Kinetic study of hemoglobin Porto Alegre [beta 9(A6)Ser----Cys] disulfide polymer reduction

The cleavage of HbPA disulfide polymer by GSH and its indirect cleavage by yeast glutathione reductase, via reduced glutathione is obtained. Decreasing the initial proportion of GSH in the hemolysate increases the formation of HbPA disulfide polymer. In the experimental conditions used, yeast glutathione reductase is unable to perform the direct cleavage of the mixed disulfide of HbPA and GSH, using it as substrate. The reduction of HbPA polymer to tetramers by DTE is analyzed by a pseudo-first-order kinetic and two rate constants are obtained. That of 265 X 10(-3) min-1 should be concerned with one disulfide of the closed ring and one of the open ring structure of dodecamer, while that of 38 X 10(-3) min-1 is related to disulfide reduction of the octamer. The enthalpy of activation values of 8.0 kcal.mol-1 an 17.4 kcal.mol-1 obtained, from the Arrhenius plot, for the "fast" and "slow" rate disulfide reduction, respectively, are indicative that a strong conformational strain of S--S bonds in the closed ring structure is maintained. The entropy of activation values of 24 e.u. and 52 e.u. are found for the activation of disulfides from dodecamers and octamers, respectively.