine uptake, from all other amino acid groups. The amino acids d-alanine, d-glutamic acid, and d-proline inhibited similarly to their l counterparts. Accelerative exchange of extravesicular [³H]glycine was demonstrated when brush border vesicles were preloaded with glycine, but not when they were preloaded with l-alanine, l-glutamic acid, or with l-proline. It is concluded that a single transport system exists at the level of the rabbit renal brush border membrane that functions to reabsorb glycine independently from other groups of amino acids.     

Bimodal effects of cellular amino acids on Na+-dependent amino acid transport in ehrlich cells

Cells depleted of amino acids show lower rates of glycine or aminoisobutyric acid uptake than do freshly isolated cells. In the amino acid-depleted cells, addition of valinomycin stimulates amino acid influx at least to the level observed in freshly isolated cells. In cells containing high levels of cellular amino acids, valinomycin has little effect on influx of amino acids. It is concluded that the transport of amino acids in freshly isolated cells is elevated compared to depleted cells because the cells are hyperpolarized by the continuous loss of cellular amino acids during the transport assay. During this hyperpolarization by amino acid loss, transport of amino acids is not further stimulated by valinomycin at low external [K⁺] ($\text{10 mM ± 5 mM}$).With the exception of preloading with glycine, cells preloaded with a single amino acid to a concentration greater than 20 mM show reduced rates of glycine and aminoisobutyric acid influx at early times (less than 15 min) compared to amino acid-depleted cells. The reduction of infiux is transient and by 30 min, influx is greater in preloaded than in amino acid-depleted cells.Knowing that increases and decreases in the membrane potential are achieved by using varying external [K⁺] in the presence of valinomycin and propranolol, and using amino acid-depleted cells, it can be shown that an increased membrane potential increases the $\text{V}$ for glycine and aminoisobutyric acid influx. A decrease in the potential difference results in a decreased $\text{V}$. Changes in $\text{K}{}_{\mathrm{<mtext>m</mtext>}}$ also occur when the membrane potential is varied.     

Stimulation of amino acid accumulation in neuroblastoma and astrocytoma cells byl-histidine

Uptake of amino acids by cultured neuroblastoma and astrocytoma cells was studied in the presence and absence ofl-histidine. Intracellularly accumulated histidine was assumed to induce accumulation of radioactively labeled amino acids from medium by means of exchange transport. Neuroblastoma cells accumulated more histidine than astrocytoma cells and were more sensitive to the enhancement of the uptake of other large neutral amino acids by histidine. Histidine also increased glutamic acid uptake in astrocytoma cells, but reduced it in neuroblastoma cells. The greatest differences between the cell lines in amino acid uptake without histidine were found with acidic amino acids (astrocytoma cells accumulated them more than neuroblastoma cells) and with taurine (the reverse was found). The uptake and exchange mechanisms for some neutral and acidic amino acids may thus be dissimilar in the plasma membranes of cultured cells of neuronal and glial origin.     

Kinetics of Tryptophan Influx into Brain Slices Depleted of Sodium and Loaded with l-Histidine

Abstract: The kinetics of tryptophan influx were studied with rat brain slices preloaded with l -histidine and/or depleted of sodium ions. The best fits of the data (velocity of influx versus tryptophan concentration) were computed by use of a model consisting of a saturable (Michaelis-Menten type) and an unsaturable (diffusional) component with an iterative nonlinear regression analysis. Sodium depletion of the slices reduced the maximal velocity of saturable influx. In histidine-preloaded slices, depleted or not depleted of sodium ions, the most marked alteration again occurred in the maximal velocity, which more than doubled. Slices preloaded with histidine contained greatly elevated levels of glutamine and histidine, which may have stimulated the influx by exchange with extracellular tryptophan even in the absence of sodium ions. The maximal velocity was higher with increasing concentration of large neutral amino acids in slices at the start of the influx measurements. The influx of tryptophan in brain cells is apparently modified by changes in the intracellular amino acid pool, which, when increased, also counteracts the effect of sodium depletion on the tryptophan influx.     

The control of lysine biosynthesis in maize

Aspartate kinase has been partially purified and characterised from germinating maize seedlings. The K_m for aspartate was 9 mM. Out of several amino acids which are potential feedback regulators of the enzymes, only lysine is markedly inhibitory, having a K_i of 13 μM and causing 100% inhibition at 0.5 mM. Lysine also protects the enzyme against heat inactivation. Dihydrodipicolinic acid synthase isolated from the same tissue is also inhibited by lysine, 1 mM causing 95% inhibition.     

Spontaneous and depolarization-induced efflux of hypotaurine from mouse cerebral cortex slices: Comparison with taurine and gaba

The spontaneous and potassium- or veratrine-stimulated efflux of [³⁵S]hypotaurine from superfused cerebral cortex slices of adult mice was compared with the release of [³H]taurine and [³H]GABA. Initially GABA was the fastest released. Hypotaurine was, however, eventually released fastest, since its spontaneous efflux did not slow down during superfusions as did taurine and GABA effluxes. More than 60 % of all preloaded labelled amino acids still remained in the slices after 80-min superfusions. The effluxes of all three amino acids were stimulated by potassium and veratrine depolarizations: GABA efflux most and hypotaurine efflux least. The veratrine-stimulated release of taurine was long-lasting, while all other responses started and ended abruptly. With respect to efflux properties hypotaurine resembled more GABA than taurine.     

Active Transport of Glutamate in Leishmania (Leishmania) amazonensis

ABSTRACT. Leishmania spp. are the causative agents of leishmaniasis, a complex of diseases with a broad spectrum of clinical manifestations. Leishmania (Leishmania) amazonensis is a main etiological agent of diffuse cutaneous leishmaniasis. Leishmania spp., as other trypanosomatids, possess a metabolism based significantly on the consumption of amino acids. However, the transport of amino acids in these organisms remains poorly understood with few exceptions. Glutamate transport is an important biological process in many organisms. In the present work, the transport of glutamate is characterized. This process is performed by a single kinetic system (K_m=0.59±0.04 mM, V_max=0.123±0.003 nmol/min per 20 × 10⁶ cells) showing an energy of activation of 52.38±4.7 kJ/mol and was shown to be partially inhibited by analogues, such as glutamine, aspartate, α‐ketoglutarate and oxaloacetate, methionine, and alanine. The transport activity was sensitive to the extracellular concentration of H⁺ but not to Na⁺ or K⁺. However, unlike other amino acid transporters presently characterized, the treatment with specific ionophores confirmed the participation of a K⁺, and not H⁺ membrane gradient in the transport process.     

Influence of phloem transport, N-fertilization and ion accumulation on sucrose storage in the taproots of fodder beet and sugar beet

To investigate the factors governing the accumulation of sucroseand amino acids in the taproots of sugar beet, their contentswere measured in the leaves, phloem sap and the taproots ofsugar beet, fodder beet and a hybrid between both, grown oneither 3.0 or 0.5 mM nitrate. In the taproots the contents ofmalate, citrate and inorganic ions were also determined. Forthe high sucrose accumulation in sugar beet as compared to theother varieties three factors were found. (a) In sugar beet,less amino acids and more sucrose are taken up into the phloemthan in fodder beet. (b) In sugar beet, the sucrose and aminoacid syntheses are less sensitive to the nitrate concentrationsthat are required for optimal plant growth than in other varieties.In fodder beet, upon raising the nitrate concentration from0.5 mM to 3 mM, the synthesis and storage of sucrose is decreasedand that of amino acids increased. The corresponding valuesin sugar beet (0.5 mM) are similar to those in fodder beet andare not much affected by an increase of nitrate. (c) The sucroseaccumulation is limited by the accumulation of inorganic ionsin the taproots. The sucrose content in the taproots is negativelycorrelated to the total ion content. Whereas sucrose representstwo-third of all solutes in the taproots of sugar beet, it amountsto only one-third of the solutes in fodder beet taproots. Key words: Amino acids, Beta vulgans L, phloem sap, potassium, sucrose storage, sugar beet, taproots, transport