Both samples are subjected to reduction of protein S-nitrosothiols as described above and labeled. By comparing probe signals between samples, S-nitrosated thiol signals that are diminished in the thioredoxin-treated samples can be identified. Although some redox proteomic methodologies make use see more of specific reduction of the cysteine modification of interest, others employ probes that react specifically with a particular modification thereby circumventing

the requirement for a reduction step. These methods and the modifications they are applied to are outlined below and the general approach is described in Figure 3d. In general, this strategy is advantageous because the methods allow for labeling within the system, affording a low chance of redox homeostasis

disruption and artifactual labeling. However, since quantification with respect to the proportion of modified to unmodified cysteine cannot be made, these methods can only determine the presence of a modification. A number of proteomic strategies have been developed for the identification of sulfenic acids using chemoselective probes based on derivatives Obeticholic Acid of 5,5-dimethyl-1,3-cyclohexadione (dimedone). Conjugation of the sulfenic acid-specific dimedone to fluorophores or biotin has allowed for proteomic screens of these conjugates [33•, 52 and 53]. More recently, Leonard et al. developed a membrane Olopatadine permeable propyl azide derivative of dimedone

capable of labeling sulfenic acids in cells while allowing for downstream selective coupling with an alkyne or phosphine biotin tag [ 12••]. This strategy foregoes the requirement for reduction of sulfenic acids and avoids potential disruption of redox homeostasis since tagging can occur within intact cells. An alternative strategy for the identification of glutathionylated proteins is based on metabolic labeling. Fratelli et al. metabolically labeled the glutathione pool of T-cells using [35S]-cysteine under a variety conditions applying exogenous oxidative stress [ 34]. Treatment with [35S]-labeled cysteine in conjunction with the protein synthesis inhibitor cycloheximide allowed for the majority of the labeled cysteines to be incorporated into the glutathione pool. Then [35S]-glutathionylated proteins were separated by two-dimensional electrophoresis and assessed by radiofluorography. Among the limitations of this approach are that proteins glutathionylated before metabolic labeling will not be detected. In addition the sensitivity of the radiofluorography system for detecting subtle changes is less robust when compared to fluorescent or MS probes that enable control and modified samples to be compared more directly.