FUNCTIONS OF THE N-TERMINAL DOMAIN OF SECRETORY LEUKOPROTEASE INHIBITOR

Secretory leukoprotease inhibitor (SLPI) comprises two homologous doma ins: the C-terminal domain contains the reactive site, while the funct ion of the N-terminal domain remains unknown. In order to elucidate th e function of the N-terminal domain, we studied the kinetics of reacti ons of human leukocyte elastase with two recombinant forms of SLPI: th e full-length inhibitor and the C-terminal domain alone. The reactions of elastase with the full-length inhibitor and the C-terminal domain share the same association rate constant, 2 X 10(6) M(-1) s(-1), but t he complex formed with the C-terminal domain is less stable, with a di ssociation rate constant of 8 x 10(4) s(-1), 5 times higher than that of, the complex with the full-length inhibitor. The binding of the ful l-length inhibitor to elastase is greatly accelerated by polyanions. I n the presence of submicromolar concentrations (1 mu g/mL) of heparin, the association rate constant is increased by more than 1 order of ma gnitude. The binding of the C-terminal domain alone to elastase shows much lower sensitivity to heparin; in the presence of 5 mu M (25 mu g/ mL) heparin, association of the C-terminal domain with elastase reache s a maximum rate of 7 X 10(6) M(-1) s(-1), about 3 times higher than t he rate in the absence of heparin. Similar differential effects of hep arin have been observed on the reactions of alpha-chymotrypsin with th e two recombinant forms of SLPI. We also found that heparin has only a small effect on the binding of elastase with elafin, an elastase-spec ific inhibitor homologous to the C-terminal domain of SLPI. These data reveal two previously unrecognized functions of the N-terminal domain : stabilizing the elastase-inhibitor complex and mediating the activat ion of the inhibitor by heparin.