Hydrolysis of artificial substrates by enterokinase and trypsin and the development of a sensitive specific assay for enterokinase in serum.

The activities of highly purified human enterokinase (enteropeptidase, EC 3.4.21.9) and bovine trypsin were tested against three synthetic substrates alpha-N-Benzoyl-L-arginine ethyl ester HCl, alpha-N-Benzoyl-DL-arginine-p-nitroanilide HCl and alpha-N-Benzoyl-DL-arginine-2-naphthylamide HCl. There was no detectable hydrolysis of these substrates by enterokinase whereas the kinetic parameters obtained for trypsin were in close agreement with those previously described by other workers. The values for Km and kcat were dependent on the Ca2+ concentration. Hydrolysis of glycine-tetra-L-aspartyl-L-lysyl-2-naphthylamide (Gly(Asp)4-Lys-Nap) by these protease was also studied. Enterokinase-catalysed hydrolysis obeyed simple steady-state kinetics and values for Km of 0.525 mM and 0.28 mM and for kcat of 21.5 s-1 and 28.3 s-1 were obtained in 0.1 mM and 10 mM Ca2+, respectively. Trypsin-catalysed hydrolysis was complex and the response to Ca2+ was sigmoidal partly due to the lability of trypsin at low Ca2+ concentrations. A sensitive specific assay for enterokinase was developed and applied to the measurement of the enzyme in serum; interference by nonspecific arylamidases was eliminated by the addition of Zn2+.