A microbioassay of leucine is achieved by the use of Leuconostoc mesenteroides and a lactate sensor based on an oxygen electrode, which is coated with immobilized lactate oxidase. The response time of the lactate sensor observed by an injection method was about 1 min. Linear relationship was obtained between the current decrease and lactate concentration in the range from. When the microbioassay media containing leucine were incubated for 6 h with L. mesenteroides and introduced into the lactate sensor system, a relationship between the current decrease and the leucine concentration was linear over the range of The current decrease was reproducible within 1% when the media containing of leucine was used. The electrochemical system was applied to the analysis of leucine in human sera. The results obtained by the present method were in good agreement with those obtained by an amino acid autoanalyzer (6 experiments, leucine concentrations in human sera; correlation coefficient; 0,98).
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