High density lipoprotein exchange reactions.
The term human serum high density lipoprotein (HDL) represents a range of lipid-protein complexes which are identified by their density and hence their relative lipid-protein content. Because HDL composition is variable, any proposed mechanisms for lipid and protein exchange must account for this variability. More importantly, since HDL has been repeatedly shown to be a negative risk factor in atherosclerosis, physical interactions have to be put in a physiological context. In this review, the lipid and protein exchange reactions of HDL with other lipoproteins, phospholipid vesicles, lipid-coated glass beads and isolated cells, will be considered. Particular emphasis will be placed on the role of the two major apoproteins of HDL, namely apo A-I and apo A-II, in these exchange reactions and a model will be presented to explain how these apoproteins might mediate lipid exchange, interconversions of lipoprotein particles, and the egress and excretion of lipid from cells.