Sulfhydryl groups activity

Disulfides. As shown in Figure 4, the and h-chains of insulin are connected by two disulfide bridges and there is an intrachain cycHc disulfide link on the -chain (see Insulin and other antidiabetic drugs). Vasopressin [9034-50-8] and oxytocin [50-56-6] also contain disulfide links (48). Oxidation of thiols to disulfides and reduction of the latter back to thiols are quite common and important in biological systems, eg, cysteine to cystine or reduced Hpoic acid to oxidized Hpoic acid. Many enzymes depend on free SH groups for activation—deactivation reactions. The oxidation—reduction of glutathione (Glu-Cys-Gly) depends on the sulfhydryl group from cysteine. 

Several mucolytics reduce the viscosity of mucus by cleaving the disulfide bonds that maintain the gel stmcture. AJ-Acet l-L-cysteine [616-91 -1] (19), introduced in 1963, and mesna [19677-45-5] (20), developed in Europe in the early 1970s (20,21), are effective compounds in this class. Whereas most mucolytics must be adrninistered by aerosol, carbocysteine [638-23-6] (21), which contains a derivatized sulfhydryl group, has shown activity by the oral route (22,23). However, carbocysteine does not reduce mucus viscosity, as does acetylcysteine, but appears to have a direct action on mucus glycoprotein production (24). 

A thiol, usually under basic catalysis, can undergo Michael addition to an activated double bond, resulting in protection of the sulfhydryl group as a substituted 5-ethyl derivative. 

Displacement of the sulfhydryl group in primary thiols, like L cysteine and 2-diethylaminoethanethiol, requires elemental fluorine, the most active oxidant Elemental sulfur is the major by-product in those reactions [7] (equation 2)... 

Taken together, these results indicate that similar to other proton-translocating membrane proteins, both types of Na /H exchangers contain critical sulfhydryl groups that are involved in the transport mechanism. These sulfhydryl groups do not appear to be present at the external transport site but may be involved in switching from an inactive to an activated state. 

Transfer constants for mercaptans with several monomers are given in Table XV. Results for the two methods described above are in satisfactory agreement. The rate of reaction with mercaptan relative to the rate of monomer addition (i.e., the transfer constant) varies considerably for different chain radicals (see Table XIV). Temperature coefBcients of the transfer constants for mercaptans are very small, which fact indicates that the activation energy for removal of a hydrogen atom from the sulfhydryl group of a mercaptan is nearly equal to that for monomer addition. 

The major limitation of nitrate therapy is the development of tolerance with continuous use. The loss of anti-anginal effects may occur within the first 24 hours of continuous nitrate therapy. While the cause of tolerance is unclear, several mechanisms have been proposed. These include depletion of the sulfhydryl groups necessary for the conversion of nitrates to nitric oxide, activation of neurohormonal systems, increased intravascular volume, and generation of free radicals that degrade nitric oxide. The most effective method to avoid tolerance and maintain the anti-anginal efficacy of nitrates is to allow a daily nitrate-free interval of at least 8 to 12 hours. Nitrates do not provide protection from ischemia during the nitrate-free period. Therefore, the nitrate-free... 

Hepatic 0.0015 0.005 (decreased glycogen, RNA, sulfhydryl groups, alterations in activities of oxidizing enzymes) ... 

One of the most convenient ways of generating sulfhydryl groups is by reduction of indigenous disulfides. Many proteins contain cystine disulfides that are not critical to structure or activity. 

Williams and Ibrahim, 1981). Other nucleophiles also are reactive. Sulfhydryl groups may attack the active species and form thioester linkages, although these are not as stable as the bond formed with an amine. 

Although the primary utility of active halogen compounds is to modify sulfhydryl groups in proteins or other molecules, the reaction is not totally specific. Iodoacetyl (and bromoacetyl) derivatives can react with a number of functional groups within proteins the sulfhydryl group of cysteine, both imidazolyl side chain nitrogens of histidine, the thioether of methionine, and... 

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