Some Characteristics of the Uptake of Glutamine by Corn Scutellum

The scuteUa separated from germinating barley grains (Hordewn vagare L. cv. Himalaya) took up the dipeptide 14Clglycylglycine (Gly-Gly) rapidly from incubation media. The pH optimum of the process was about 4.5, and the rate of uptake conformed to Michaelis-Menten kinetics with an apparent Km of 2.3 mm and V. of 41 imole gram-l hour-'. The uptake was strongly inhibited by dinitrophenol and cyanide and by lack of 02.After incubation of the scuteUa with Gly-Gly, no intact Gly-Gly was detectable in the scuteila but the level of free glycine increased. The poorly hydrolyzed "model" dipeptide glycylsarcosine, which is actively taken up and accumulated by the scutella, was a competitive inhibitor of the uptake of Gly-Gly and completely inhibited the uptake at infinitely high inhibitor concentration. This suggests that Gly-Gly is taken up by the same mechanism as glycylsarcosine as an intact dipeptide (without hydrolysis in the membrane) and is hydrolyzed to free glycine by the abundant peptidases of the scutefla.The uptake of Gly-Gly was not affected by glycine or leucine, but was strongly inhibited by all of the 10 dipeptides tested for inhibition. The three dipeptides tested for uptake, Ala-Gly, Pro-Gly, and Gly-Pro, were all taken up by the scuteDa. Thus, the uptake mechanism for the dipeptides seems to be rather nonspecific with respect to the side chains of the amino acids. The high rates of the uptake suggest that this process has an essential role in the mobilization of reserve proteins in the germinating grain.Recent experiments (2, 6, 15) have shown that the N-methylated peptides, Gly-Sar3 and Gly-Sar-Sar, which are very resistant to hydrolysis by peptidases, were actively taken up and accumulated against a concentration gradient by the scutella of germinating barley grains. Gly-Gly, the easily hydrolyzed, unsubstituted analog of Gly-Sar, inhibited the uptake of both Gly-Sar and Gly-SarSar, presumably by competition for the peptide transport system (15). Since the uptake of Gly-Sar was not inhibited by free glycine, the peptide transport system appeared to be separate from the system(s) for the transport of free amino acids.We have now studied more closely the uptake of Gly-Gly and some other peptides by the scutellum. This paper reports an investigation of some factors affecting the uptake of Gly-Gly by the scutellum of germinating barley and its interaction with amino acids and with other peptides. data, suggest that the hydrolysis products of the reserve proteins of the starchy endosperm are taken up by the scutellum as a mixture of amino acids and small peptides and the latter are hydrolyzed to free amino acids in the scutellum before "long distance" transport to the growing tissues of the seedling. MATERIALS AND METHODSPlant Material. Grains of a huskiess variety of barley, Hordeum vulgare L. cv. Himalaya, were obtained from Agronomy Club, Washington State University, Pullman, Washington. They were surface-sterilized with sodium hypochlorite (10 g/l) for 20 mi, rinsed several times in sterile ...

Section: Resultsmentioning

confidence: 88%

Uptake of Glycylglycine by the Scutellum of Germinating Barley Grain

Sopanen¹,

Burston²,

Taylor³

et al. 1978

“…(---), Nonmediated uptake. '-5 Intercept: 1 of corn scutella (28) and in cotyledons of castor bean (20). In all three cases, the uptake is rapid and appears to be carrier-mediated active transport, the carriers having a broad specificity.…”

Section: Discussionmentioning

confidence: 99%

“…Glutamine has earlier been shown to be rapidly taken up into slices of corn scutella, the uptake being carrier-mediated active transport (28). Here, we report some general properties of the uptake of glutamine into whole barley scutella.…”

mentioning

confidence: 92%

Uptake of Glutamine by the Scutellum of Germinating Barley Grain

Väisänen¹,

Sopanen²

1985

Scutella separated from germinating grains of barley (Hordeum vulgare L. cv Himalaya) took up '4Cqglutamine at an initial rate of about 10 micromoles-gram-'.hour-' in the standard assay conditions (pH 5, 30°C, 1 millimolar glutamine). Inhibition by unlabeled glutamine and by dinitrophenol indicated that about 95% of the uptake was due to carriermediated active transport. The pH optimum of the uptake was 5, and after correction for a nonmediated component the uptake appeared to conform to Michaelis-Menten kinetics with an apparent K. of about 2 millimolar and a V.. of about 25 micromoles -gram-'. hour-'.The uptake of glutamine was inhibited by all of the 18 amino acids tested; the mode of inhibition was studied only with proline and was competitive. Eight of the ten amino acids tested at high concentrations appeared to be able to inhibit the mediated uptake of glutamine virtually completely. However, when the inhibitory effect of asparagine was extrapolated to an infinitely high concentration of asparagine, about 24% of the mediated uptake of glutamine remained uninhibited. These results suggest that glutamine is taken up by two (or more) rather unspecific amino acid uptake systems, the minor one having no affinity for asparagine.Glutamine and alanine could completely inhibit the mediated uptake of I millimolar leucine, but about 12% of the mediated uptake appeared to be uninhibitable by asparagine. Furthermore, the ratio of the mediated uptake of glutamine to that of leucine changed from 0.9 to 1.7 between days I and 3 of germination. These results give further support for the presence of two unspecific amino acid uptake systems in barley scutella.During germination of a barley grain the reserve proteins in the starchy endosperm are hydrolyzed into a mixture of short oligopeptides and free amino acids (16,17). These are taken up into the scutellum, most probably into the epithelial cells (11,26, and references cited therein), from where the amino acids taken up or liberated from the peptides are transferred to the growing seedling.Previously, some properties of the uptake of amino acids into the scutellum were studied using leucine as substrate (18,26). The uptake of leucine is separate from the uptake of peptides and is due to active transport. The pH optimum is near 5, the pH ofthe starchy endosperm. The rate ofuptake increases rapidly in the beginning of germination and attains a high level in comparison to the rates of uptake in other parts of the seedling. Glutamine has earlier been shown to be rapidly taken up into slices of corn scutella, the uptake being carrier-mediated active transport (28). Here, we report some general properties of the uptake of glutamine into whole barley scutella. Using kinetic analysis of the inhibitory effects of other amino acids, we have also attempted to find out whether glutamine is taken up by one or more uptake systems.Some preliminary results have already been reported (23, 27 Uptake Assay. The standard procedure was modified from the leucine uptake assay used earl...

“…Cysteine and methionine are presumably absorbed by the scutellum much like glutamine (26). However, their fate after absorption is unknown.…”

mentioning

confidence: 99%

Cysteine, γ-Glutamylcysteine, and Glutathione Levels in Maize Seedlings

Rauser

Schupp²,

Rennenberg³

1991

The levels of cysteine (Cys), -y-glutamylcysteine (yEC), and glutathione (GSH) were measured in the endosperms, scutella, roots, and shoots of maize (Zea mays L.) seedlings. GSH was the major thiol in roots, shoots, and scutella, Cys predominated in endosperms. The endosperm, scutellum, and functional phloem translocation were required for maintenance of GSH pools in roots and shoots of 6-day-old seedlings. and shoots in proportion to the tissue fresh weight. Taken together, the data supported the hypothesis that Cys from the endosperm is absorbed by the scutellum and used to synthesize GSH for transfer through the phloem to the root and shoot. The estimated flux of GSH to the roots was 35 to 60 nanomoles per gram per hour, which totally accounted for the small gain in GSH in roots between days 6 and 7. For Cd-treated roots the GSH influx was similar, yet the GSH pool did not recover to control levels within 24 hours. The estimated flux of GSH to the entire shoot was like that to the roots; however, it was low (11-13 nanomoles per gram per hour) to the first leaf and high (76-135 nanomoles per gram per hour) to the second and younger leaves.