Transport in the General Circulation

Blood serum is an aqueous environment incompatible with the hydrophobic lipid molecules.[1,8,10] Lipids are transported in lipoprotein molecules composed of several proteins and various lipid components (Table 1). Protein and lipid components are exchanged and some lipoproteins are synthesized from others. The chylomicrons are very large particles synthesized by the gut and present in lymph and the general circulation. They contain a large lipid component, mostly TAG, and little protein. The gut and the liver synthesize very low-density lipoproteins (VLDL) that also contain considerable amounts of lipid, mostly TAG and C+CE, and 10% protein. The low-density lipoproteins (LDL) are synthesized in the serum from degraded VLDL and chylomicrons. They are composed of large amounts of C + CE with 20% protein. In humans and pigs, but not all species, most of the C+CE is carried in LDL. The high-density lipoproteins (HDL) are synthesized in the serum, liver, and gut. They are composed of 45% protein, a small amount of TAG, 40% C + CE, and 45% PL. Finally, the FA is carried on albumin, a protein with multiple FA binding sites.

The lipoproteins deliver complex lipids to the tissues where they are absorbed to be used to build membranes, for storage as energy reserves or as precursors for many biologically active regulatory molecules (steroid hormones, eicosanoids, etc.). The animal cell cannot absorb intact TAG. It is digested in the intestinal tract to simpler components. In tissues that rely on FA as an oxidative substrate, e.g., cardiac and skeletal muscle and adipose, there is a specialized enzyme, lipoprotein lipase (LPL), that is secreted by these cells to cleave TAG to FAs. The LPL moves to the surface of the capillary endothelial cell, where it cleaves TAG in lipoproteins, particularly the chylomicrons and VLDL. The FAs are absorbed by the capillary endothelial cell and then pass to the underlying tissue cells.

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