| STORAGE AND TRANSPORT OF FATTY ACIDS: THE SYNTHESIS OF TRIACYLGLYCEROLS
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| Fatty acids derived from endogenous synthesis or from the diet, are stored and transported as triacylglycerols (triglycerides).
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| Figure 15.4 The malate shuttle. The provision of acetyl-CoA and reducing equivalents for fatty acid biosynthesis and the malate shuttle. Fru-6-P, fructose-6-phosphate; Glc-6-P, glucose-6-phosphate; NADH, reduced nicotinamide dinucleotide. (See also 'Electron shuttles' box on p. 100.) |
In both the liver and the adipose tissue, triacylglycerols (TAGs) are produced by a pathway involving glycerol-3-phosphate (glycerol-3-P) and phosphatidic acid as intermediates (Fig. 15.7). However, glycerol-3-P is of different origin in the two tissues: in the liver, glycerol itself provides the source, via the action of glycerol kinase, but in the adipose tissue (which lacks this enzyme), glucose is the source of glycerol-3-P, via glycolysis and its immediate precursor dihydroxyacetone phosphate (DHAP) (Fig. 15.7). Thus, the storage of fatty acids in the adipose tissue can take place only when glycolysis is activated in the fed state.
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| The TAG produced in smooth endoplasmic reticulum of the liver is then, together with cholesterol and phospholipids, associated with apolipoprotein B100,which is also synthesized on the endoplasmic reticulum, to form very-low-density lipoprotein (VLDL). The VLDL is then processed in the Golgi apparatus and released into the bloodstream, transporting triacylglycerols to other tissues (see Chapter 17). In the bloodstream, VLDL is acted upon by lipoprotein lipase (LPL), an enzyme attached to the basement membrane glycoproteins of the capillary endothelial cells. LPL hydrolyses TAG, releasing fatty acids to tissues. LPL synthesis is induced by insulin (Fig. 15.8 and Chapter 17).
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