Kinetics of reaction of human activated complement c 1 inhibitor with the human complement system protease activated complement c 1r and protease activated complement c 1s
Sim, R.B.; Arlaud, G.J.; Colomb, M.G.
Biochimica et Biophysica Acta 612(2): 433-450
ISSN/ISBN: 0005-2728 Accession: 005784338
The interactions of the 2 classical serine proteases of the complement [C] system with their natural inhibitor, C.hivin.1[activated C1]-inhibitor, were studied. C.hivin.1r and C.hivin.1s react with C.hivin.1-inhibitor to form complexes which contain 1 mol of C.hivin.1-inhibitor per mol of protease monomer. The complexes are not degraded in the presence of excess protease and are not dissociated by strong denaturing agents. Rate constants and dissociation constants for these reactions fall within the normal range for protease-protease inhibitor interactions. The affinity of both proteases for C.hivin.1-inhibitor is similar; C.hivin.1s reacts more rapidly than does C.hivin.1r. The presence of Ca2+ decreases the rate at which C.hivin.1r complexes with C.hivin.1-inhibitor but does not affect the reactivity of C.hivin.1s. The C.hivin.1r-(C.hivin.1-inhibitor) reaction is inhibited by extremes of ionic strength and has a more marked temperature-dependence than the C.hivin.1s-(C.hivin.1-inhibitor) interaction. Heparin stimulates the rate of the reaction of C.hivin.1s with C.hivin.1-inhibitor by a factor of 14-15. The C.hivin.1r-(C.hivin.1-inhibitor) interaction is accelerated by heparin; the effect is much smaller than for C.hivin.1s. Neither protease is inhibited by heparin alone. Flufenamic acid inhibited the action of C.hivin.1-inhibitor. The incorporation of C.hivin.1r into C.hivin.1 bound to immune complexes increased the reactivity of C.hivin.1r towards C.hivin.1-inhibitor. C.hivin.1s reactivity was not stimulated by this treatment. The enhancement of C.hivin.1r activity by strong binding to antibody-antigen-C1q complexes and to C.hivin.1s parallels earlier work on the activation of the C1r proenzyme, and demonstrates that the other components of the antibody-antigen-C1 complex act as modifiers of the activities of activated and proenzymic C1r.