results obtained in this work for cysteine-BSSB differ by a factor of about two from the values expected on the basis of probability and are in agreement with this conclusion.Despite the fact that the constants are by no means unusual, the behavior of cysteine-BSSB is different from that of systems previously studied, and this serves to point out the important role which may be played by the buffer medium. At pH 7.0, for instance, BSH is almost completely ionized, while RSH is not. As a result, BSSB is almost completely reduced by an equivalent amount of RSH. Qualitative observations of similar import had been made by Eldjarn and Pihl, who reported that cystine was iiot reduced at pH 7.4 by aromatic mercaptans. It is iinportaritJ to realize this is not due to significant differences in the stabilities of the disulfide bonds involved, at least in the case of BSSB and RSSR, but to the fact that the acid-base reactions taking place with the buffer components provide the driving force for the reaction producing BS-.The case of BSH is of course an extreme one. With aliphatic mercaptans, little or no ionization will take place below pH 7, and hence there will be no effect on the interchange reaction. Above p H 7, e.g., in the work of Eldjarn and Pihl that was conducted at pH 7.4, ionizations would certainly be appreciable. Their effect 011 the point of equilibrium is difficult to evaluate, however, since this depends on the difference between the ionization characteristics of the reactants and products, which are iiot known for the most part.The reaction of 3,5-diimino-1,2,4-dithiazoline ( DSZ) with cysteine, glutathione, and 2-aminoethanethiol is first order in DSz and second order in mercaptan; the specific rate constants are 9.4, 6.9, and 17.0 X lOJ moles2 1.-2 min.-', respectively, at pH 4.60 and 30". At pH 4.6-3.5, the rate is approximately proportional to the reciprocal of hydrogen ion activity. A mechanism is proposed.
