Glycerol 3-phosphate, adipose

The glycerol produced by the action of hormone-sensitive lipase in the adipose tissue cannot be utilized by adipose tissue itself. Adipose cells lack the enzyme glycerol kinase, which is necessary to convert glycerol to glycerol phosphate. 

Glycerol phosphate comes from glycerol (not in adipose) or from dihydroxyacetone phosphate (in liver and adipose). Nitrogen-containing phospholipids are made from diglyceride. Other phospholipids are made from phosphatidic acid. 

Pathways for production of glycerol phosphate in liver and adipose tissue. 

PA is the precursor of many other phosphoglycerides. The steps in its synthesis from glycerol phosphate and two fatty acyl CoAs were illustrated in Figure 16.14, p. 187, in which PA is shown as a precursor of triacylglycerol. [Note Essentially all cells except mature ery-. throcytes can synthesize phospholipids, whereas triacylglycerol synthesis occurs essentially only in liver, adipose tissue, lactating mammary glands, and intestinal mucosal cells.]... 

The monoacylglycerol pathway has also been demonstrated in the adipose tissue of the hamster and the rat. Under certain conditions it appears to compete with the glycerol phosphate pathway for acyl groups and may serve to regulate the activity of the latter pathway. The origin of the monoacylglycerol substrate in adipose tissue is not known. 

Glycerol is released from adipose tissue as a result of lipolysis, and only tissues such as liver and kidney that possess glycerol kinase, which catalyzes the conversion of glycerol to glycerol 3-phosphate, can utihze it. Glycerol 3-phosphate may be oxidized to dihydroxyacetone... 

Insulin stimulates lipogenesis by several other mechanisms as well as by increasing acetyl-CoA carboxylase activity. It increases the transport of glucose into the cell (eg, in adipose tissue), increasing the availability of both pyruvate for fatty acid synthesis and glycerol 3-phosphate for esterification of the newly formed fatty acids, and also converts the inactive form of pyruvate dehydrogenase to the active form in adipose tissue but not in liver. Insulin also—by its ability to depress the level of intracellular cAMP—inhibits lipolysis in adipose tissue and thereby reduces the concentration of... 

Figure 25-7. Metabolism of adipose tissue. Hormone-sensitive lipase is activated by ACTH, TSH, glucagon, epinephrine, norepinephrine, and vasopressin and inhibited by insulin, prostaglandin E, and nicotinic acid. Details of the formation of glycerol 3-phosphate from intermediates of glycolysis are shown in Figure 24-2. (PPP, pentose phosphate pathway TG, triacylglycerol FFA, free fatty acids VLDL, very low density lipoprotein.)...

Adipose tissue releases fat by activation of the hormone-sensitive lipase. The glycerol released by adipose tissue can provide some glucose equivalents to the liver. Adipose tissue itself can t use glycerol—it s missing glycerol kinase to make glycerol 3-phosphate. 

Triglycerides, the storage form of fatty acids, are formed by attaching three fatty adds (as fetty acyl CoA) to glycerol. Triglyceride formation from fatty acids and glycerol 3-phosphate occurs primarily in liver and adipose tissue. 

Reduction of dihydroxyacetone phosphate (DHAP) from glycolysis by glycerol 3-P dehydrogenase, an enzyme in both adipose tissue and liver... 

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