Application of the reaction of dithioesters with epsilon-amino groups in lysine to the chemical modification of proteins

The reaction of lysine with dithioesters was applied to horseradish peroxidase donor: hydrogen-peroxide oxidoreductase, EC 1.11.1.7) using carboxymethyl dithiotridecanoate: three to four lysine residues were modified. The modified enzyme was soluble and active in diethyl ether. Papain (EC 3.4.22.2) was modified with carboxymethyl dithiobenzoate: two lysine residues were modified. The modified enzyme was soluble and active in dimethylsulfoxide. From these results it is concluded that dithioesters are efficient reagents for the modification of peripheral lysine residues of proteins. Aromatic dithioesters, less reactive but more selective, should be recommended for thiol-dependent enzymes such as papain.