Triacylglycerol movement

Muscle Fast twitch Slow twitch Rapid movement Sustained movement Glycolysis Aerobic pathways, eg, p-oxidation and citric acid cycle Glucose Ketone bodies, triacylglycerol in VLDL and chylomicrons, free fatty acids Lactate Lipoprotein lipase. Respiratory chain well developed. 

The major role of skeletal muscle is movement, which is described and discussed in Chapter 13). Nevertheless, since muscle comprises 40% of the body it is large enough to play a part in control of the blood concentrations of the major fuels glucose, fatty acids, triacylglycerol and some amino acids. Skeletal muscle contains the largest quantity of protein in the body, which is used as a source of amino acids under various conditions (e.g. starvation, trauma, cancer see above). It plays an important part in the metabolism, in particular, of branched-chain amino acids, glutamine and alanine, which are important in the overall metabolism of amino acids in the body (discussed below). 

Chylomicrons, discussed in Chapter 17 in connection with the movement of dietary triacylglycerols from the intestine to other tissues, are the largest of the lipoproteins and the least dense, containing a high... 

Figure 21-1 Movement of triacylglycerols from liver and intestine to body cells and lipid carriers of blood. VLDL very low density lipoprotein which contains triacylglycerols, phospholipids, cholesterol, and apolipoproteins B, and C. IDL intermediate density lipoproteins found in human plasma. LDL low density lipoproteins which have lost most of their triacylglycerols. ApoB-100, etc., are apolipoproteins listed in Table 21-2. LCAT, lecithin cholesterol acyltransferase CETP, cholesteryl ester transfer protein (see Chapter 22).

A second lipothrophic factor is betaine, which is effective because the transfer of at least one of its methyl groups to homocysteine is very efficient and can replenish methionine for choline formation. In the absence of sufficient lipotrophic factors, a fatty liver develops, and there is insufficient movement of fats either ingested or synthesized in the liver to the adipose tissue. As fats enter or are synthesized in the liver, they are repackaged or packaged as VLDLs to be moved out for transport from the blood to adipose tissue. The VLDLs contain protein, triacylglycerol, cholesterol, cholesterol esters, and phospholipids, especially phosphatidylcholine (lecithin). If one has either a protein deficiency or a lipotrophic factor deficiency, the movement of triacylglycerol s from the liver to adipose is ineffective and a fatty liver can develop. Choline can be present in the diet and need not be synthesized de novo. Phospholipid synthesis has been discussed previously (Chapter 15). 

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