Autolysis of bovine enteropeptidase heavy chain: evidence of fragment 118-465 involvement in trypsinogen activation.

Variations in bovine enteropeptidase (EP) activity were shown to result from autolysis caused by the loss of calcium ions; the cleavage sites were determined. The native enzyme preferred its natural substrate, trypsinogen (KM=2.4 microM), to the peptide and fusion protein substrates (KM=200 and 125 microM, respectively). On the other hand, the truncated enzyme composed of the C-terminal fragment 466-800 of EP heavy chain and intact light chain did not distinguish these substrates. The results suggest that the N-terminal fragment 118-465 of the enteropeptidase heavy chain contains a secondary substrate-binding site that interacts directly with trypsinogen.