Absence of evidence for an intermediate in the deacetylation of acetylchymotrypsin

A recent paper [Chibber, B. A. K., Tomich, J. M., Mertz, E. T. & Viswanatha, T. (1977)Proc. Natl. Acad. Sci. USA 74, 510-514] presented evidence that was taken to support the existence of an intermediate in the deacetylation of acetylchymotrypsin. It was observed that deacylation, as measured by following the decrease in [14 C]acetylchymotrypsin (decrease in acid-precipitable radioactivity), occurred at 1/10 the rate of reactivation, as measured by return of activity towardN -acetyl-L-tyrosine ethyl ester. Our experiments have shown that, at pH 6, the deacylation rate constant (measured by the loss of [14 C]acetylchymotrypsin and by the formation of [14 C]acetate) is identical (within experimental error) with the rate constant for reactivation (measured by determining the activity of aliquots of reactivating enzyme againstN -acetyl-L-tryptophan ethyl ester) and withK cat for the turnover ofp -nitrophenyl acetate by α-chymotrypsin. Part of the 10-fold greater reactivation rate observed by Chibberet al. has been shown to be due to the presence of 10% (vol/vol) isopropanol in their reactivation mixture, and it is argued that the balance of the effect is a manifestation of the “indole effect” produced by the simultaneous presence of 10 mMN -acetyl-L-tyrosine ethyl ester throughout the reactivation experiments. The results presented are entirely consistent with the three-step mechanism of catalysis by α-chymotrypsin and negate the existence of the proposed additional acetyl-enzyme intermediate.