Cyanuration of papain : Activity and fluoroescence of the products

1. 1. A simple equation is derived which describes the degree of modification as a function of amount of (quickly hydrolyzed) reagent. The cyanuration of tyrosine with cyanuric fluoride obeys this equation. 2. 2. Of the nineteen tyrosines of succinyl papain ten are cyanurated at pH 9.5, twelve at pH 10.5. By application of the theoretical equation the reacting tyrosines can be subdivided into two sets: those reacting at an equal or even higher rate than free tyrosine, and those reacting at a lower rate. 3. 3. Ultraviolet titration of the tyrosines shows that the more easily ionizing phenolic groups are preferentially cyanurated. 4. 4. Cyanuration of tyrosines shifts the basic branch of the curve of fluorescence of papain versus pH towards higher pH. The fluorescence of papain is explained in terms of Trp 26 and the ionization of tyrosines and of Asp 57. 5. 5.Cyanuration gave no essential loss of activity towards benzoyl- -arginine ethyl ester and only a small loss of activity towards casein.