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1.
A reevaluation of the specificity of system y+, the classical transporter for cationic amino acids is presented. System y+ has been defined as a transporter for cationic amino acids that binds neutral amino acids with lower affinity in the presence
of Na+. The discovery of other transporters for cationic amino has suggested that some properties, originally attributed to system
y+, may relate to other transport systems. Uncertainty concerns mainly, the affinity for neutral amino acids and the cation
dependence of this interaction. Neutral amino acids (13 analogues tested) were found to bind to system y+ in human erythrocytes with very low affinity. Inhibition constants (Kiy, mm) ranged between 14.2 mm and >400 mm, and the strength of interaction was similar in the presence of Na+, K+ or Li+ (145 mm). In choline medium, no interaction was detected up to 20 mm of the neutral amino acid. Guanidinium ion (5 mm, osmolarity maintained with choline) potentiated neutral amino acid binding; the effect was most important in the case of
l-norvaline which aligned with guanidinium ion is equivalent to arginine. This suggests cooperative interaction at the substrate
site. The specificity of system y+ was shown to be clearly distinct from that of system y+L, a cationic amino acid transporter that accepts neutral amino acids with high affinity in the presence of Na+ and which influenced the classical definition of system y+.
Received: 28 September 1998/Revised: 21 December 1998 相似文献
2.
System y+L is a broad-scope amino acid transporter which binds and translocates cationic and neutral amino acids. Na+ replacement with K+ does not affect lysine transport, but markedly decreases the affinity of the transporter for l-leucine and l-glutamine. This observation suggests that the specificity of system y+L varies depending on the ionic composition of the medium. Here we have studied the interaction of the carrier with various
amino acids in the presence of Na+, K+, Li+ and guanidinium ion. In agreement with the prediction, the specificity of system y+L was altered by the monovalent cations. In the presence of Na+, l-leucine was the neutral amino acid that interacted more powerfully. Elongation of the side chain (glycine - l-norleucine) strengthened binding. In contrast, bulkiness at the level of the β carbon was detrimental. In K+, the carrier behaved as a cationic amino acid specific carrier, interacting weakly with neutral amino acids. Li+ was found to potentiate neutral amino acid binding and in general the apparent affinities were higher than in Na+; elongation of the nonpolar side chain made a more important contribution to binding and the carrier was more tolerant towards
β carbon substitution. Guanidinium stimulated the interaction of the carrier with neutral amino acids, but the effect was
restricted to certain analogues (e.g., l-leucine, l-glutamine, l-methionine). Thus, in the presence of guanidinium, the carrier discriminates sharply among different neutral amino acids.
The results suggest that the monovalent cations stabilize different carrier conformations.
Received: 22 January 1996/Revised: 26 April 1996 相似文献
3.
The transport specificity of system y+L of human erythrocytes was investigated and the carrier was found to accept a wide range of amino acids as substrates. Relative rates of entry for various amino acids were estimated from their trans-effects on the unidirectional efflux of l-[14C]-lysine. Some neutral amino acids, l-lysine and l-glutamic acid induced marked trans-acceleration of labeled lysine efflux; saturating concentrations of external l-leucine and l-lysine increased the rate by 5.3±0.63 and 6.2±0.54, respectively. The rate of translocation of the carrier-substrate complex is less dependent on the structure of the amino acid than binding. Translocation is slower for the bulkier analogues (l-tryptophan, l-phenylalanine); smaller amino acids, although weakly bound, are rapidly transported (l-alanine, l-serine). Half-saturation constants (±sem) calculated from this effect (l-lysine, 10.32±0.49 m and l-leucine, 11.50±0.50 m) agreed with those previously measured in cis-inhibition experiments. The degree of trans-acceleration caused by neutral amino acids did not differ significantly in Na+, Li+ or K+ medium, whereas the affinity for neutral amino acids was dramatically decreased if Na+ or Li+ were replaced by K+. The observation that specificity is principally expressed in substrate binding indicates that the carrier reorientation step is largely independent of the forces of interaction between the carrier and the transport site.We wish to thank Dr C.A.R. Boyd for helpful discussions and Prof. H.N. Christensen for sharing with us very relevant bibliographic material. We are grateful to FONDECYT (1282/91) and DTI (B 2674) (Chile) for financial assistance. 相似文献
4.
Uptake of glyeine,l-cysteine,l-leucine,l-methionine,l-aspartic acid andl-lysine was investigated in resting cells ofSaccharomyces cerevisiae treated with 0.3mm actidione for blocking protein synthesis. The amino acids were taken up against substantial concentration gradients (up to
nearly 1,000∶1 for μm l-cysteine and glycine). They were present in the free form inside the cells. Their unidirectional transmembrane fluxes were
under a negative feedback control by the intracellular concentration of the amino acid involved. The amino acids tested apparently
employed more than one transport agéncies for their membrane passage, the half-saturation constants being 6.2–7.7×10−4
m for glycine, 2.5×10−4
m forl-cysteine, 6×10−5 and 4×10−4
m forl-lysine, 3×10−5 and 6×10−4
m forl-methionine, 7–18×10−5 and 1.6×10−3
m forl-aspartic acid and 6×10−5 and 2×10−3
m forl-leucine. The specificities of the transport systems are overlapping but there emerges a wide-affinity transport system for
glycine, alanine, leucine, methionine, serine, cysteine, phenylalanine, aspartic acid, asparagine, glutamic acid and tryptophan
(and possibly for other amino acids), and more specific systems for each of the following: glycine, lysine, methionine, histidine,
arginine, and aspartic and glutamic acids. Proline had the peculiar effect of stimulating the transport of all the amino acids
tested. The amino acids apparently interacted in the uptake not only by competition for the binding site but also by allotopic
inhibition (e.g.l-cysteine) and possibly stimulation (l-proline). The initial rate of uptake of amino acids and their steady-state level of distribution were characterized by identical
activation energies: 7.5 kcal/mole forl-lysine, 6.9 kcal/mole forl-aspartic acid, and 13.2 kcal/mole for glycine. 相似文献
5.
The effect of l-arginine on transepithelial ion transport was examined in cultured M-1 mouse renal cortical collecting duct (CCD) cells using
continuous short circuit current (I
SC
) measurements in HCO3
−/CO2 buffered solution. Steady state I
SC
averaged 73.8 ± 3.2 μA/cm2 (n= 126) and was reduced by 94 ± 0.6% (n= 16) by the apical addition of 100 μm amiloride. This confirms that the predominant electrogenic ion transport in M-1 cells is Na+ absorption via the epithelial sodium channel (ENaC). Experiments using the cationic amino acid l-lysine (radiolabeled) as a stable arginine analogue show that the combined activity of an apical system y+ and a basal amino acid transport system y+L are responsible for most cationic amino acid transport across M-1 cells. Together they generate net absorptive cationic
amino acid flux. Application of l-arginine (10 mm) either apically or basolaterally induced a transient peak increase in I
SC
averaging 36.6 ± 5.4 μA/cm2 (n= 19) and 32.0 ± 7.2 μA/cm2 (n= 8), respectively. The response was preserved in the absence of bath Cl− (n= 4), but was abolished either in the absence of apical Na+ (n= 4) or by apical addition of 100 μm amiloride (n= 6). l-lysine, which cannot serve as a precursor of NO, caused a response similar to that of l-arginine (n= 4); neither L-NMMA (100 μm; n= 3) nor L-NAME (1 mm; n= 4) (both NO-synthase inhibitors) affected the I
SC
response to l-arginine. The effects of arginine or lysine were replicated by alkalinization that mimicked the transient alkalinization
of the bath solution upon addition of these amino acids. We conclude that in M-1 cells l-arginine stimulates Na+ absorption via a pH-dependent, but NO-independent mechanism. The observed net cationic amino acid absorption will counteract
passive cationic amino acid leak into the CCD in the presence of electrogenic Na+ transport, consistent with reports of stimulated expression of Na+ and cationic amino acid transporters by aldosterone.
Received: 11 September 2000/Revised: 6 December 2000 相似文献
6.
Sequential Amino Acid Exchange across b0,+-like System in Chicken Brush Border Jejunum 总被引:2,自引:0,他引:2
Torras-Llort M Torrents D Soriano-García JF Gelpí JL Estévez R Ferrer R Palacín M Moretó M 《The Journal of membrane biology》2001,180(3):213-220
In the small intestine, cationic amino acids are transported by y+-like and b0,+-like systems present in the luminal side of the epithelium. Here, we report the characterization of a b0,+-like system in the apical membrane of the chicken jejunum, and its properties as an amino acid exchanger. Analysis of the
brush border membrane by Western blot points out the presence of rBAT (protein related to b0,+ amino acid transport system) in these membranes. A functional mechanism for amino acid exchange across this system was established
by kinetic analysis measuring fluxes at varying substrate concentrations both in internal (in) and external (out) vesicle
compartments. This intestinal b0,+-like system functions for l-arginine as an obligatory exchanger since its transport capacity increases 100–200 fold in exchange conditions, thus suggesting
an important role in the intestinal absorption of cationic amino acids. The kinetic analysis of Argin efflux velocities is compatible with the formation of a ternary complex and excludes a model involving a ping-pong mechanism. The binding affinity of Argout is higher than that of Argin, suggesting a possible order of binding (Argout first) for the formation of the ternary complex during the exchange cycle. A model of double translocation pathways with
alternating access is discussed.
Received: 23 March 2000/Revised: 29 December 2000 相似文献
7.
M. Torras-Llort R. Ferrer J.F. Soriano-García M. Moretó 《The Journal of membrane biology》1996,152(3):183-193
The properties of l-lysine transport in chicken jejunum have been studied in brush border membrane vesicles isolated from 6-wk-old birds. l-lysine uptake was found to occur within an osmotically active space with significant binding to the membrane. The vesicles
can accumulate l-lysine against a concentration gradient, by a membrane potential-sensitive mechanism. The kinetics of l-lysine transport were described by two saturable processes: first, a high affinity-transport system (K
mA= 2.4 ± 0.7 μmol/L) which recognizes cationic and also neutral amino acids with similar affinity in the presence or absence
of Na+ (l-methionine inhibition constant KiA, NaSCN = 21.0 ± 8.7 μmol/L and KSCN = 55.0 ± 8.4 μmol/L); second, a low-affinity transport mechanism (KmB= 164.0 ± 13.0 μmol/L) which also recognizes neutral amino acids. This latter system shows a higher affinity in the presence
of Na+ (KiB for l-methionine, NaSCN = 1.7 ± 0.3 and KSCN = 3.4 ± 0.9 mmol/L). l-lysine influx was significantly reduced with N-ethylmaleimide (0.5 mmol/L) treatment. Accelerative exchange of extravesicular labeled l-lysine was demonstrated in vesicles preloaded with 1 mmol/L l-lysine, l-arginine or l-methionine. Results support the view that l-lysine is transported in the chicken jejunum by two transport systems, A and B, with properties similar to those described
for systems b
0,+ and y+, respectively.
Received: 14 August 1995/Revised: 2 April 1996 相似文献
8.
An inherited amino acid transport deficiency results in low concentrations of glutathione (GSH) in the erythrocytes of certain sheep. Earlier studies based on phenotyping according to GSH concentrations indicated that the gene Tr
H, which controls normal levels of GSH, behaves as if dominant or incompletely dominant to the allele Tr
h, which controls the GSH deficiency. The present paper shows that when sheep are classified according to amino acid transport activity, the Tr
H gene behaves as if codominant to Tr
h. Erythrocytes from sheep homozygous for the Tr
H gene exhibit rapid saturable l-alanine influx (apparent K
m ,21.6mm; V
max, 22.4 mmol/liter cells/hr). Cells from sheep homozygous for the Tr
h gene exhibit slow nonsaturable l-alanine uptake (0.55 mmol/liter cells/hr at 50mm extracellular l-alanine). Cells from heterozygous sheep show saturable l-alanine uptake with a diminished V
max (apparent K
m, 19.1mm; V
max, 12.7 mmol/liter cells/hr). These erythrocytes have a significantly lower GSH concentration than cells from Tr
H, TrH sheep but similar intracellular levels of dibasic amino acids.The authors are grateful to the M.R.C. for a Project Grant. 相似文献
9.
Summary Intracellular calcium was measured in single olfactory neurons from the channel catfish (Icatalurus punctatus) using the fluorescent Ca2+ indicator fura 2. In 5% of the cells, olfactory stimuli (amino acids) elicited an influx of calcium through the plasma membrane which led to a rapid transient increase in intracellular calcium concentration. Amino acids did not induce release of calcium from internal stores in these cells. Some cells responded specifically to one stimulus (l-alanine,l-arginine,l-norleucine andl-glutamate) while one cell responded to all stimuli. An increase in intracellular calcium could also be elicited in 50% of the cells by direct G-protein stimulation using aluminum fluoride. Because the fraction of cells which respond to direct G-protein stimulation is substantially larger than the fraction of cells responding to amino acids, we tested for possible damage of receptor proteins due to exposure of the olfactory neurons to papain during cell isolation. We find that pretreatment with papain does not alter specific binding ofl-alanine andl-arginine to olfactory receptor sites in isolated olfactory cilia. The results are discussed in terms of their relevance to olfactory transduction. 相似文献
10.
Peter C. Hubbard Eduardo N. Barata Rodrigo O. A. Ozório Luisa M. P. Valente Adelino V. M. Canário 《Journal of comparative physiology. A, Neuroethology, sensory, neural, and behavioral physiology》2011,197(8):839-849
The current study investigated the olfactory sensitivity of the blackspot sea bream to amino acids, odorants associated with
food detection in fish, and compared the efficacy of two different experimental methods: multi-unit recording from the olfactory
nerve and the electro-olfactogram (EOG). Twenty essential amino acids plus l-DOPA evoked clear, concentration-dependent olfactory responses using both methods, with estimated thresholds of 10−8.5–10−6.2 M (nerve recording) and 10−7.5–10−4.8 M (EOG). The most potent amino acids were l-cysteine, l-methionine (both sulphur-containing), l-alanine, l-leucine (both neutral), l-glutamine (amide-containing) and l-serine (hydroxyl-containing). The least potent were l-proline (secondary α-amino group), the aromatic amino acids and glycine (simplest). Although the rank order of olfactory
potency was similar for the two methods used, and the calculated thresholds given by the two methods were positively correlated,
the sensitivity of the EOG was consistently lower than multi-unit recording by approximately one order of magnitude, presumably
due to the electrical shunting effect of seawater. As in freshwater, the EOG could be a valid method for comparing olfactory
potency of different odorants in stenohaline marine fish; however, for absolute ‘biological’ thresholds, a more invasive recording
technique, such as multi-unit recording from the olfactory nerve, should be used. 相似文献
11.
Conrad EM Ahearn GA 《Journal of comparative physiology. B, Biochemical, systemic, and environmental physiology》2007,177(3):297-307
The intestine of the American lobster, Homarus americanus, was isolated and perfused in vitro with a physiological saline, based on the ion composition of the blood, to characterize
the mechanisms responsible for transmural transport of zinc and how the amino acid, l-histidine, affects the net movement of the metal across the tissue. Previous studies with this preparation, focusing on the
characteristics of unidirectional mucosa to serosa (M to S) fluxes of 65Zn2+ and 3H-l-histidine, indicated the presence of a brush border co-transport process responsible for simultaneously transferring the
metal and amino acid across this tissue as an apparent bis-complex (Zn-[His]2) using a PEPT-1-like dipeptide carrier mechanism. In addition, both zinc and l-histidine were also transferred toward the blood by separate transporters that were independent of the other substrate. The
focus of the present study was to characterize the serosa to mucosa (S to M) flux of 65Zn2+ under a variety of conditions, and use these values in conjunction with those from the previous study, to assess the direction
and magnitude of net metal movement across the tissue. Transmural S to M transport of 65Zn2+ was markedly reduced with the addition of the serosal inhibitors ouabain (32%), excess K+ (25%), excess Ca2+ (30%), Cu2+ (38%), nifedipine (21%), and vanadate (53%). In contrast, this flux was markedly stimulated with the serosal addition of
ATP (24%) and excess Na+ (28%). These results suggest that S to M fluxes of zinc occurred by the combination of the basolateral Na/Ca exchanger (NCX),
where zinc replaced calcium, and a basolateral nifedipine-sensitive calcium channel. Transmural M to S 65Zn2+ fluxes (5–100 μM) were threefold greater than S to M metal transport, and the addition of luminal l-histidine doubled the net M to S zinc flux over its rate in the absence of the amino acid. The results of this paper and
those in its predecessor indicate that zinc transport by the lobster intestine is absorptive and significantly enhanced by
luminal amino acids. 相似文献
12.
The release of neurotransmitters and modulators has been studied mostly using labeled preloaded compounds. For several reasons,
however, the estimated release may not reliably reflect the release of endogenous compounds. The basal and K+-evoked release of the neuroactive endogenous amino acids GABA, glycine, taurine, l-glutamate and l-aspartate was now studied in slices from the hippocampus and brain stem from 7-day-old and 3-month-old mice under control
and ischemic conditions. The release of synaptically not active l-glutamine, l-alanine, l-threonine and l-serine was assessed for comparison. The estimates for the hippocampus and brainstem were markedly different and also different
in developing and adult mice. GABA release was much greater in 3-month-old than in 7-day-old mice, whereas with taurine the
situation was the opposite, in the hippocampus in particular. K+ stimulation enhanced glycine release more in the mature than immature brain stem while in the hippocampus the converse was
observed. Ischemia enhanced the release of all neuroactive amino acids in both brain regions, the effects being relatively
most pronounced in the case of GABA, aspartate and glutamate in the hippocampus in 3-month-old mice, and taurine in 7-day-old
and glycine in 3-month-old mice in the brain stem. These results are qualitatively similar to those obtained on earlier experiments
with labeled preloaded amino acids. However, the magnitudes of the release cannot be quite correctly estimated using radioactive
labels. In developing mice only taurine release may counteract the harmful effects of excitatory amino acids in ischemia in
both hippocampus and brain stem. 相似文献
13.
l-Amino acid dehydrogenases are a group of enzymes that catalyze the reversible oxidative deamination of l-amino acids to their corresponding 2-oxoacids, using either nicotinamide adenine dinucleotide (NAD+) or nicotinamide adenine dinucleotide phosphate (NADP+) as cofactors. These enzymes have been studied widely because of their potential applications in the synthesis of amino acids
for use in production of pharmaceutical peptides, herbicides and insecticides, in biosensors or diagnostic kits, and development
of coenzyme regeneration systems for industrial processes. This article presents a review of the currently available data
about the recently discovered amino acid dehydrogenase superfamily member l-aspartate dehydrogenase (l-AspDH), their relevant catalytic properties and speculated physiological roles, and potential for biotechnological applications.
The proposed classification of l-AspDH on the basis of bioinformatic information and potential role in vivo into NadB (NAD biosynthesis-related) and non-NadB
type is unique. In particular, the mesophilic non-NadB type l-AspDH is a novel group of amino acid dehydrogenases with great promise as potential industrial biocatalysts owing to their
relatively high catalytic properties at room temperature. Considering that only a few l-AspDH homologs have been characterized so far, identification and prodigious enzymological research of the new members will
be necessary to shed light on the gray areas pertaining to these enzymes. 相似文献
14.
l-Glutamic acid (l-Glu) and other excitatory amino acids and amino acid analogs enhanced [35S]thiocyanate (SCN–) uptake in isolated-resealed synaptic membrane vesicles. The SCN– uptake was used as a measure of membrane depolarization to evaluate the characteristics of functional excitatory amino acid receptors in the synaptic membranes.N-Methyl-d-aspartate (NMDA) andl-Glu produced additive effects on SCN– accumulation indicating the presence of distinctl-Glu and NMDA receptors. On the other hand, kainic acid (KA) andl-Glu shared either common receptor sites or ion channels. The effects of antagonists on NMDA,l-Glu, and KA stimulation of SCN– influx were consistent with previously reported electrophysiologic observations in intact neurons. 相似文献
15.
Peter M. Taylor Harinder S. Hundal Michael J. Rennie 《The Journal of membrane biology》1989,112(2):149-157
Summary We have investigated transport of the amino acid glutamine across the surface membranes of prophase-arrestedXenopus laevis oocytes. Glutamine accumulation was linear with time for 30 min; it was stereospecific with aK
m
of 0.12±0.02mm andV
max of 0.92±0.17 pmol/oocyte · min forl-glutamine. Transport ofl-glutamine was Na+-dependent, the cation not being replaceable with Li+, K+, choline, tris(hydroxymethyl)-aminomethane (Tris), tetramethylammonium (TMA) or N-methyld-glucamine NMDG); external Cl– appeared to be necessary for full activation of Na+-dependent glutamine transport. Two external Na+ may be required for the transport of one glutamine molecule.l-glutamine transport (at 50 m glutamine) was inhibited by the presence of other amino acids:l-alanine,d-alanine,l-leucine,l-asparagine andl-arginine (about 60% inhibition at 1mm);l-histidine,l-valine and glycine (25 to 40% inhibition at 1mm);l-serine,l-lysine,l-phenylalanine andl-glutamate (45 to 55% inhibition at 10mm). N-methylaminoisobutyric acid (meAIB) had no effect at 10mm, but 2-aminobicyclo[2,2,1]heptane-2-carboxylic acid (BCH) inhibited Na+/glutamine transport by about 50% at 10mm.l-glutamine was a competitive inhibitor of the Na+-dependent transport ofl-alanine,d-alanine andl-arginine; this evidence is consistent with the existence of a single system transporting all four amino acids. Glutamine uptake in oocytes appears to be catalyzed by a transport system distinct from the cotransport Systems A, ASC, N and Gly, although it resembles System B0,+. 相似文献
16.
Nutritional and medicinal aspects of <Emphasis Type="SmallCaps">d</Emphasis>-amino acids 总被引:2,自引:0,他引:2
This paper reviews and interprets a method for determining the nutritional value of d-amino acids, d-peptides, and amino acid derivatives using a growth assay in mice fed a synthetic all-amino acid diet. A large number of
experiments were carried out in which a molar equivalent of the test compound replaced a nutritionally essential amino acid
such as l-lysine (l-Lys), l-methionine (l-Met), l-phenylalanine (l-Phe), and l-tryptophan (l-Trp) as well as the semi-essential amino acids l-cysteine (l-Cys) and l-tyrosine (l-Tyr). The results show wide-ranging variations in the biological utilization of test substances. The method is generally
applicable to the determination of the biological utilization and safety of any amino acid derivative as a potential nutritional
source of the corresponding l-amino acid. Because the organism is forced to use the d-amino acid or amino acid derivative as the sole source of the essential or semi-essential amino acid being replaced, and
because a free amino acid diet allows better control of composition, the use of all-amino-acid diets for such determinations
may be preferable to protein-based diets. Also covered are brief summaries of the widely scattered literature on dietary and
pharmacological aspects of 27 individual d-amino acids, d-peptides, and isomeric amino acid derivatives and suggested research needs in each of these areas. The described results
provide a valuable record and resource for further progress on the multifaceted aspects of d-amino acids in food and biological samples. 相似文献
17.
We report the application of multiple time regression analysis with the in situ brain perfusion technique to measure the rates
of passage between blood and brain for [14C]l-proline, [14C]l-alanine, and [14C] α-aminoisobutyric acid (AIB) and their rapidly reversible volumes following perfusion of these amino acids from 10 to 60
seconds. We also report on their mechanism of transport. Proline diffused through the blood-brain barrier with a transfer
coefficient (Kin) of 0.55 ± 0.15 × 10−4 ml/s/g and had no reversible compartment. AIB had a low Kin of 0.68±0.14×10−4 ml/s/g and a significant reversible volume of 4.34±0.51×10−3 ml/g in parietal cortex.l-alanine had the highest transfer coefficient, 3.11±0.26 × 10−4 ml/s/g, and a reversible volume of 10.03±0.93×10−3 ml/g in the same cerebral region. Postwash procedures which remove any radiotracer in the vasculature and capillary depletion
were performed for alanine and AIB, as they had significant reversible compartments, to test the possibility of rapid efflux
from the endothelial cells. Results obtained from wash and capillary depletion procedures suggest that a rapid efflux could
occur from endothelial cells after entry of alanine and AIB. Mechanisms of transport forl-alanine and AIB were investigated using amino acids (5 mM) as substrates and inhibitors of different amino acid transport
systems. AIB transport was reduced by plasma andl-leucine and unchanged by sodium-free buffer, confirming its passage by the L1 system.l-alanine uptake was sodium-independent and not reduced by plasma.l-serine,l-cysteine,l-leucine andl-phenylalanine produced similar inhibition (66%) whilel-alanine produced a lower inhibition (41%).l-arginine increased alanine uptake in cortex and thalamus. Addingl-serine tol-phenylalanine reduced the uptake only in cortex and hippocampus. These data suggest thatl-alanine is transported by another L transport system different from the L1 system at the luminal membrane. 相似文献
18.
The mycelium of the wood-rotting fungus,P. ostreatus, contains NAD-dependent glutamate synthase inhibited by azaserine.l-Glutamine andl-glutamate are the most important free amino acids in the mycelium. Feeding of the mycelium with nitrogenous substrates showed
thatl-glutamate,l-aspartate andl-alanine are interconnected by way of transaminases. After the inhibition of glutamine synthetase by methionine-S-sulfoximine
the synthesis ofl-glutamate was inhibited and the level of all free amino acids decreased. The15N-NMR spectra of mycelia after the addition of15NH4Cl confirmed that the GS/GOGAT is the only pathway of ammonia assimilation inP. ostreatus and NAD-glutamate dehydrogenase should be the deaminating enzyme. 相似文献
19.
A capillary assay was employed to quantify positive chemotactic responses in the motile, unicellular, marine algaDunaliella tertiolecta. Among a wide range of inorganic and organic compounds tested, only ammonium ion,l-tyrosine,l-tryptophan, andl-phenylalanine were found to be major atractants for the chlorophyte.l-Methionine andl-cysteine weakly attracted the alga at 10−3 M. The minimum concentration of the major attractants needed to elicit an observable chemotactic response was approximately
10−6 M. The maximum response occurred when the capillaries contained 10−5 Ml-tyrosine orl-tryptophan, 10−4 Ml-phenylalanine, and 10−3 M ammonium chloride. The other amino acids, carbohydrates, B-vitamins, urea, and nitrate were among the chemicals that failed
to attractD. tertiolecta. The alga apparently possesses one chemoreceptor that binds ammonium ion only, and another chemoreceptor that binds the three
aromatic amino acids. 相似文献
20.
Norbert Sauer 《Planta》1984,161(5):425-431
Glucose or non-metabolizable glucose analogues induce two systems of amino-acid transport in Chlorella vulgaris: an arginine-lysine system and a proline system. An additional third system of amino-acid transport is induced when glucose and an inorganic nitrogen source are present during glucose induction. The transport rates in glucose-NH
4
+
-treated cells are 10 to 80 times higher than in untreated cells. The transport system shows a rather broad specificity and catalyses the transport of at least ten neutral and acidic amino acids. Three of these amino acids (l-alanine, l-serine and glycine) are transported by the proline system as well. The system is specific for l-amino acids and has a pH optimum between 5 and 6. Transport by this system seems to be active, since amino acids are accumulated inside the cells. 相似文献