MECHANISM OF ACTIVATION OF PAPAIN

The predominant form of inactive papain prepared by the method of Kimmel and Smith (1) is shown to be a mixed disulfide formed between a sulfhydryl group on the enzyme and cysteine. Reduction or other nucleophilic cleavage of this bond frees the active thiol of papain and stoichiometric amounts of free cysteine or cysteine whose sulfur atom is covalently bound to the activating nucleophile. It is shown specifically that K 14CN activation of papain produces 14C labeled 2-iminothiazolidine-4-carboxylic acid (ITC) and a stoichiometric amount of free SH on the protein. Performic acid oxidation of inactive papain yields an amount of cysteic acid approximately equivalent to the quantity of ITC obtained in the cyanide activation experiment. © 1969.