Preparation and properties of guanidinated mercuripapain.

0-methylisourea has recently been shown to be a useful reagent for the modification of alkali-stable enzymes primarily by conversion of e-amino groups of lysine to guanidinium groups. Chymotrypsinogen (2), ribonuclease (3), and lysozyme (4) have been treated with this reagent. Examination of the properties of these modified proteins has provided information relating structure to enzymic activity. In this paper, experiments are reported which describe the reaction of 0-methylisourea with mercuripapain and the effects of guanidination on several properties of the enzyme. The results obtained show that although seven of the eight lysine residues in papain can be converted to homoarginine residues, there is no alteration of the enzymic properties. In addition, although mercuripapain is rapidly and completely inactivated by nitrous acid, the guanidinated derivative is only partially and slowly inactivated by this treatment.