362 GENERAL DISCUSSION 



PASSYNSKY : In connection with the formulation of the problem under 

 discussion, I would like to point out the comparatively simple depen- 

 dence which is characteristic of the action of thiol compounds. 



When thiols are oxidized by ferricyanide, cysteine-oxidase, or peni- 

 cillin (which, as we have shown, is also an oxidizing agent Avith e'o = 

 + 0-3 V), one can establish the following sequence of thiol compounds 

 in terms of the ease of oxidation. 



I > II > III 



Cysteine Glutathione Methionine 



Cysteine methyl ester Thioglycolic acid S-Methylcysteine 



Thiolactic acid Cystine 



Cysteine ethyl ester N-Acetylcysteuie Serine 



The protective properties of these thiols are distributed in the same 

 way. The redox potentials of the compounds of groups I and II are 

 close (e'o— 0-06 to 0-08 V) so that the similarity of oxidation of the 

 SH-grou])s in these compounds depends upon the degree of polarity 

 of the SH -group, or the proportion of the ion composition in this link — 

 S~ . . . H+. The higher the polarization, the stronger is the form — S~, 

 presented, the higher the reaction capacity of the thiols. 



In the compounds of group I, the possil)ility exists of transferring a 

 proton from — SH to the — NH2 group with the formation of the dipo- 

 lar form — NH3+ and — S~ similar to the zwitterion form +NH3 — R — 

 COO^ of the ammo acids. 



Group II contains compounds in which this proton transition is, 

 for various reasons, complicated: in glutathione by the distance of the 

 ~NH2 from the ~SH, in acetylcysteine (the acetyl substituent in 

 the — NHo group, in the thiols and thiolactic acids) by the absence of 

 the —NH2 group. 



Finally, in Group III compounds proton transference is in general 



absent in view of the replacement of the hydrogen of the — SH groups 



in methionine, S-methylcysteine and cystine and in serine — as a 



result of the fact that the — OH link has energy of 110 kcal, against 82 



kcal in the — SH group and the expulsion of the hydrogen atom is 



much more difficult. In the organism, however, the methionine and 



the cystine may produce some action due to the partial metabolic 



transition to cysteine. 



NH9 

 If we compare cysteine HS — CH2 — CH < ^(/-j/^Vt with the best defence 



substance, cysteamine HS — CHo — CH2 — NH2, then we may see that 

 in the latter the transition of the proton with the formation of a di- 



