Dithiothreitol and amino acids interfere with the fluorometric determination of glutathione with orthophthaldehyde.

The reaction of orthophthaldehyde with glutathione at pH 8.0 is commonly employed as a specific fluorometric assay for glutathione. However, this method has been reported to overestimate the glutathione concentration when compared with either enzymatic or HPLC-based methods. We have found that amino acids and other thiols interfere in the reaction of glutathione with orthophthaldehyde, causing erroneous estimates of glutathione concentration. The extent of this interference is described.