Disulfide alkali treatment, -elimination

Figure 16. / -Elimination scheme for disulfides on alkali treatment (13)...

In previous papers, we have (a) reviewed elimination reactions of disulfide bonds in amino acids, peptides, and proteins under the influence of alkali (5) (b) analyzed factors that may operate during alkali-induced amino acid crosslinking and its prevention (6) (c) demonstrated inhibitory effects of certain amino acids and inorganic anions on lysinoalanine formation during alkali treatment of casein, soy protein, wheat gluten, and wool and on lanthionine formation in wool ( 7, 9) (d) demonstrated that... 

Although the results are consistent with a B-elimination reaction leading to formation of dehydroalanine, the conclusions are based on the assumption that the absorbance at 241 nm is associated with dehydroalanine side chains derived from cystine residues. This assumption may not always be justified for the following reasons. First, alkali treatment of casein which has very few or no disulfide bonds also yields significant amounts of dehydroalanine residues (52). These presumably arise from serine side chains. Second, Nashef et al. (41) cite evidence that other functionalities may contribute to the 241 nm absorption. These considerations suggest that there is a need to directly measure dehydroalanine in proteins. This is now possible with our method (52), whereby the dehydroalanine residues are first transformed to S-pyridylethyl side chains by reaction with 2-mercaptopyridine (Figure 12). Amino acid analysis of the acid-hydrolyzed protein permits estimation of the dehydroalanine content as S-B-(2-pyridylethyl)-L-cysteine along with the other amino acids. 

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