Identification of reactive tyrosine residues in cysteine‐reactive dehydrogenases Differences between liver sorbitol, liver alcohol and Drosophila alcohol dehydrogenases

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Abstract
Modification of tyrosine residues with tetranitromethane and reversible sulphite protection of cysteine residues were tested on three dehydrogenases of two families. In liver alcohol dehydrogenase no Tyr residue is appreciably labelled, while in the homologous sorbitol dehydrogenase Tyr‐109 is specifically labelled; the difference corresponds to a segment correlating with subunit interactions and the different quaternary structures of the proteins. In Drosophila alcohol dehydrogenase. Tyr modification is multiple, and the results show the presence of two different states of Cys residues, reactive in the presence and absence of cupric ions, respectively. Super‐activation with cyanide was also noticed after S‐sulphocysteine protection. The results demonstrate the possibility of identification of specific Tyr residues in proteins with reversibly protected Cys residues.