ACTIVITIES OF DIRECTLY AND INDIRECTLY ACETYLATED CHYMOTRYPSINS

Abstract

Introduction of a number of acetyl groups into [alpha]-chymotrypsin by treatment with acetic anhydride reduced both proteolytic and esterolytic activities. The latter activity, but not the former, was restored, usually to about the level found for the unacetylated enzyme, by standing in neutral or slightly alkaline solution. The protein lost one active acetyl group during this treatment, thus acting like monoacetyl chymotrypsin prepared with nitrophenylacetate. Acetylated chymotrypsinogen could be activated by trypsin, but only under special conditions that are not necessary for the naturally occurring protein. The presence of an electrolyte or else the addition of some natural (i.e., unacetylated) chymotrypsin was required. When either of these conditions was fulfilled, the end product of activation showed about the same proteolytic acitivity as directly acetylated chymotrypsin, but the esterolytic acitivty was nearly double that found for either the directly acetylated or the natural enzyme. The effect of natural chymotrypsin in this activation was at least in part on the acetylated chymotrypsinogen itself before the attack thereon by trypsin. The failure of hydroxylamine to change the potential activity of the zymogen indicated that the markedly increased esterase activity of the activation product is owing to the presence of an N-acetyl derivative. A simple viewpoint that fits the presently known facts is that such an N-acetyl group is so siturated as to offer "steric hindrance" to the rearrangement or further degradation of the enzyme to a less active form.