Kinetics and Equilibria of the Formation and Reduction of the Disulfide Bonds in Arginine-Vasopressin and Oxytocin by Thiol/Disulfide Interchange with Glutathione and Cysteine

Article abstract of DOI:10.1021/jo00094a039

Rate and equilibrium constants are reported for reduction of the disulfide bonds in the neurohypophyseal peptide hormones oxytocin (OT) and arginine-vasopressin (AVP) by thiol/disulfide interchange with glutathione (GSH) and cysteine (CySH) and for formation of the disulfide bonds by thiol/disulfide interchange with oxidized glutathione (GSSG) and cystine (CySSCy).The reactions take place in two steps.In the first step of the reduction reactions, AVP and OT react with GSH and CySH to form peptide-GSH and peptide-CySH mixed disulfides, which in turn react with another molecule of GSH or CySH to give the reduced dithiol form of the peptide and GSSG or CySSCy.Analysis of the forward and reverse rate constants indicates that which step is rate determining depends on the concentration of GSH or CySH.At physiological concentrations of GSH and CySH, intramolecular thiol/disulfide interchange in the mixed disulfides to reform the native disulfide bonds is faster than reaction with another molecule of GSH or CySH, even though intramolecular thiol/disulfide interchange involves closure of 20-membered rings.Rate constants for reaction of GSH and CySH with the disulfide bonds of AVP and OT are 1-2 orders of magnitude larger than for reaction with disulfide bonds formed by two cysteine-containing peptides, which suggests that the disulfide bonds in the neurohypophyseal peptide hormones are strained.Equilibrium constants are also reported for reaction of GSH with the hexapeptide analogs of AVP and OT, pressinoic acid (PA), and tocinoic acid (TA).A reduction potential of -0.216 V was calculated for the disulfide bonds of OT and TA from the thiol/disulfide interchange equilibrium constants.Reduction potentials of -0.229 V and -0.227 V were calculated for the disulfide bonds in AVP and PA, respectively.The similarity of the reduction potentials for OT and TA and for AVP and PA indicates that the acyclic tripeptide tails of OT and AVP have little affect on the redox properties of their disulfide bonds.