Elevated plasma homocysteine leads to alterations in fibrin clot structure and stability: implications for the mechanism of thrombosis in hyperhomocysteinemia

Summary. Elevated plasma homocysteine is associated with an increased risk of atherosclerosis and thrombosis. However, the mechanisms by which homocysteine might cause these events are not understood. We hypothesized that hyperhomocysteinemia might lead to modification of fibrinogen in vivo, thereby causing altered fibrin clot structure. New Zealand White rabbits were injected intraperitoneally (i.p.) every 12 h through an indwelling catheter with homocysteine or buffer for 8 weeks. This treatment raised the plasma homocysteine levels to about 30 µmol L^?1 compared with 13.5 µmol L^?1 in control rabbits by the end of the treatment period. The fibrinogen levels were 3.2 ± 0.6 in homocysteine‐treated and 2.5 ± 1.1 mg mL^?1 in control rabbits. The reptilase time was prolonged to 363 ± 88 for plasma from homocysteine‐treated rabbits compared with 194 ± 48 s for controls (P < 0.01). The thrombin clotting time (TCT) for the homocysteine‐treated rabbits was significantly shorter, 7.5 ± 1.7 compared with 28.6 ± 18 s for the controls (P < 0.05). The calcium dependence of the thrombin clotting time was also different in homocysteinemic and control plasmas. Clots from plasma or fibrinogen of homocysteinemic rabbits were composed of thinner fibers than control clots. The clots formed from purified fibrinogen from homocysteine‐treated rabbits were lyzed more slowly by plasmin than comparable clots from control fibrinogen. Congenital dysfibrinogenemias have been described that are associated with fibrin clots composed of thin, tightly packed fibers that are abnormally resistant to fibrinolysis, and recurrent thrombosis. Our results suggest that elevated plasma homocysteine leads to a similar acquired dysfibrinogenemia. The formation of clots that are abnormally resistant to fibrinolysis could directly contribute to the increased risk of thrombosis in hyperhomocysteinemia.