Decreased binding of apolipoprotein (a) to familial defective apolipoprotein B-100 (Arg3500-->Gln). A study of the assembly of recombinant apolipoprotein (a) with mutant low density lipoproteins.

In familial defective apolipoprotein B-100 (FDB), glutamine is substituted for arginine at position 3500 of the amino acid sequence. This mutation alters the structure of low density lipoproteins (LDL) and reduces their binding to LDL receptors. We studied the assembly in vitro of FDB-LDL with two recombinant apo(a) (r-apo(a)) isoforms containing 17 or 18 kringle IV-type repeats, respectively. R-apo(a) complexed to LDL in a concentration- and time-dependent manner. When we mixed normal LDL at protein concentrations from 1 to 10 mg/liter with 200 micrograms/liter r-apo(a) and incubated for 20 h, 15-44% of r-apo(a) were bound to LDL, forming an artificial Lp(a)-like particle. With LDL from a homozygous FDB patient, only 2-16% of r-apo(a) were complexed; heterozygous FDB-LDL bound 2-30% of r-apo(a). We also studied the effect of r-apo(a) on the interaction of the monoclonal antibody MB47 with normal and mutant apoB-100. FDB-LDL displayed enhanced binding of MB47. Adducts generated from normal LDL and r-apo(a) had an increased affinity for MB47, when compared to LDL alone. In contrast, r-apo(a) did not change MB47 reactivity when incubated with FDB-LDL. Altogether, our findings suggest that domains in apolipoprotein B which are important for the interaction with the LDL receptor play a role in the assembly of Lp(a) as well. They provide, in addition, an explanation for the observation that in Lp(a) of heterozygous FDB patients, the ratio of defective to normal apoB-100 is significantly smaller than in LDL from the same patients.