Apolipoprotein A-II
lipid transporter activity
Isolation and partial characterization of lipoprotein A-II (LP-A-II) particles of human plasma.
High density lipoproteins (HDL) consist of a mixture of chemically and functionally distinct families of particles defined by their characteristic apolipoprotein (Apo) composition. The two major lipoprotein families are lipoprotein A-I (LP-A-I) and lipoprotein A-I:A-II (LP-A-I:A-II). This study describes the isolation of a third minor HDL family of particles referred to as lipoprotein A-II (LP-A-II) because it lacks ApoA-I and contains ApoA-II as its main or sole apolipoprotein constituent. Because ApoA-II is an integral protein constituent of three distinct lipoprotein families (LP-A-I:A-II, LP-A-II: B:C:D:E and LP-A-II), LP-A-II particles were isolated from whole plasma by sequential immunoaffinity chromatography on immunosorbers with antisera to ApoA-II, ApoB and ApoA-I, respectively. In normolipidemic subjects, the concentration of LP-A-II particles, based on ApoA-II content, is 4-18 mg/dl accounting for 5-20% of the total ApoA-II not associated with ApoB-containing lipoproteins. The lipid composition of LP-A-II particles is characterized by low percentage of triglycerides and cholesterol esters and a high percentage of phospholipids in comparison with lipid composition of LP-A-I and LP-A-II: A-II. The major part of LP-A-II particles contain ApoA-II as the sole apolipoprotein constituent; however, small subsets of LP-A-II particles may also contain ApoD and other minor apolipoproteins. The lipid/protein ratio of LP-A-II is higher than those of LP-A-I and LP-A-I:A-II. In homozygous ApoA-I and ApoA-I/ApoC-III deficiencies, LP-A-II particles are the only ApoA-containing high density lipoprotein with levels found to be within the same range (7-13 mg/dl) as those of normolipidemic subjects. However, in contrast to normal LP-A-II, their lipid composition is characterized by higher percentages of triglycerides and cholesterol esters and a lower percentage of phospholipids and their apolipoprotein composition by the presence of ApoC-peptides and ApoE in addition to ApoA-II and ApoD. These results show that LP-A-II particles are a minor HDL family and suggest that, in the absence of ApoA-I-containing lipoproteins, they become an efficient acceptor/donor of ApoC-peptides and ApoE required for a normal metabolism of triglyceride-rich lipoproteins. Their other possible functional roles in lipid transport remain to be established in future experiments.