Sequences within Apolipoprotein(a) Kringle IV Types 6−8 Bind Directly to Low-Density Lipoprotein and Mediate Noncovalent Association of Apolipoprotein(a) with Apolipoprotein B-100

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Abstract
Lipoprotein(a) [Lp(a)] particle formation is a two-step process in which initial noncovalent interactions between apolipoprotein(a) [apo(a)] and the apolipoprotein B-100 (apoB-100) component of low-density lipoprotein (LDL) precede disulfide bond formation. To identify kringle (K) domains in apo(a) that bind noncovalently to apoB-100, the binding of a battery of purified recombinant apo(a) [r-apo(a)] species to immobilized human LDL has been assessed. The 17K form of r-apo(a) (containing all 10 types of kringle IV sequences) as well as other truncated r-apo(a) derivatives exhibited specific binding to a single class of sites on immobilized LDL, with Kd values ranging from ∼340 nM (12K) to ∼7900 nM (KIV5-8). The contribution of kringle IV types 6−8 to the noncovalent interaction of r-apo(a) with LDL was demonstrated by the decrease in binding affinity observed upon sequential removal of these kringle domains (Kd ∼ 700 nM for KIV6-P, Kd ∼ 2000 nM for KIV7-P, Kd ∼ 5100 nM for KIV8-P, and no detectable specific binding of KIV9-P). Interestingly, KIV9 also appears to participate in the noncovalent binding of apo(a) to LDL since the binding of KIV5-8 (Kd ∼ 7900 nM) was considerably weaker than that of KIV5-9 (Kd ∼ 2000 nM). Finally, it is demonstrated that inhibition of Lp(a) assembly by proline, lysine, and lysine analogues, as well as by arginine and phenylalanine, is due to their ability to inhibit noncovalent association of apo(a) and apoB-100 and that these compounds directly exert their effects primarily through interactions with sequences contained within apo(a) kringle IV types 6−8. On the basis of the obtained data, a model is proposed for the interaction of apo(a) and LDL in which apo(a) contacts the single high-affinity binding site on apoB-100 through multiple, discrete interactions mediated primarily by kringle IV types 6−8.