A method for studying export of glutathione S-conjugates from cells is proposed based on the use of the spin label tempo-maleimide, This compound is conjugated intracellularly with glutathione and the concentration of the exported conjugate is measured in cell supernatants in an electron spin resonance spectrometer after reoxidation with ferricyanide. This method allows for measurements of micromolar concentrations of the conjugate and requires low amounts of cells.
Pułaski, Ł., & Bartosz, G. (1996). An electron spin resonance assay of glutathione S-conjugate transport. Journal of Biochemical and Biophysical Methods, 33(2), 65–71. https://doi.org/10.1016/0165-022X(95)00036-Q