In addition to promoting trypsinoge

In addition to promoting trypsinogen activation, the heavy chain also affects the reactions of enteropeptidase with macromolecular protease inhibitors. BEK purified from intestine was reported to be inhibited by BPTI with a dissociation constant of '5 nM (7), and recombinant HL-BEK also was found to be inhibited by BPTI with a final Ki* of 1.8 nM (Table II). In addition, the kinetics were typical of slow binding inhibition (Fig. 5). Detailed analysis (Fig. 6) was consistent with a mechanism in which a low affinity complex was formed rapidly with a Ki of 99 nM, and this complex slowly isomerized to the final high affinity complex with a forward rate constant (k3) of 0.88
min21. Deletion of the heavy chain had little effect on k3, modestly increased the final Ki* to 6.2 nM, and increased the initial Ki '7-fold to '700 nM (Table II). An opposite pattern was observed for STI. BEK is completely resistant to STI (7),
and HL-BEK also was not inhibited by STI. However, deletion of the heavy chain rendered L-BEK susceptible to slow inhibition by STI with a Ki* of 1.6 nM (Table II). In addition, the kinetics of the L-BEK reaction with STI did not indicate the existence of an initial low affinity complex.