[Hydrolysis by enteropeptidase of nonspecific (model) peptide sequences and possible physiological role of this phenomenon].

Enteropeptidase (enterokinase) (EC 3.4.21.9), a highly specific processing protease, initiating a cascade of reactions activating the digestion enzymes. Catalyzing trypsinogen activation enteropeptidase exhibits unique properties for high efficiency hydrolysis of the polypeptide chain after lysine-15 residue in the -DDDDK15- sequence. In 1998 we found an unusual calcium-dependent autolysis of the enteropeptidase heavy chain leading to the drastic loss of its activity towards trypsinogen: after lysine-360 (-NNYEK360-INCN-), -), arginine-384 (-NEWER384-TQGS-), arginine-422 (-GRRER422-VGLL-) and lysine-465 (-QNMEK465-TIFQ-) residues. We used hepta-nona-peptides as the model substrates for autolysys: human angiotensin II--DRVYIHPF and cattle hemoglobin b-chain fragments: LTAEEKA and MLTAEEKAA. Kinetic parameters of enteropeptidase hydrolysis for these substrates were determined. Recent study demonstrates the ability of enteropeptidase to hydrolyze peptide bonds formed by carboxyl groups of Lys or Arg residues if less than four but at least one negative charged amino acid residue is in any of substrate P2-P5 positions. Ca(2+)-dependent autolysis of enteropeptidase heavy chain and of trypsin were compared; the second one serves as the natural defense mechanism against the undesirable premature proenzymes activation in pancreas leading to pancreatitis. The corresponding enteropeptidase inactivation in low Ca2+ environment ought to be the component of the same protective mechanism.