Activation of lecithin cholesterol acyltransferase by a disulfide-linked apolipoprotein A-I dimer

Apolipoprotein A-I(Milano) is a molecular variant of apoA-I, containing the Arg173→Cys substitution that forms a disulfide linked homodimer (A-I(M)/A-I(M)). To assess the effect of this structural modification on a major function of the apolipoprotein, its activation of lecithin cholesterol acyltransferase (LCAT), we prepared well-defined complexes of A-I(M)/A-I(M) and apoA-I with phospholipids and cholesterol and compared their reactivities with LCAT. Particles with A-I(M)/A-I(M) had very similar diameters to apoA-I particles (7.8 and 12.5 nm) but had distinct apolipoprotein and phospholipid contents and protein secondary structures; they bound LCAT with comparable affinities, but were less efficient substrates for the enzyme (40 to 70% less reactive). We conclude that the local structural constraints in A-I(M)/A-I(M) do not prevent the formation of well-defined complexes with phospholipids and do not influence the binding of the enzyme to the particles, but have an inhibitory effect on LCAT activation.