Reaction of arginine residues in basic pancreatic trypsin inhibitor with phenylglyoxal

Abstract

Many topographical studies have been oriented towards explaining the mechanism of complex formation between basic pancreatic trypsin inhibitor (BPTI)** and trypsin. With respect to the specificity of trypsin, the arginine and Iysine residues of BPTI can play an essential role in this interaction. There are six arginine (one of them N-terminal) and four lysine residues in the molecule. The N-terminal arginine residue is not important for activity, because after its degradative elimination or substitution the remaining molecule retains inhibitory activity [ 1, 21. The four lysine residues can be guanidylated or amidinated without loss of inhibitory activity [ 1,2] . In contrast to this kind of substitution, which does not alter the charge of the residues, acetylation or succinylation of lysine residues destroys activity [ l-31 . Since trypsin does not hydrolyse peptide bonds involving homoarginine or e-acetamidyllysine [4-61, it appears that the free e-amino groups of lysine are not necessary for the activity of the inhibitor. On the other hand, reactions of the trypsin-inhibitor complex and of the free inhibitor with N-carboxy-DL-alanine anhydride have shown that lysine residue no. 15 is buried in the complex (fig. 1) [7] . Consequently, this residue has been regarded as the specific site of interaction with trypsin. The role of arginine residues nos. 17,20,39,42 and 53 is not known. In the alkaline pH range, the inhibitor-trypsin complex is essentially undissociated at pH 10.5 and only partially dissociated at pH 11.8 [8] , which seems to indicate that the critical interaction involves a guanidinium rather than an e-amino group. The three-dimensional model resulting from