On the mechanism of enzyme action. LVIII. Acetyltrypsin, a stable trypsin derivative

• Published in: Archives of biochemistry and biophysics Vol. 52; no. 2; pp. 464 - 477
• Main Authors: Sri Ram, J., Terminiello, L., Bier, M., Nord, F.F.
• Format: Journal Article
• Language: English
• Published: Elsevier Inc 01.10.1954
• ISSN: 0003-9861; 1096-0384
• DOI: 10.1016/0003-9861(54)90146-0

Acetylated trypsin is homogeneous in the ultracentrifuge and presents a slight inhomogeneity in electrophoretic migration. Its isoelectric point is about pH 3.8. Owing to blocking of the ϵ-amino groups of lysine, acetylated trypsin is stable in alkaline media and is not subject to self-digestion. Its stability in acid media is, however, smaller than that of the unmodified enzyme. Its thermal stability is also slightly lower than that of trypsin. Ca ++ ions stabilize both enzymes. The pH optimum of digestion for casein is shifted to more alkaline pH values when the enzyme is acetylated. The temperature optimum is slightly lower. The Michaelis-Menten constant for trypsin and acetyltrypsin with casein is K m = 4.4 × 10 −6 M and K m = 3 × 10 −5 M, respectively. Acetyltrypsin combines also with ovomucoid and is inhibited by it, though to a much lesser degree than the unmodified enzyme. The dissociation constant of the equimolecular acetyltrypsin-ovomucoid is 1.3 × 10 −6 M using hemoglobin as substrate, and 3.5 × 11 −7 M with casein. The inhibition seems therefore to be competitive.