Affinity of Chicken Cystatin for S- (Carbamoylmethyl)-Papain, Measured by Fluorescence at Acidic ph

We have characterized the interaction between S-(carbamoylmethyl)-papain and chicken cystatin by means of fluorescence titration at acidic pH. The calculated binding constant was 32 x 10 6, which is less than the tight affinity at pH 7.4 (∼7 x 10 10). Results have been discussed in terms of the burial of Trp-104 of cystatin and the possible role of the charge of the carbamoylmethyl moiety.