Lipid transport function of lipoproteins in blood plasma

Fatty acid and cholesterol transport in plasma lipoproteins evolved in the context of an open circulatory system in which lipoprotein particles are secreted directly into the blood and have ready access to cells in various tissues. In higher vertebrates with closed capillary beds, hydrolysis of triglycerides at capillary surfaces is required for efficient uptake of their component fatty acids into cells. Likewise, hydrolysis of cellular triglycerides in cells of adipose tissue precedes mobilization of the fatty acids and permits large amounts to be transported in the blood. However, in all Metazoa lipoproteins are secreted primarily from cells adjacent to an open microvascular bed. Uptake of lipoprotein particles as such into cells occurs in invertebrates and vertebrates alike, facilitated by binding to high-affinity receptors on cell surfaces. In vertebrates, a concentration gradient created between cholesterol in cells and lipoproteins by a cholesterol-esterifying enzyme that acts on lipoproteins promotes movement of cholesterol into the plasma compartment. Thus the strategies to transport poorly soluble lipids include enzymatic reactions at cell surfaces and in blood plasma as well as the processes of exocytosis and endocytosis.