Separation of swine plasma LDL from Lpb2/3 heterozygotes into two apoB allelic haplotypes, Lpb2 and Lpb3, with apoB epitope specific antibodies

Studies were performed to investigate the separation of Lpb (lipoprotein B) species present in plasma of heterozygous swine bearing the Lpb2 and Lpb3 apoB mutant genes. Low density lipoprotein (LDL) fractions from Lpb2/2 and Lpb3/3 homozygotes were coupled to a matrix and used to isolate affinity-purified antibodies anti-Lpb2 and anti-Lpb3 from swine alloimmune sera, one with specificity for the Lpb2 epitope(s) and the other for Lpb3. These antibodies in turn were used to construct two immunosorbers, anti-Lpb2 and anti-Lpb3 Sepharose columns. To separate the two Lpb haplotype populations present in LDL, a density gradient ultracentrifuge subfraction (d 1.032-1.043 g/ml) obtained from Lpb2/3 heterozygous pigs was applied to the specific immunosorbers. The retained fraction from the anti-Lpb2 column reacted in the double immunodiffusion test with anti-Lpb2 and anti-Lpb13 immune sera but not with either anti-Lpb3 or anti-Lpb12, while the unretained fractions reacted with anti-Lpb3 and anti-Lpb12 but not with either anti-Lpb2 or anti-Lpb13. The reaction patterns obtained with the two sets of alloimmune sera indicate the existence of two separate lipoprotein populations in LDL: one lipoprotein carrying the Lpb2 and Lpb13 epitopes corresponding to the Lpb2 apoB allele, and the other carrying the Lpb3 and Lpb12 allotypes specified by the Lpb2 gene. Immunoblotting with anti-Lpb2 and anti-Lpb3 and silver staining showed that the epitopes of both isolated LDL subpopulations are associated with apoB-100. Neutral lipid analyses showed no differences between the isolated Lpb2 and Lpb3 lipoprotein species from the Lpb2/3 heterozygotes. These studies demonstrate that plasma LDL subfractions from Lpb heterozygous swine can be separated into two haplotype populations, each corresponding to the product of one apoB gene, and reveal a new insight into the phenotypic expression of plasma LDL, and the LDL phenotype-genotype relationship. Furthermore, this approach will facilitate studies on metabolic differences of two structurally distinct LDL, unaffected by in vitro manipulation, exposed to the metabolic milieu of one individual.