Mutant rat trypsin Asp(189)Ser was prepared and complexed with highly
purified human alpha(1)-proteinase inhibitor. The complex formed was p
urified to homogeneity and studied by N-terminal amino acid sequence a
nalysis and limited proteolysis with bovine trypsin. As compared to un
complexed mutant trypsin, the mutant enzyme complexed with alpha(1)-pr
oteinase inhibitor showed a highly increased susceptibility to enzymat
ic digestion. The peptide bond selectively attacked by bovine trypsin
was identified as the Arg(117)-Val(118) one of trypsin. The structural
and mechanistic relevance of this observation to serine proteinase-su
bstrate and serine proteinase-serpin reactions are discussed.