ENZYME PACKED-BED SYSTEM FOR THE ONLINE MEASUREMENT OF GLUCOSE, GLUTAMATE, AND LACTATE IN BRAIN MICRODIALYSATE

Rapid measurement of glucose, glutamate, and lactate Is important in understanding the dynamics of the energy balance of the brain. Glutamate Is also the main excitatory neurotransmitter. A general immobilized enzyme-based flow injection assay system is described which uses oxidase and peroxidase enzymes to convert the analyte into an oxidized ferrocene species which is detected electrochemically by reduction. The enzymes glucose oxidase, glutamate oxidase, lactate oxidase, and horseradish peroxidase are immobilized with near 100 % efficiency onto 10-mu-m tresyl-activated silica beads (1000- and 500-angstrom pore size). The beads are slurry-packed into 2- X 20-mm columns to give beds for glucose, glutamate, or lactate which are stable for >40 days. The flow injection assays described have detection limits from 1.8 to <20 pmol and have been configured to have linear calibration responses over the range of basal and stimulated levels of the three compounds found in 5-mu-L microdialysate samples from the rat striatum. The assays are used for automated on-line measurement of glucose, glutamate, and lactate in striatal microdialysate at 2.5-min intervals.