LOW-MOLECULAR-WEIGHT KININOGEN BINDS TO PLATELETS TO MODULATE THROMBIN-INDUCED PLATELET ACTIVATION

The kininogens, high molecular weight kininogen (HK) and low molecular weight kininogen (LK), are multifunctional, single-gene products that contain bradykinin and identical amino-terminal heavy chains. Studies were performed to determine if LK would bind directly to platelets. I-125-LK specifically bound to gel-filtered platelets inthe presence of 50-mu-M Zn2+. HK effectively competed with I-125-LK for the same binding site (K(i) = 27 +/- 9 nM, n = 5). Similarly, the K(i) for LK inhibition of I-125-LK binding was 12 +/- 1 nM (n = 3). Albumin, fibrinogen, factor XIII, and kallikrein did not inhibit I-125-LK binding to unstimulated platelets. I-125-LK (66 kDa) was not cleaved upon binding to platelets. The binding of I-125-LK to unstimulated platelets was found to be fully reversible by the addition of a 50 molar excess of unlabeled LK at both 10 and 20 min. LK binding to platelets was saturable with an apparent K(d) of 27 +/- 2 nM (mean +/- S.E., n = 9) and 647 +/- 147 binding sites/platelet. Both LK and HK at plasma concentrations inhibited thrombin-induced platelet aggregation. LK and HK at about 5% of plasma concentration also inhibited thrombin-induced secretion of both stirred and unstirred platelets. Both kininogens were found to be noncompetitive inhibitors of proteolytically active thrombin binding to platelets. The kininogens did not inhibit D-phenylalanyl-prolyl-arginine chloromethyl ketone-treated thrombin from binding to platelets. These studies indicated that both kininogens have a region on their heavy chain which allows them to bind to platelets. Further,kininogen binding by its heavy chain modulates thrombin activation of platelets since it prevents proteolytically active thrombin from binding to its receptor.