Comparison of the inhibitory effects of ApoB100 and tissue factor pathway inhibitor on tissue factor and the influence of lipoprotein oxidation

The procoagulant activity of tissue factor is regulated by circulating inhi bitors such as tissue factor pathway inhibitor (TFPI) and LDL. These 2 inhi bitors also readily associate making the distinction between their activiti es difficult. We have examined the relative contributions of intact and C-t erninal truncated TFPI and ApoB100. By following the inhibitory potential o f the preparations, over a period of 120 minutes, it was demonstrated that TFPI and LDL-resembling particles inhibited tissue factor at different rate s. TFPI was found to be a short, fast-acting inhibitor, whereas the action of LDL-resembling particles was more prolonged but slower. The oxidation of LDL has been closely associated with the development of cardiovascular dis ease, including atherosclerosis and thrombosis. Positively charged amino ac ids, particularly lysine residues, are prone to alterations via the formati on of adducts by lipid peroxidation products. These residues are important in the inhibition of tissue factor activity by ApoB100. They also play an i mportant role in the inhibitory Kunitz domains of TFPI. We have shown that the decline in the ability of LDL to inhibit tissue factor was as a result of modifications in LDL arising from oxidation. By examining the effects of oxidation on full-length and C-terminal truncated TFPI bound to LDL-resemb ling particles, we found that TFPI is only affected when in close associati on with ApoB100. C-terminal truncated TFPI was not affected significantly b y oxidation. Finally, chemical modification of lysine and arginine residues reduced the overall inhibition of tissue factor by TFPI. We propose that T FPI and LDL act separately to inhibit tissue factor in vivo. However, the o xidation of LDL can alter both the endogenous activity of ApoB100 and reduc e that of closely associated TFPI, compromising normal hemostasis.