Analysis of structure--function relationships in human apolipoprotein(a).

Elevated levels of lipoprotein(a) (Lp(a)) have been strongly correlated with the development of atherosclerosis in human populations. Lp(a) is distinguishable from low density lipoprotein by the presence of the unique protein component apolipoprotein(a) (apo(a)), which contains repeated domains that closely resemble that of plasminogen kringle IV. Using human embryonic kidney cells, we have expressed a recombinant form of apo(a) (r-apo(a)) containing 17 kringle IV-like domains. We have utilized this recombinant expression system to study the assembly of Lp(a) particles. We have demonstrated that Lp(a) particles containing r-apo(a) can be assembled extracellularly in plasma by covalent linkage to low density lipoprotein. Using site-directed mutagenesis, we have demonstrated that a cysteine residue present at position 4057 of the apo(a) protein (i.e., in the penultimate kringle IV repeat) mediates this covalent linkage. Using polymerase chain reaction amplification of liver apo(a) complementary DNA, we have demonstrated the presence of a polymorphism in apo(a) kringle IV type 10, which results in the substitution of a threonine for a methionine. Preliminary studies indicate that the presence of a threonine at this position may enhance the interaction of Lp(a) with lysine-Sepharose.