Novel regulations of glutamate and aspartate uptake by HeLa cells

Glutamine: do the data support the cause for glutamine supplementation in humans?

This review examines the preclinical rationale for using glutamine supplements and reviews the prospective randomized trials using glutamine to improve outcomes in patients. A special role for glutamine in gut physiology and in management of a variety of serious illnesses has been suggested, because it is the most abundant extracellular amino acid, and is used at high rates by the gut, liver, central nervous system, and immune cells. A state of relative Gln deficiency has been postulated in humans based on the decrease in plasma Gln in acute critical illness, but the decrease in plasma Gln is not specific for that amino acid, predicts only poorer outcome, and has not been validated to identify a deficiency state. Current evidence does not necessarily predict a special need or role for Gln in critical illness. Clinical efficacy of supplemental Gln has been difficult to demonstrate, possibly related to the lack of a Gln deficiency state, the wide range of end points used that reflect the lack of certainty of the predicted effect of supplementation, the heterogeneous patient populations studied, the lack of stable clinical course during the study, the lack of adequate power, and the relatively short follow-up period. Prospective randomized clinical trials of Gln supplementation were reviewed in patients with short-bowel syndrome, during cancer chemotherapy and in bone marrow transplantation, and in surgical, burn, and intensive care unit patients. No firm recommendation can be made at this time. Future studies should seek to develop a more standard and stable design for intervention in sufficiently powered studies.

Novel carboxylated N-glycans contain oligosaccharide-linked glutamic acid

We previously reported that N-glycans from bovine lung contain novel carboxylate groups. Here, we provide evidence that the carboxylated glycans contain glutamic acid. We labeled HeLa cells with [2,3-(3)H]glutamate and used a carboxylate-specific monoclonal antibody to enrich for the desired proteins. PNGaseF digestion of these proteins released labeled N-glycans with a free amino group and 1-3 carboxylates. Mild acid hydrolysis had no effect, but strong acid hydrolysis of the glycans released >80% of the (3)H as glutamate. Reducing the carboxylates to alcohols prior to hydrolysis eliminated the [(3)H]glutamate and generated [(3)H]4-amino 5-hydroxy pentanoic acid, suggesting that [(3)H]glutamate was linked to the glycan through its gamma-carboxyl. The glutamate-containing N-glycans resisted exoglycosidase digestion and oligosaccharide processing inhibitors greatly reduced [(3)H]glutamate incorporation. These results demonstrate that mammalian cells synthesize complex-type N-glycans with glutamate linked to their antennae, further expanding their potential for covalent or ionic interactions.

New mutations and phenotypes associated with glutamate and aspartate transport in Chinese hamster ovary (CHO-K1) cells

Two new Chinese hamster ovary cell (CHO-K1) mutants lacking amino acid transport System X-AG activity were isolated by [3H]aspartate suicide selection. These null mutants, Dd-B6 and Dd-B7, were analyzed by somatic cell hybridization, along with previously described partial-function mutants, Ed-A1 and Ed-B8. With respect to System X-AG activity, all four mutations fell into a single complementation group. By quantitative assay, the mutations in Ed-A1 and Ed-B8 behaved as simple recessives in fusions with wild type cells, while those in Dd-B6 and Dd-B7 were codominant. We have discovered that Ed-A1 and Ed-B8 are highly permeable to small neutral molecules. This high permeability phenotype was dominant to wild-type. Northern, Southern, and Western analyses indicated that System X-AG in CHO is not closely related to any of the three well characterized glutamate transporters represented by GLT-1, EAACI or GLAST.

Glutamate 404 is involved in the substrate discrimination of GLT-1, a (Na+ + K+)-coupled glutamate transporter from rat brain

Sodium-coupled glutamate transporters, located in the plasma membrane of nerve terminals and glial processes, serve to keep its extracellular glutamate concentration below extracellular levels. Moreover, they help in conjunction with diffusion to terminate the transmitter's action in synaptic transmission. We have investigated the role of negatively charged amino acid residues of GLT-1, a cloned (Na+ + K+)-coupled glutamate transporter from rat brain. Using site-directed mutagenesis we modified these negative residues, which are located in hydrophobic surroundings and are highly conserved within the glutamate transporter family. Out of five residues meeting these criteria, three, aspartate 398, glutamate 404, and aspartate 470, are critical for heterologously expressed glutamate transport. This defective transport cannot be attributed to the mere requirement of a negative charge at these positions. After prelabeling of the proteins with [35S]methionine, immunoprecipitation of all mutant transporters indicates that their expression levels are similar to that of wild type. No cryptic activity was revealed by reconstitution experiments aimed to monitor the activity of transporter molecules not located in the plasma membrane. Significantly, whereas all of the mutants at the glutamate 404 position exhibit impaired transport of glutamate, they possess considerable transport of D- and L-aspartate, up to 80% of wild type values. Binding of glutamate is not impaired in these mutants. Our observations indicate that the glutamate 404 residue may be located in the vicinity of the glutamate-aspartate permeation pathway.