Apolipoprotein A-I1 (ApoA-11) is a major apoprotein
of human plasma high density lipoproteins (HDL).
The apoprotein has two identical chains of known amino
acid sequence; the chains are linked by a single disulfide
bond. In the present study, we have developed a specific radioimmunoassay
for apoA-I1 that provides a convenient and
reproducible method for measuring 10-100 ng of apoprotein.
The assay was based on the competition of apoprotein
with isotopically labeled [1251]apoA-IID. ioxane (52%) was
used to separate antibody bound [1251]apoA-II from the free apoprotein. The assay has enabled us to begin studies
directed toward mapping-out of the antigenic reactive regions
of apoA-11. It has also allowed us to determine the interrelationship
between the lipid-binding sites and the antigenic
sites of the molecule. The antigenic reactivity of
apoA-I1 was approximately the same in'HDL and phospholipid-
apoA-I1 complexes as that of the free apoprotein.
However, succinylation of apoA-I1 was associated with a
marked decrease in antigenic reactivity.
