Accurate measurement of glutathione for disease diagnosis and drug metabolite screening

A method of assessing toxicological properties in a drug candidate by performing a laboratory assay, said assay comprising the steps of:

• a) adding to a sample collection container all three of 1) isotopically enriched glutathione (GSH), 2) isotopically enriched glutathione disulfide (GSSG) and 3) a measured amount of a thiol-group blocking agent, followed by
• b) adding to said sample collection container of step a) an aliquot of a drug to be assessed for toxins, which drug forms a conjugate, said conjugate's significantly reducing but not stopping the oxidation or transformation of said GSH in said conjugate;
• c) assaying said aliquot for the ratio of GSH and GSSG originally present in said aliquot at the time of collection and correcting the assayed ratio for oxidation or transformation of said GSH by direct IDMS (D-IDMS) accounting for species' transformations and conjugates and direct speciated isotope dilution mass spectrometry by calculating transitions based on the behavior of said isotopically enriched GSH and said isotopically enriched GSSG;
• and
• d) calculating conjugated GSH (CGSH) for constituents of said aliquot, using the formula
• CGSH=ΣiCGSH1+CGSH2+ . . . CGSHi
• by adding an enzyme to the sample collection container containing said aliquot of step b) and producing one or more data of chemically reactive metabolites of CGSH concentrations which are calculated by a computer and output to a user according to the above calculations carried out on measurements performed by the mass spectrometer, wherein resulting mass spectrometry data distinguish by cluster location benign forms of conjugates versus toxic forms.