Background Disulfide bond formation is a key posttranslational modification with implications for structure function and stability of numerous proteins. aspects of current methodology together with identifying potential troubles inherent in their experimental implementation. Major conclusions While many reagents have been described for the measurement and manipulation of the redox status of thiols and disulfides a number of these methods remain underutilized. The ability to effectively quantify changes in redox conditions in living cells presents a continuing challenge. General Significance Many unresolved questions in the metabolic interconversion of thiols and disulfides remain. For example while pool sizes of redox pairs and their intracellular distribution are being uncovered very little is known about the flux in thiol-disulfide exchange pathways. New tools are needed to address this important aspect of cellular metabolism. [20 21 Fig. 2 Addition of thiols to maleimides together with selected exchange and ring opening reactions. The reaction of thiol (R1-SH) with maleimides (in this case NEM) is usually reversible albeit shifted strongly toward the adduct formation (Reaction A). In the presence … Mammalian cultured cells AS-604850 are permeable to NEM and this has motivated its use for quenching thiols in intact cells. However the inclusion of a denaturant such as SDS may be necessary to make sure rapid labeling of all free cysteine residues because about 20% of AS-604850 total cellular protein thiols are not susceptible to modification by NEM under native conditions . Vinyl pyridine like NEM reacts with thiols at the double bond and was previously widely used. Since vinyl pyridine reacts more than 500-fold slower than NEM both high concentration and long reaction times are required for complete reaction [25 26 Cyanylation using 1-cyano-4-dimethylamino-pyridinium salts (CDAP) represents an efficient means of thiol blocking [27-30] (Fig. 3A). The reaction is rapid at pH 4-5 leading to quantitative derivatization of thiols using low mM concentrations of CDAP [30 31 These properties are useful because they allow efficient alkylation at low pH where thiol exchange is usually minimal. An additional feature of CDAP is usually that cyanylated peptide-thiol AS-604850 adducts are susceptible to specific cleavage in the presence of ammonia (Fig. 3B). Here a cyclization involving the cyanylated side-chain results in cleavage of the peptide chain N-terminal to the target cysteine residue. In combination with mass spectroscopy this procedure allows for mapping of disulfide bond patterns in proteins . On the other hand the cyanylated proteins are intrinsically unstable above pH 7. CDAP itself is usually stable in polar aprotic Jun solvents such as acetonitrile but is usually prone to hydrolysis in aqueous solutions above pH 5 . Fig. 3 Cyanylation using CDAP. Reaction (A) of CDAP with cysteinyl peptide results in formation of a cyanylated species. This species can react further in 1.5 M NH4OH to cleave the adjacent N-proximal peptide bond (B). Although rapid and indiscriminate alkylation of thiols is usually often the desired outcome of labeling protocols less reactive reagents have been recently used very effectively in proteomic approaches for the identification of proteins made up of hyper-reactive cysteine residues [33 34 3 AS-604850 Reduction of disulfide bonds In the reduction of thiols for further analysis there are three major concerns: a) that this disulfide reduction is usually quantitative and rapid b) that this reducing agent is usually specific and does not show significant side reactions and c) that this reductant selected does not complicate down-stream reactions and processes. Disulfide reduction is usually accomplished primarily by thiol exchange type reagents (like dithiothreitol DTT or 2-mercaptoethanol ME) or by various substituted phosphines such as tris(2-carboxyethyl)phosphine TCEP . 3.1 Phosphine- and Thiol-based reductants Unlike thiol reagents for all those practical purposes phosphines are irreversible reductants of disulfide bonds in aqueous solutions (Fig. 4). Here the phosphine performs a nucleophilic attack on one of the two sulfur atoms forming a phosphonium ion sulfur adduct which is usually subsequently hydrolyzed.