Determinants of lipoprotein(a) assembly: A study of wild-type and mutant apolipoprotein(a) phenotypes isolated from human and rhesus monkey lipoprotein(a) under mild reductive conditions

We previously observed that rhesus monkey lipoprotein(a) [Lp(a)], is lysine-binding defective (Lys(-)) and attributed this deficiency to the presence of Arg72 in the lysine-binding site (LBS) of kringle IV-10 of apolipoprotein(a) [apo(a)] [Scanu, A. M., Miles, L. A., Fless, G. M., Pfaffinger, D., Eisenbart, J., Jackson, E., Hoover-Plow, J. L., Brunck, T., & Plow, E. F. (1993) J. Clin. Invest. 91, 283-291]. We also identified human mutants having Arg72 instead of Trp72 (wild type) in the LBS of kringle IV-10 [Scanu, A. M., Pfaffinger, D., Lee, J. C., & Hinman, J. (1994) Biochim. Biophys. Acm 1227, 41-45]. Unique to the human mutant phenotype were the very low levels of plasma Lp(a), suggesting structural differences between human and rhesus apo(a) and a possible divergent mode of Lp(a) assembly. In order to explore the possibility of a relationship between apo(a) LBS and Lp(a) assembly, we developed a novel method for isolating wild-type and mutant apo(a) phenotypes in a free form by subjecting each parent Lp(a) to mild reductive conditions using 2 mM dithioerythritol (DTE) and 100 mM of the lysine analogue, epsilon-aminocaproic acid (EACA). The application of this method to the study of wild-type and mutant apo(a) species showed that regardless of the source of Lp(a), i.e., positive lysine binding (Lys(+)) or negative lysine binding (Lys(-)), all of the isolated free apo(a)s were Lys(+). Moreover, incubation of free apo(a)s with their autologous human or rhesus low-density lipoproteins (LDL) generated Lp(a) complexes which were structurally and functionally indistinguishable from their parent native Lp(a). In each instance, the reassembly process was inhibited by the presence of either EACA or proline. These two reagents had a minimal effect on either Lp(a) or reassembled Lp(a) [RLp(a)]. Free apo(a) bound to apoB100 of very low density lipoproteins (VLDL) to form a triglyceride-rich Lp(a). These results show that (1) both human and rhesus Lp(a) are amenable to dissassembly and reassembly, (2) the presence of Arg72 in the LBS of kringle IV-10 is not involved, at least directly, in this process, (3) its cleavage from apoB100 opens up in apo(a) a domain that is both EACA and proline sensitive and involved in Lp(a) assembly, and (4) the apoB 100 of VLDL is also competent to bind apo(a). Our observations also suggest that the difference in plasma Lp(a) levels between the rhesus and the human mutant, both having Arg72 in the LBS of apo(a) kringle IV-10, is not related to the assembly process, but more likely to a divergence in production/secretion rates between the two apo(a) phenotypes.