[Lipid metabolism. I. Role of insulin in lipid metabolism].

The triacylglyceroles that comprise the bulk of lipids in the diet are hydrolyzed to free fatty acids, monoacylglyceroles and glycerol in the intestinal tract. During absorption through the intestinal tract mucosa, triacylglyceroles are resynthesized from free fatty acids, and glycerol-3-phosphate is formed in the intestinal mucose. these globules, called chylomicrons, pass through the liver and adipose tissue, they are reduced in size by an enzyme, lipoprotein lipase (LPL). In the postabsorptive period, free fatty acids and glycerol are released from adipocytes by neural and hormonal stimulation. The free fatty acids can be burned by almost all tissues of the body except the brain. They are burned in the mitochondria by a process of b-oxidation to acetyl-CoA, which can then enter the citrate acid cycle for conversion to CO2, adenosine triphosphate, and water. When excessive quantities of glucose are ingested, the glucose can be converted to a storage form, triacylglycerol. Fatty acids are synthesized by a series of reactions in which acetyl-CoA and malonylo-CoA residues sequentially condense until the fatty acid chain is completed. The fatty acids are then combined with glycerol-3-phosphate, generated in the liver, to form the neutral triacylglyceroles. The insulin has effects on both the synthetic (estrification) and breakdown (lipolysis) pathways. The promotion of triacylglycerol storage in fat is one of the most important of the actions of insulin.