| Summary: | A new method is developed to determine the concentrations of reduced (GSH) and oxidized glutathione (GSSG), and hence the GSH:GSSG ratios in human plasma and saliva samples. The assay is based on the masking of GSH in a GSH and GSSG mixture via a 1,4-addition reaction with p-benzoquinone (BQ), followed by enzymatic kinetic measurement. The enzyme, glutathione reductase, is highly specific to glutathione. Excess BQ can thus be easily removed by the addition of non-GSH thiols. The assay takes less than 2 min, is suitable for short timescale study and minimizes the in-vitro underestimation of the GSH:GSSG ratio arising from the degradation of GSH and formation of GSSG. We further show in this paper that the stability of the total glutathione content (GSH + GSSG) and GSH in saliva is significantly greater than in plasma, encouraging the development of non-invasive saliva sensing.
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