Flow injection methods for determination of L-glutamate using glutamate decarboxylase and glutamate dehydrogenase reactors with spectrophotometric detection

Abstract

The use of glutamate decarboxylase and glutamate dehydrogenase reactors in flow injection (FI) systems for the determination of glutamate in food is described. In a gas diffusion FI system, glutamate was decarboxylated by glutamate decarboxylase immobilized on a carrier. The produced carbon dioxide was measured spectrophotometrically after gas diffusion using an acid–base indicator as acceptor. The linear range of calibration was from 2 to 20 mmol l^–1 with r= 0.9978 and the RSD was 2.8%(n= 5) for 10 mmol l^–1L-glutamate. Using a glutamate dehydrogenase reactor the NADH formed by the enzymic reaction was detected at 340 nm. For the normal FI approach the calibration graph was linear from 0.05–0.06 mmol l^–1 with r= 0.9991 and the RSD was 2.4%(n= 5) for 0.3 mmol l^–1L-glutamate. For the stop flow approach with a significant reduction in NAD⁺ consumption the linear range was from 0.2 to 1.0 mmol l^–1L-glutamate with r= 0.9994. The RSD was 1.9%(n= 5) for 0.6 mmol l^–1L-glutamate. A sample frequency of 30 h^–1 using these three approaches was reached. Results from food samples using these three approaches were in good agreement with each other. Recoveries were 94.6–101.2%, 96.7–99.3% and 95.9–98.2% for L-glutamate decarboxylase, L-glutamate dehydrogenase with the normal FI approach and L-glutamate dehydrogenase with the stop flow approach, respectively. The assay results of one sample compared well with those given by the manufacturer.