Apolipoprotein H (ApoH) is a plasma glycoprotein isolated from human s
erum. The interactions of ApoH with lipid membrane were reported to be
essential for its physiological and pathogenic roles. In this paper w
e studied the ability of ApoH to insert into phospholipid membranes us
ing the monolayer approach. The results show that ApoH is surface acti
ve and can insert into the lipid monolayers. The insertion ability of
ApoH is stronger when a higher content of negatively charged lipids is
present in the membrane. The acidic-pH and low-ionic-strength conditi
ons will also enhance ApoH insertion, but these factors may not have m
uch influence on the final insertion ability of ApoH, suggesting that,
in the mechanism of ApoH insertion, not only electrostatic forces, bu
t also hydrophobic interactions, are evidently involved. Modification
by heat inactivation and reduction/alkylation does not change the crit
ical insertion pressure (pi(c)) of ApoH, suggesting a stable domain, m
aybe a linear sequence motif, but not the native three-dimensional str
ucture of ApoH, is responsible for its insertion. The extent to which
insertion of ApoH into phospholipid membranes may facilitate the 'immu
ne cleaning' of plasma liposomes is discussed.