Phenotyping of human apolipoprotein E from whole blood plasma by immunoblotting

Human apolipoprotein E (apoE) exists in the population in three common genetically determined isoforms, apoE-2, E-3, and E-4, that are coded for by three alleles epsilon-2, epsilon-3 and epsilon-4 at the apoE structural gene locus resulting in six phenotypes, three homozygotes (E 2/2, E 3/3, and E 4/4) and three heterozygotes (E 2/3, E 2/4, and E 3/4). A new procedure is described that allows identification of apoE isoforms and phenotypes from whole plasma or serum without the need for isolating apoE-containing lipoproteins or two-dimensional gel electrophoresis of serum. This rapid method combines cysteamine treatment of apoE in plasma, separation in parallel of cysteamine-treated and untreated hydrophobic serum proteins by charge-shift electrophoresis, and isoelectric focusing of apolipoproteins with immunoblotting. Compared to phenotyping of apoE after isolation of VLDL, the new procedure agreed in most cases and may be of special value in detecting apoE mutants that differ in their cysteine residues or either are spun off during isolation of lipoproteins or cofocus with other apoproteins and thus escape detection by conventional one-dimensional techniques. The method provides a simple tool to screen apoE isoforms that are known to have a major impact on individual plasma cholesterol levels.