4810
Yol. ITS
COMMUNICATIONS T O T H E EDITOR THE REACTION OF p-NITROPHENYL WITH CHYMOTRYPSIN' Sir :
ACETATE
T h e reaction of NPA with chymotrypsin a t pH 5.0 results in a stable monoacetyl derivative which is an intermediate in the catalytic hydrolysis of the ester a t higher pH.? Phosphorylating agents (e.g., DFP) have a similar effect and give rise to stable monophosphoryl enzyme derivatives, the hydroxyl group of a serine residue being the final point of attachment of the phosphoryl residue. 3 , 4 , 5 The reaction of NP-4 with chymotrypsin is a two-phase process,6 a rapid initial "burst" of p nitrophenol being followed by a slow liberation until NPA is exhausted. At low temperature and pH, using rapid mixing techniques, the initial acetylation reaction in isolation has been followed in the Cary recording spectrophotometer and its kinetics have been found to' correspond to those of a bimolecular reaction. At higher pH and temperature, the slow zero order reaction, which represents the turnover of acetylchymotrypsin (deacetylation being rate-limiting), has been followed. The acetylation reaction, with a maximum a t pH S-9, occurs with a velocity approximately one hundred fold t h a t of de-acetylation (maximal a t PH 8.5-10). T h e energy of activation for the acetylation reaction a t pH 6.0 was found to be 13,700 cal. per mole and for the de-acetylation a t p H 7.5, 15,700 cal. per mole (as compared to 18,400 cal. per mole for the base catalyzed hydrolysis of NPAGj. The following series of experiments were undertaken to study the attachment of the acetyl group to the enzyme. -1difference spectrum (600-230 rnp) between two portions of the same acetylchymotrypsin solution, one of which had been allowed to de-acetylate a t FH S.0, 25", showed t h a t the two proteins were spectrally identical. Since a histidine side chain has been implicated in the action of proteolytic and since the properties of acetyl-imidazole are known to include a characteristic ultraviolet absorption with a peak a t 245 mp,7 the changes a t 243 mp were carefully followed during the acetylation reaction. The quantity of enzyme was chosen t o provide an easily ( 1 J p - S i t r o p h e n y l acetate will he ahbret-iated t o NP.4, a n d diiS(Jpropyl phosphofluoridate t o DFP This work waa performed under C o n t r a c t i-io. N~lnr-477-04between t h e University of Washington a n d t h e Office of h'aval Kesearch, Department of t h e h'avy, a n d was supported also b y f u n d s made available b y t h e people of t h e S t a t e of Washington, Initiative 171. Our t h a n k s are d u e t o Miss Dorothy K a u f f m a n f o r technical assistancc. Details of this work will be given in a paper nuw in preparatirm (2) A . K. Balls a n d F. I,. nldrich, P v x . S a / . Acnil. SrI.. 41, 190 4 K . Balls and H. h-. Wood, J . B i d . Chent., 219, 245 (19581. (1955); . ( 3 ) N K , Schaffer, S C . M a y a n d W. H. Summerson, ibid., '206, 201 (19AlJ. (-1) I
