The effect of urea and guanidine hydrochloride on activity and optical rotation of crystalline papain.

The catalytic activity of several proteolytic enzymes is altered by high concentrations of urea and guanidine salts (2-4). These studies, in conjunction with certain physical data, indicate that hydrogen bonding in the secondary structure of these enzymes is essential for maintenance of their catalytic function. Because of the increasing amount of information that is available concerning the structure of papain (5, 6), and its mechanism of action (7-lo), it was of interest to determine whether this enzyme is affected by concentrations of urea or guanidine hydrochloride which alter secondary, structural hydrogen bonding. This paper presents the results of such studies. In contrast to the work of Lineweaver and Schwimmer (11) who reported no inactivation of papain by 9 M urea, our results demonstrate that even lower levels of urea cause considerable inactivation. Although urea reversibly inactivates papain, high levels of guanidine hydrochloride were found to cause an irreversible inactivation, accompanied by significant unfolding of the enzyme.