The essential tryptophan residues of pig kidney aminoacylase

The tryptophan residues in pig kidney aminoacylase (N-acylamino acid amido hydrolase, EC 3.5.1.14) have been modified by N-bromosuccinimide (NBS) at low pH. The modification of eight tryptophan residues as measured by spectrophotometric and spectrofluorimetric methods leads to complete loss of enzymatic activity. The decreases in absorption at 280 nm and fluorescence emission at 337 nm indicate the modification of tryptophan residues. Both the inactivation and tryptophan residual modification are monophasic, first-order reactions. Quantitative treatment of the data (Tsou, C. L., Sci. Sin., 1962, 11, 1535-1558) shows that among the tryptophan residues modified, two are essential for aminoacylase catalytic activity. Kördel and Schneider (Hoppe-Seyler's Physiol. Chem. 1976, 357, 1109-1115) reported that the modification of tryptophan residues led to inactivation of aminoacylase, and suggested that tryptophan residues are essential for enzymatic activity. We have now shown that eight tryptophan residues can be modified by N-bromosuccinimide and that two of them are essential for the catalytic activity of this enzyme.