Total cysteine and glutathione determination in hemolymph of individual adult D. melanogaster

Determination of thiols, glutathione (GSH) and cysteine (Cys) are important due to their roles in oxidative stress and aging. Oxidants such as soluble O2 and H2O2 promote oxidation of thiols to disulfide (—S—S—) bonded dimers affecting quantitation accuracy. The method presented here reduces disulfide-bonded species followed by fluorescence labelling of the 29.5 (18.2) nL hemolymph volumes of individual adult Drosophila Melanogaster. The availability of only tens of nanoliter (nL) samples that are also highly volume variant requires efficient sample handling to improve thiol measurements while minimizing sample dilution. The optimized method presented here utilizes defined lengths of capillaries to meter tris (2-carboxyethyl)phosphine reducing reagent and monobromobimane derivatizing reagent volumes enabling Cys and GSH quantitation with only 20-fold dilution. The nL assay developed here was optimized with respect to reagent concentrations, sample dilution, reaction times and temperatures. Separation and identification of the nL thiol mixtures were obtained with capillary electrophoresis-laser induced fluorescence. To demonstrate the capability of this method total Cys and total GSH were measured in the hemolymph collected from individual adult D. Melanogaster. The thiol measurements were used to compare a mutant fly strain with a non-functional cystine–glutamate transporter (xCT) to its background control. The mutant fly, genderblind (gb), carries a non-functional gene for a protein similar to mammalian xCT whose function is not fully understood. Average concentrations obtained for mutant and control flies are 2.19 (0.22) and 1.94 (0.34) mM Cys and 2.14 (0.60) and 2.08 (0.71) mM GSH, respectively, and are not significantly different (p > 0.05). Statistical analysis showed significant differences in total GSH of males and females independent of the xCT mutation. Overall, the method demonstrates an approach for effective chemical characterization of thiols in nL sample volumes.