A role for apolipoprotein(a) in protection of the low-density lipoprotein component of lipoprotein(a) from copper-mediated oxidation

Low-density lipoprotein (LDL) oxidation is stimulated by copper. Addition of a recombinant form of apolipoprotein(a) (apo(a); the distinguishing protein component of lipoprotein(a)) containing 17 plasminogen kringle IV-like domains (17K r-apo(a)) protects LDL against oxidation by copper. Protection is specific to apo(a) and is not achieved by plasminogen or serum albumin. When Cu2+ is added to 17K r-apo(a), its intrinsic fluorescence is quenched in a concentration-dependent and saturable manner. Quenching is unchanged whether performed aerobically or anaerobically and is reversible by ethylenediaminetetraacetate, suggesting that it is due to equilibrium binding of Cu2+ and not to oxidative destruction of tryptophan residues. The fluorescence change exhibits a sigmoid dependence on copper concentration, and time courses of quenching are complex. At copper concentrations below 10 μM there is little quenching, whereas above 10 μM quenching proceeds immediately as a double-exponential decay. The affinity and kinetics of copper binding to 17K r-apo(a) are diminished in the presence of the lysine analogue ε-aminocaproic acid. We propose that copper binding to the kringle domains of 17K is mediated by a His–X–His sequence that is located about 5 Å from the closest tryptophan residue of the lysine binding pocket. Copper binding may account for the natural resistance to copper-mediated oxidation of lipoprotein(a) relative to LDL that has been previously reported and for the protection afforded by apo(a) from copper-mediated oxidation of LDL that we describe in the present study.