Glutamate removal

Removal of glutamate occurs by both high-affinity transporters (with Kms in the low micromolar range) and low-affinity transporters (with Kms around 0.5mmol/l). Only the former have been characterized on a molecular level. Glutamate removal has two components binding of... 

Glutamate removal. Glutamate s actions ate stopped not by enzymatic breakdown, as in other neurotransmitter systems, but by removal by two transport pumps. The first of these pumps is a presynaptic glutamate transporter, which works as do all the other neurotransmitter transporters already discussed for monoamine neurotransmitter systems such as dopamine, norepinephrine, and serotonin. The second transport pump, located on nearby glia, removes glutamate from the synapse and terminates its actions there. Glutamate removal is summarized in Figure 10—22. 

Synthesis of glutamate removes a-ketoglutarate from the TCA cycle, thereby decreasing the regeneration of oxaloacetate in the TCA cycle. Because oxaloacetate is necessary for the oxidation of acetyl CoA, oxaloacetate must be replaced by anapierotic reactions. There are two major types of anapierotic reactions (1) pyruvate carboxylase and (2) the degradative pathway of the branched-chain amino acids, valine and isoleucine, which contribute succinyl CoA to the TCA cycle. This pathway uses B12 (but not folate) in the reaction catalyzed by methylmalonyl CoA mutase. 

In E. coli GTP cyclohydrolase catalyzes the conversion of GTP (33) into 7,8-dihydroneoptetin triphosphate (34) via a three-step sequence. Hydrolysis of the triphosphate group of (34) is achieved by a nonspecific pyrophosphatase to afford dihydroneopterin (35) (65). The free alcohol (36) is obtained by the removal of residual phosphate by an unknown phosphomonoesterase. The dihydroneoptetin undergoes a retro-aldol reaction with the elimination of a hydroxy acetaldehyde moiety. Addition of a pyrophosphate group affords hydroxymethyl-7,8-dihydroptetin pyrophosphate (37). Dihydropteroate synthase catalyzes the condensation of hydroxymethyl-7,8-dihydropteroate pyrophosphate with PABA to furnish 7,8-dihydropteroate (38). Finally, L-glutamic acid is condensed with 7,8-dihydropteroate in the presence of dihydrofolate synthetase. 

Sodium poly(a-L-glutamate). It was washed with acetone, dried, dissolved in water and ppted with isopropanol at 5°. Impurities and low molecular weight fractions were removed by dialysis of the aqueous solution for 50h, followed by ultrafiltration through a filter impermeable to polymers of molecular weights greater the 10. The polymer was recovered by freeze-drying. [Mori et al. J Chem Soc, Faraday Trans I 2583 1978.]... 

The salt ions are captured by the ion exchange membranes that are present. The applications are limited to desalting amino add solutions, eg removal of HQ from L-glutamic add solution. 

The digestion of the protein, after heme removal, using Glu-C endoproteinase was also carried out. This enzyme cleaves the polypeptide backbone on the carboxyl terminus of a glutamic acid residue and in this case yielded twelve chromatographic responses. Despite two of these arising from unresolved components, molecular weight information was obtained from 15 polypeptides, one of which was the intact protein, covering the complete sequence, as shown in Table 5.10. 

The ammonia produced by enteric bacteria and absorbed into portal venous blood and the ammonia produced by tissues are rapidly removed from circulation by the liver and converted to urea. Only traces (10—20 Ig/dL) thus normally are present in peripheral blood. This is essential, since ammonia is toxic to the central nervous system. Should portal blood bypass the liver, systemic blood ammonia levels may rise to toxic levels. This occurs in severely impaired hepatic function or the development of collateral links between the portal and systemic veins in cirrhosis. Symptoms of ammonia intoxication include tremor, slurred speech, blurred vision, coma, and ultimately death. Ammonia may be toxic to the brain in part because it reacts with a-ketoglutarate to form glutamate. The resulting depleted levels of a-ketoglutarate then impair function of the tricarboxylic acid (TCA) cycle in neurons. 

Figure 29-8. The glutaminase reaction proceeds essentially irreversibly in the direction of glutamate and NH/ formation. Note that the amide nitrogen, not the a-amino nitrogen, is removed.

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