Inhibiting platelet-stimulated blood coagulation by inhibition of mitochondrial respiration

Abstract

Platelets are important mediators of blood coagulation that lack nuclei, but contain mitochondria. Although the presence of mitochondria in platelets has long been recognized, platelet mitochondrial function remains largely unaddressed. On the basis of a small amount of literature that suggests platelet mitochondria are functional, we hypothesized that the inhibition of platelet mitochondria disrupts platelet function and platelet-activated blood coagulation. To test this hypothesis, members of the tetrazole, thiazole, and 1,2,3-triazole families of small molecule heterocycles were screened for the ability to inhibit isolated mitochondrial respiration and coagulation of whole blood. The families of heterocycles screened were chosen on the basis of the ability of the heterocycle family to inhibit a biomimetic model of cytochrome c oxidase (CcO). The strength of mitochondrial inhibition correlates with each compound's ability to deter platelet stimulation and platelet-activated blood clotting. These results suggest that for this class of molecules, inhibition of blood coagulation may be occurring through a mechanism involving mitochondrial inhibition. Platelets are directly involved in a number of functions necessary for clotting, including recognition of vascular lesions, triggering activation of the coagulation cascade, and activation of other platelets. The platelet membrane serves as a scaffold for clot formation, and platelets are involved in the activation and cocatalysis of reactions involving many of the soluble clotting factors (1). Like red blood cells, platelets lack nuclei and consequently are unable to replace damaged proteins encoded in the nuclear genome. However, unlike red blood cells, platelets contain actively metabolizing mitochondria (2). Some hints as to the role these mitochondria play in platelet function have been elucidated (3). Along with glycogen granules, platelet mitochondria provide energy that is needed at least indirectly for platelet aggregation and secretion of procoagulant molecules (4). More direct evidence of a role for mitochondria in coagulation rests on observations that changes in the permeability of mitochondrial membranes are linked to changes in coagulation activity (5, 6). These facts imply that inhibition of platelet mitochondrial function should have an inhibitory effect upon platelet-activated blood coagulation.Experimental investigation led to the discovery of three families of small molecule heterocycles that reversibly inhibit mitochondrial respiration and attenuate platelet-activated blood coagulation. These three families of compounds comprise unique examples of a class of anticoagulants proposed to inhibit blood clotting through a mitochondrial mechanism (Fig. 1).The discovery of these particular families of platelet inhibitory molecules occurred after initial work from the Collman laboratory related to biomimetic modeling of cytochrome c oxidase (CcO). CcO is the terminal enzyme in the electron transport chain that catalyzes the four-electron reduction of O 2...

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