Journal of Lipid Research, Vol 30, 865-875, Copyright © 1989 by Lipid Research, Inc.
Protein polymorphism of a human plasma apolipoprotein D antigenic epitope
R Camato, YL Marcel, RW Milne, S Lussier-Cacan and PK Weech
Laboratory of Lipoprotein Metabolism, Institut de Recherches Cliniques de Montreal, Quebec, Canada.
Based on our previous observation that monoclonal antibody anti-apoD- 4E11
reacted with several HDL proteins we studied them further with three
questions in mind: i) is there common protein polymorphism in healthy
individuals? ii) how many proteins are present and what are their
characteristics? iii) are they all apolipoproteins and do they have the
same lipoprotein distribution as apoD? Isolated, delipidated apoD was used
as a standard for radioimmunometric assay of plasma with antibody 4E11. The
antigen varied from 3 to 11 mumol-equivalents of apoD per liter of plasma
(equivalent to 5-20 mg apoD/dl plasma) with means of 6.1 and 6.8 mumol/l in
men and women, respectively. Two- dimensional electrophoresis of plasma
found up to eight 4E11-antigenic- proteins of different Mr, each
heterogeneous in pI. All plasmas tested contained apoD and an Mr 38,000
antigen, the latter being the most immunoreactive. Six proteins of Mr
70,000-94,000 were found, but the number varied between subjects. Eighty
nine percent of the plasma antigen was associated with lipoproteins: 83%
with HDL and VHDL, 5% with LDL and VLDL. Lipoproteins of all sizes,
separated by polyacrylamide gradient gel electrophoresis, contained the
antigen. ApoD was almost the only 4E11-antigen in LDL, and was in two
states: the one free, the other an apoD-apoB mixed disulfide complex. The
apparent proportions of higher Mr antigens increased with increasing
lipoprotein density, and the proportion of apoD decreased reciprocally.
None of these 4E11-antigenic-proteins cross-reacted with antiserum to
retinol-binding protein.