Solubility, immunochemical, and lipoprotein binding properties of apoB-100-apo[a], the protein moiety of lipoprotein[a].

The protein moiety of Lp[a] consisting of apoB and apo[a] covalently linked to each other, once freed of lipids by delipidation at pH 8.0 with mixtures of diethyl ether and ethanol, is freely water-soluble at pH values above 6.4. This is in contrast to apoB which, if prepared by similar delipidation techniques, is only soluble at alkaline pH, indicating that the coupling of the carbohydrate-rich apo[a] to apoB confers water solubility to this apolipoprotein that it does not possess on its own. When probed in a sandwich ELISA with antibodies specific to apo[a], the results suggest that some apo[a] epitopes in Lp[a] are masked by lipid but are freely accessible to antibodies in the lipid-free apoB-apo[a] complex. Examination of apoB-apo[a] with an ELISA specific for apoB showed a decreased and altered immunoreactivity of apoB when compared to either low density lipoprotein (LDL) or Lp[a]. These results are consistent with a model in which the hydrophobic lipid binding domains of apoB in apoB-apo[a] self-associate and are shielded from the aqueous environment by the hydrophilic portions of apoB and by an envelope of apo[a]. The apoB-apo[a] complex has lipophilic properties as shown by its interaction with the phospholipid-stabilized triglyceride emulsion, Intralipid. In addition, it has an avidity for all types of lipoproteins although displaying a preference for triglyceride-rich particles. In the presence of plasma, the interaction of apoB-apo[a] with all lipoproteins is reduced. Neither iodinated apo[a] nor iodinated Lp[a] nor LDL had an affinity for lipoproteins, suggesting that the lipophilic properties of apoB-apo[a] are probably due to apoB since apo[a] is rather hydrophilic and is unable to bind to lipids. Thus, the apoB-apo[a] complex has amphipathic properties with apo[a] providing the hydrophilic capacity to interact with the aqueous environment and apoB providing the hydrophobic interactions necessary to bind lipids.