alpha-Chymotrypsin superactivity in aqueous solutions of cationic surfactants

alpha-Chymotrypsin (alpha-CT) activity was tested in aqueous media with the following cetyltrialkylammonium bromide surfactants in the series methyl, ethyl, propyl and butyl, different in the head group size, and for the sake of comparison also with the anionic sodium n-dodecyl sulfate and the zwitt erionic myristyldimethylammonium propanesulfonate. N-glutaryl-L-phenylalani ne p-nitroanilide hydrolysis rate was monitored at surfactant concentration above the critical micellar one. Only some cationic surfactants gave super activity and the head group size had a major weight. The highest superactiv ity was measured in the presence of cetyltributylammonium bromide. The effe ct of both nature and concentration of three different buffers was also inv estigated. There is a dependence of enzyme superactivity on buffer type. Mi chaelis-Menten kinetics were found. The binding constants of substrate with micellar aggregates were determined in the used buffers and the effective improvement of reaction rate (at the same free substrate concentration in t he medium) was calculated. k(cat) significantly increased while K-m was lit tle changed after correction to free substrate concentration. The ratio of k(cat) to K-m was between 12 and 35 times higher than in pure buffer, depen ding on buffer and surfactant concentrations. The increase of alpha-CT acti vity (30%) was less important in the presence of 1 x 10(-2) M tetrabutylamm onium bromide, a very hydrophobic salt, unable to micellise. Fluorescence s pectra showed differences of enzyme conformation in the presence of various surfactants. (C) 1999 Published by Elsevier Science B.V. All rights reserv ed.