Isolation and characterization of the two major apoproteins in human lipoprotein a.

Human Lpa was isolated in preparative amounts from two donors; the native lipoprotein and its constituent apoproteins, apoa and apoB, were characterized extensively. Based on differences in apparent molecular weight, four different isoforms of apoa, a1-a4, were observed between the two donors. The number and relative distribution of these isoforms varied between donors but were constant for each donor. Each apoa isoform was shown to be derived from a discrete apoa-B100 disulfide-linked complex present before reduction. Complete delipidation of Lpa was followed by solubilization, reduction, and carboxamidomethylation of the constituent apoproteins. These apoproteins were then separated by immunoaffinity chromatography using anti-apoa- or anti-apoB-Sepharose; their purity and structural integrity were demonstrated by Western blot analysis. ApoB isolated by this procedure was essentially identical to apoB from autologous LDL with respect to molecular weight, secondary structure, amino acid composition, and sialic acid content. However, apoa differed from apoB in that it exhibited: a much less alpha-helical, less beta, but much more disordered structure; a lower proportion of aspartate, isoleucine, leucine, phenylalanine, and lysine, but a higher proportion of proline, glycine, and threonine; and a much higher content of sialic acid. These results indicate that apoa is not a superglycosylated form of apoB but is distinctly different in its composition and structure.