Suicide inactivation of chymotrypsin by benzoxazinones

Abstract

The benzoxazines 2-ethoxy-4H-3,1-benzoxazine-4-one (1a) and 2-(trifluoromethyl)-4H-3,1-benzoxazin-4-one (1d) inactivate chymotrypsin. The inactivation is stoichiometric and proceeds with rate constants of 7 .times. 105 and > 4 .times. 106 M-1 min-1, respectively. The inactivated enzyme recovers catalytic activity slowly, k = 2.3 .times. 10-3 min-1 and 3.7 .times. 10-2 min-1 (pH 7.1). When the enzyme regains catalytic activity, 2-[N-(ethoxycarbonyl)amino]benzoic acid is released from enzyme inactivated with 1a and N-(trifluoroacetyl)anthranilic acid from enzyme inactivated with 1d. The mechanism of inactivation involves attack of the active site serine on the C-4 carbonyl of the inactivator which leads to ring opening and formation of an o-substituted benzoylchymotrypsin, which hydrolyzes slowly due to electron releasing ability of the substituents. The rate of hydrolysis of the benzoylchymotrypsin from 1a or 1d is in close agreement with those predicted from the Hammett parameters (.sigma.,.rho.) for hydrolysis of their p-substituted analog. The inactivation of chymotrypsin by 2-methyl-4H-3,1-benzoxazin-4-one (1b) is an equilibrium process (kinact = 1 .times. 104 M-1 min-1 and Keq = 2 .times. 106 M-1). Formation of a benzoylchymotrypsin is demonstrated by spectral changes and methanol trapping. The benzoylchymotrypsin can also decay by direct hydrolysis to N-acetylanthranilic acid. Chymotrypsin is also inactivated by 2-amino-4H-3,1-benzoxazin-4-one (1c). The inactivation is rapid (k = 8.7 .times. 104 M-1 min-1), but reactiativation is also relatively fast (k = 0.14 min-1). Upon reactivation benzoyleneurea is released. Reaction of 1c with chymotrypsin leads to the formation of (o-ureidobenzoyl)-chymotrypsin, which does not undergo hydrolysis but decomposes through intramolecular attack of the ureido NH2 upon the carbonyl group of the benzoyl ester. Compound 1a also inactivates trypsin and elastase. Compound 1b was a poor inactivator of trypsin and does not inhibit elastase. Both trypsin and elastase catalyze the conversion of 1c to benzoyleneurea.