Oxidized low-density lipoprotein reduces plasma coagulation in vitro

Oxidation of low-density lipoprotein (LDL) was shown to occur in vivo and involved lipid peroxidation and apolipoprotein modification. We studied the effect of oxidized-LDL (Ox-LDL) on plasma coagulation by measuring prothrombin time (PT) and partial thromboplastin time (PTT) following the addition of Ox-LDL to normal plasma. Ox-LDL, but not native LDL, caused prolongation of PT and PTT by 30% in a dose- and time-dependent pattern. This effect was also shown to be present following lipoprotein delipidation, suggesting that it was the apolipoprotein fraction of Ox-LDL, but not its lipid fraction, that was responsible for the prolongation of PT and PTT. This was further substantiated since similar effect could be obtained by adding LDL treated with trinitrobenzenesulphonic acid to block the lysine groups, as occurs in oxidized LDL. Ox-LDL, unlike LDL, was found to reduce plasma ionized calcium by 33%. Moreover, adding calcium ions to Ox-LDL negated its effect on PT and PTT, suggesting that Ox-LDL apolipoprotein may influence coagulation by binding calcium ions.