SEQUENCES WITHIN APOLIPOPROTEIN(A) KRINGLE-IV TYPES 6-8 BIND DIRECTLYTO LOW-DENSITY-LIPOPROTEIN AND MEDIATE NONCOVALENT ASSOCIATION OF APOLIPOPROTEIN(A) WITH APOLIPOPROTEIN B-100

Lipoprotein(a) [Lp(a)] particle formation is a two-step process in whi ch initial noncovalent interactions between apolipoprotein(a) [apo(a)] and the apolipoprotein B-100 (apoB-100) component of low-density lipo protein (LDL) precede disulfide bond formation. To identify kringle (K ) domains in apo(a) that bind noncovalently to apoB-100, the binding o f a battery of purified recombinant apo(a) [r-apo(a)] species to immob ilized human LDL has been assessed. The 17K form of r-apo(a) (containi ng all 10 types of kringle TV sequences) as well as other truncated r- apo(a) derivatives exhibited specific binding to a single class of sit es on immobilized LDL, with K-d values ranging from similar to 340 nM (12K) to similar to 7900 nM (KIV5-8). The contribution of kringle IV t ypes 6-8 to the noncovalent interaction of r-apo(a) with LDL was demon strated by the decrease in binding affinity observed upon sequential r emoval of these kringle domains (K-d similar to 700 nM for KIV6-P, K-d similar to 2000 nM for KIV7-P, K-d similar to 5100 nM for KIV8-P, and no detectable specific binding of KIV9-P). Interestingly, KIV9 also a ppears to participate in the noncovalent binding of apo(a) to LDL sinc e the binding of KIV5-8 (K-d similar to 7900 nM) was considerably weak er than that of KIV5-9 (K-d similar to 2000 nM). Finally, it is demons trated that inhibition of Lp(a) assembly by proline, lysine, and lysin e 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 i nteractions with sequences contained within apo(a) kringle IV types 6- 8. On the basis of the obtained data, a model is proposed for the inte raction of apo(a) and LDL in which apo(a) contacts the single high-aff inity binding site on apoB-100 through multiple, discrete interactions mediated primarily by kringle IV types 6-8.