Characterization of three human apolipoprotein E isoforms (E2, E3 and E4) expressed in Escherichia coli.

Apolipoprotein E is one of the apolipoproteins involved in cholesterol metabolism. Three major isoforms are present in men: E2, E3, E4 corresponding to the products of three alleles. They have different affinities for receptors and the epsilon 4 allele is a risk factor for cardiovascular diseases and more recently for Alzheimer's disease. We describe here the production, by heterologous expression in Escherichia coli, of the three apolipoprotein E isoforms for use in both research and clinical laboratories. By Surface Plasmon Resonance, the purified recombinant apolipoprotein E isoforms were able to recognize three monoclonal anti-human apolipoprotein E antibodies with affinity constants close to those of purified human apolipoprotein E. For receptor binding studies, the recombinant apolipoprotein E isoforms were associated with VLDL isolated from apolipoprotein E knockout mice. Although the association of the recombinant apolipoproteins E with the mouse VLDL was less efficient than that of human plasma apolipoprotein E3, the recombinant apolipoprotein E3 and apolipoprotein E4 complexes competed efficiently with 125I-labelled LDL for binding to the LDL receptor in J774 macrophages, whereas the recombinant apolipoprotein E2-VLDL complexes did not. These results suggest that the recombinant apolipoprotein E isoforms have biological properties similar to the human apolipoprotein E isoforms.