Development and characterization of a polymer gel with an immobilized enzyme to measure L-glutamate

Urea Biosensors Based on PVC Membrane Containing Palmitic Acid

A new urea biosensor was prepared by immobilizing urease with four different procedures on poly(vinylchloride) (PVC) ammonium membrane electrode containing palmitic acid by using nonactine as an ammonium-ionophore. The analytical characteristics were investigated and were compared those of the biosensor prepared by using carboxylated PVC. The effect of pH, buffer concentration, temperature, urease concentration, stirring rate and enzyme immobilization procedures on the response to urea of the enzyme electrode were investigated. The linear working range and sensitivity of the biosensor were also determined. The urea biosensor prepared by using the PVC membranes containing palmitic acid showed more effective performance than those of the carboxylated PVC based biosensors. Additionally, urea assay in serum was successfully carried out by using the standard addition method.

Engineering temperature-sensitive poly(N-isopropylacrylamide) polymers as carriers of therapeutic proteins

This study was carried out to engineer N-isopropylacrylamide (NiPAM) polymers that contain protein-reactive N-acryloxysuccinimide (NASI) and hydrophobic alkylmethacrylates (AMAs). These thermoreversible, protein-conjugating polymers hold potential for retention of therapeutic proteins at an application site where tissue regeneration is desired. The lower critical solution temperatures (LCST) of the polymers were effectively controlled by the AMA mole content. The AMAs with longer side-chains were more effective in lowering the LCST. Polymers without NASI exhibited a stable LCST in phosphate buffer and in serum over a 10-day study period. The LCST of polymers containing NASI was found to increase over time in phosphate buffer, but not in serum-containing medium. The LCST increase in phosphate buffer was proportional to the AMA content. The feasibility of localizing a therapeutic protein, recombinant human bone morphogenetic protein-2 (rhBMP-2), to a site of application was explored in a rat intramuscular injection model. The results indicated that polymers capable of conjugating to rhBMP-2 were most effective in localizing the protein irrespective of the LCST (13-25 degrees C). For polymers with no NASI groups, a lower LCST resulted in a better rhBMP-2 localization. We conclude that thermosensitive polymers can be engineered for delivery of therapeutic proteins to improve their therapeutic efficacy.

Glutamate measured by 6-s resolution brain microdialysis: capillary electrophoretic and laser-induced fluorescence detection application

In the present experiment the combination of brain microdialysis and CZE-LIFD permitted the measurement of glutamate in 100 nl microdialysis samples collected every 5 or 6 s. Samples were collected every 6 s, in rats anesthetized with two different anesthetic agents (ketamine and sodium thiopental). A microdialysis probe was inserted in the cortex of an anesthetized rat in the territory irrigated by the middle cerebral artery. The artery was clamped for 30 s and then released. The samples were derivatized with fluorescein isothiocyanate I (FITC) by means of a continuous-flow reactor, collected and injected into a home-made CZE-LIFD instrument. Glutamate decreased immediately after clamping the artery in ketamine anesthetized rats and increased 1 min after the onset of the ischemia in sodium thiopental anesthetized rats. In another experiment a 60 mM KCl solution was injected through a microdialysis probe inserted in the hippocampus of an anesthetized rat. In the first 5 s after the KCl solution reached the tissue, glutamate increased but gamma-aminobutytic acid and glutamine did not. The experiments show that time resolution of brain microdialysis can be reduced to a few seconds if the analytical technique is the proper one.

Application of a fluorometric method to measure glutamate release from single retinal photoreceptors

Release of the endogenous transmitter, glutamate, was measured from individual cone photoreceptors using a microfluorometric technique. The assay for glutamate was conducted within the lumen of a suction pipette, and was based on the fluorometric measure of the production of NADH from NAD+. This reaction was catalyzed by glutamate dehydrogenase contained in the pipette. Upon introduction of glutamate to the pipette, an increase in the NADH fluorescence was observed, representing the stoichiometric conversion of glutamate to NADH. The fluorescent signal was quantified, allowing an estimate of glutamate release from a single cone upon depolarization. The release observed was elicited upon depolarization of the cell with extrinsic current, and was detectable simultaneous with stimulation of the cell. Depolarization-induced release of endogenous glutamate was from the synaptic pedicle of the cell, and this release decreased with subsequent stimulations. The decrease in the release could be briefly reversed by an increase in the depolarization current used, or by allowing the cell to rest for several minutes.

Electrophysiology of glutamate and sodium co-transport in a glial cell of the salamander retina

1. Müller cells were isolated from salamander retinas and their membrane voltage was controlled with a whole-cell voltage clamp. External D-aspartate, L-aspartate and L-glutamate each induced a membrane current. D-Glutamate, kainate, quisqualate and N-methyl-D-aspartate were more than 100x less effective than L-aspartate. Kynurenic acid had no effect on the current produced by L-glutamate, L-aspartate or D-aspartate. 2. The current induced by an acidic amino acid (AAA) was completely dependent on the presence of external Na+. Neither Li+, Cs+, choline nor TEA+ were able to substitute for Na+. The relationship between external Na+ concentration and current amplitude can be explained if the binding of three Na+ ions enabled transport. The apparent affinity constant for Na+ binding was 41 mM. Altering K+, H+ and Cl- concentrations demonstrated that these ions are not required for transport. 3. The shape of the current-voltage relation did not depend on the external amino acid concentration. The relationship between D-aspartate concentration and current amplitude can be described by the binding of D-aspartate to a single site with an apparent affinity constant of 20 microM. 4. Influx and efflux of AAA were not symmetric. Although influx was electrogenic, efflux did not produce a current. Moreover, influx stimulated efflux; but efflux inhibited influx. 5. Removing external Na+ demonstrated that Na+ carried a current in the absence of an AAA. Li+ was a very poor substitute for Na+. This current may be due to the uncoupled movement of Na+ through the transporter. The relationship between the external Na+ concentration and the amplitude of the uncoupled current can be explained if the binding of two or three Na+ ions enabled the translocation of Na+ in the absence of an AAA. The apparent affinity constant for Na+ binding was approximately 90 mM. 6. The temperature dependence of the AAA-induced current had a Q10 between 8 and 18 degrees C of 1.95. The Q10 is consistent with a rate constant for influx of 10(4) s-1 (at -70 mV and 20 degrees C). The maximum rate of influx was measured following a concentration jump produced by the photolysis of 'caged' L-glutamate. The onset of the observed current was limited by the 1.3 ms resolution of the recording system. Hence, the rate constant for influx must be faster than 10(3) s-1.(ABSTRACT TRUNCATED AT 400 WORDS)