Characterization of the basis of lipoprotein [a] lysine-binding heterogeneity

Although elevated plasma concentrations of lipoprotein a] (Lpa]) are considered to be a risk factor for atherosclerosis, the mechanisms by which Lpa] mediates its pathogenic effects have not been conclusively determined. The apolipoprotein a] (apoa]) component of Lpa] confers unique structural properties to this lipoprotein, including the ability to bind to lysine residues in biological substrates. It has been shown, however, that only a fraction of plasma Lpa] (Lpa]-Lys(+)) binds to lysine-Sepharose in vitro. The nature of the non-lysine-binding Lpa] fraction in plasma (Lpa]-Lys(-)) is currently unknown. In the present study, the Lpa]-Lys(+) fraction was determined in the plasma of six unrelated individuals; the Lpa]-Lys(+) fraction in these plasma samples ranged from approximately 37 to approximately 48%. Interestingly, purification of the Lpa] by density gradient ultracentrifugation followed by gel filtration and ion-exchange chromatography resulted in progressive increases in the Lpa]-Lys(+) fraction. Addition of either purified low density lipoprotein (LDL) or fibronectin to the purified Lpa] at a 1:1 molar ratio reduced the Lpa]-Lys(+) fraction (maximal decrease of 34 and 20%, respectively) whereas addition of both fibronectin and LDL to the purified Lpa] resulted in a further decrease (45% maximally) in this fraction. Similar results were obtained by using a recombinant expression system for apoa]: addition of a 4-fold molar excess of either LDL or fibronectin to conditioned medium containing metabolically labeled recombinant apoa] reduced the Lys(+) fraction by 49 and 23%, respectively.Taken together, our data suggest that the lysine-binding heterogeneity of plasma Lpa] is not primarily an intrinsic property of the lipoprotein, but rather results in large part from its ability to noncovalently associate with abundant plasma components such as LDL and fibronectin. These interactions appear to mask the lysine-binding site in apoa] kringle IV type 10, which mediates the interaction of Lpa] with lysine-Sepharose. The contribution of these interactions to the function of Lpa] in vivo remains to be investigated.