Glutamate sensors

Glutamate Sensors. Glutamate occurs in unusually high concentrations in the brain and has been shown to stimulate neuronal activity (35). The role of this species as a neurotransmitter is not completely understood, however, and there has been a great deal of research aimed at studying its excitatory function in the brain. A selective biosensor for rapid determination of glutamate would be of great importance to neurochemical researchers. 

Figure 8 shows a preliminary response curve for a glutamate biosensor. For this sensor, glutamate dehydrogenase has been added to the biocatalytic layer in a sensor like that shown in Figure 6. 

The next group of multimembrane systems comprises membranes sensitized biologically using immobilized enzymes or microorganisms. Species that are directly sensed by an ISE are produced in the enzymatic reaction of the analyte. Examples of such sensors are those used for determination of urea in milk, based on immobilized urease and measurement of a pH change. An example of the application of bacteria strains is the use of immobilized recombinant Escherichia coli coupled with a pH electrode. Such electrodes have been used for determination of cephalosporins. When this bacterial strain is coupled with a CO2 gas sensor, glutamic acid determination can be carried out. 

Structural and functional evidence clearly demonstrates that family C receptors function as dimers, either as homodimers or as heterodimers. The metabotropic glutamate receptors and the calcium sensors, as discussed in Section 2.6.1, are found as covalently connected dimers in which there is a disulfide bridge between a Cys residue located in a loop in the N-terminal extracellular domain of each monomer. This disulfide bridge apparently serves only to hold the monomers in close proximity, as the loop is so unstructured that it does not resolve in the x-ray structure. 

Blankenstein G., Preuschoff F., Spohn U., Mohr K.H., Kula M.R., Determination of L-glutamate and L-glutamine by flow-injection analysis and chemiluminescence detection comparison of an enzyme column and enzyme membrane sensor, Anal. Chim. Acta 1993 271 231-237. 

Iida T., Kawabe T., Noguchi F., Mitamura T., Nagata K., Tomita K., ISFET-type -glutamate sensor using thermophilic glutamine-synthetase from a thermophile, Nippon-Kagaku-Kaishi 1987 10 1817-1821. 

Figure 5 Five-channel enzyme sensor for the simultaneous determination of glucose, lactate, glutamate, glutamine, and ammonium. MFM, microfiltration module WV, valves P, pumps DC, dialysis cell B, blank reactors MC, reactor D, biosensor flow cell. (Adapted with permission from Ref. 34.)...

S. Kar and M. A. Arnold, Fiber-optic ammonia sensor for measuring synaptic glutamate and extracellular ammonia, Anal. Chem. 64, 2438-2443 (1992). 

Figure 5.5 — Flow-through biosensor for the determination of L-glutamate. (A) Flow injection manifold. (B) Sensing microzone of the probe sensor (optrode), incorporated in the flow-cell (FTC). P pump IV injection valve MC mixing chamber AD air damper BFB bifurcated fibre bimdle LS light source PMT photomultiplier R recorder GLU L-glutamate 0-Glu 2-oxoglutarate E enzyme layer I optical insulator S sensing layer PS polyester support. For details, see text. (Adapted from [6] with permission of Elsevier Science Publishers).

The carrier used for this purpose consisted of a 0.1 M phosphate buffer of pH 7. The appearance of the sensing microzone is shown in Fig. 5.5.B. The oxygen optrode used was based on a 10-pm silicone rubber film containing dissolved decacyclene as indicator (S) that was fixed on a 110-pm thick polyester support (PS). A 9-pm black PTFE membrane (I) was used for optical insulation. The dye fluorescence was found to be markedly dependent on the concentration of oxygen, which exerted a quenching effect on it. The enzyme (glutamate oxidase) was immobilized on a 150-pm thick immunoaffmity membrane (E). The sensor was prepared similarly as reported by Trettnak et al. [7]. 

Another problem is the cahbration of BOD-sensor to enable the comparison with the conventional BOD. For cahbrating a BOD-sensor, a glucose-glutamic acid standard solution (the so-called GGA-standard), which is also used as standard solution for the BODj-method [71], or aqueous solutions of glucose or glycerol with a defined BODj [43,54] are used. 

The microbial sensor for acetic acid was applied to a fermentation broth of glutamic acid. The concentration of acetic acid was determined by the microbial sensor and by a gas chromatographic method. Good agreement was obtained the regression coefficient was 1.04 for 26 experiments. 

Glutamic Acid Sensor. A rapid automatic measurement of glutamic acid in fermentation media is required in fermentation industries. 

The concentrations of glutamic acid in some fermentation broths were determined by the microbial sensor and by the Auto-analyzer method. The results were in good agreement. The response of the sensor was constant for more than 3 weeks and 1500 assays. Thus the microbial sensor appears to be very attractive for the determination of glutamic acid. 

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