Structure of 2-hydroxy-5-nitrobenzylated carboxypeptidase A

Bovine pancreatic carboxypeptidase A (EC 3.4.12.2) was treated with dimethyl (2-hydroxy-5-nitrobenzyl)sulfonium chloride at pH 7.5, resulting in a preparation which consisted primarily of a monohydroxynitrobenzylated derivative of the enzyme. Samples of the hydroxynitrobenzylated enzyme were subjected to tryptic digestion and to cyanogen bromide cleavage, and resulting peptides were isolated chromatographically. One tryptic hydroxynitrobenzyl-containing peptide was isolated; its amino acid composition was that of the N-terminal tryptic segment of carboxypeptidase Agamma (residues 8--35). Likewise, CNBr cleavage of the hydroxynitrobenzylated enzyme revealed that the hydroxynitrobenzyl group resided in the N-terminal fragment, FN (residues 8--22). Neither of these hydroxynitrobenzylated peptides contains Trp, the amino acid residue which is characteristically the site of hydroxynitrobenzylation in proteins, and each was found to contain approximately one less Asx than the corresponding native peptide. Both dansylation and automated Edman degradation procedures revealed that the N-terminal Asn of carboxypeptidase Agamma had been modified by hydroxynitrobenzylation of the enzyme. Thus the sulfonium salt reacts with carboxypeptidase A in the same manner as that established earlier for 2-hydroxy-5-nitrobenzyl bromide (Radhakrishnan, T.M., Bradshaw, R.A., Deranleau, D.A. and Neurath, H. (1970) FEBS Lett. 7, 72--76). Such reactivity of the alpha-amino group presumably reflects its unique location with respect to Trp residues in the tertiary structure of the enzyme.