Formation of persulfide groups in alkaline treated insulin

Alkaline treatment of disulfidecontaining proteins is known to produce a number of unusual compounds related with the degradation of the cystinyl residues. Lanthionine [l] and lysinoalanine [2] are among the products identified with certainly, while dehydroala- nine [2] , thiocysteine [3,4] , cysteine sulfenic acid [S] and other products are supposed, by indirect evidence, to rise in the meantime. Although for experimental purposes the effect of alkali on proteins is usually studied under heavy alkaline conditions, sensitivity of proteins to alkali is sufficiently high to represent a possible nuisance to proteins under the mild alkaline conditions employed in many biochemical studies [6] . The mechanism of degradation of the cystinyl sulfur by alkali is still controversial and it is not yet clear whether the main reaction is hydrolysis of disulfide bond or Pelimination of the cystinyl sulfur with the formation of persulfide groups (R-SSH). We have re- cently detected persulfide groups by spectral analysis of proteins in the presence of sodium sulfide [7]. The results of that study encouraged us to extend the in- vestigation to the formation of persulfide groups dur- ing the alkaline cleavage of proteins. In the present note we report the results obtained by using insulin as a representative disulfidecontaining protein. 2.