Triacylglycerols are stored in adipose tissue in large lipid droplets and are continually undergoing lipolysis (hydrolysis) and reesterification. These two processes are entirely different pathways involving different reactants and enzymes. This allows the processes of esterification or lipolysis to be regulated separately by many nutritional, metabolic, and hormonal factors. The balance between these two processes determines the magnitude of the FFA pool in adipose tissue, which in turn determines the level of FFA circulating in the plasma. Since the latter has most profound effects on the metabolism of other tissues, particularly liver and muscle, the factors operating in adipose tissue that regulate the outflow of FFA exert an influence far beyond the tissue itself. Moreover, since the discovery in the last 20 years that adipose tissue secretes hormones such as leptin and adiponectin, known as adipokines, its role as an endocrine organ has been recognized. Leptin regulates energy homeostasis by stimulating energy use and limiting food intake. If it is lacking, food intake may be uncontrolled, causing obesity. Adiponectin modulates glucose and lipid metabolism in muscle and liver, and enhances the sensitivity of tissues to insulin.

The Provision of Glycerol-3-Phosphate Regulates Esterification: Lipolysis Is Controlled by Hormone-Sensitive Lipase

 Triacylglycerol is synthesized from acyl-CoA and glycerol 3-phosphate. Since the enzyme glycerol kinase is not expressed in adipose tissue, glycerol cannot be utilized for the provision of glycerol-3-phosphate, which must be supplied from glucose via glycolysis (Figure1).

Fig1. Triacylglycerol metabolism in adipose tissue. Hormone-sensitive lipase is activated by ACTH, TSH, glucagon, epinephrine and norepinephrine, and inhibited by insulin, prostaglandin E1 , and nicotinic acid.. (FFA, free fatty acids; PPP, pentose phosphate pathway; TG, triacylglycerol; VLDL, very-low-density lipoprotein.)

Triacylglycerol undergoes hydrolysis by a hormone sensitive lipase to form FFA and glycerol. This lipase is distinct from lipoprotein lipase, which catalyzes lipoprotein triacyl glycerol hydrolysis before its uptake into extrahepatic tissues (see earlier). Since the glycerol cannot be utilized, it enters the blood and is taken up and transported to tissues such as the liver and kidney, which possess an active glycerol kinase. The FFA formed by lipolysis can be reconverted in adipose tis sue to acyl-CoA by acyl-CoA synthetase and reesterified with glycerol-3-phosphate to form triacylglycerol. Thus, there is a continuous cycle of lipolysis and reesterification within the tissue (see Figure 1). However, when the rate of reesterification is not sufficient to match the rate of lipolysis, FFAs accumulate and diffuse into the plasma, where they bind to albumin and raise the concentration of plasma-free fatty acids.

Increased Glucose Metabolism Reduces the Output of FFA

 When the utilization of glucose by adipose tissue is increased, the FFA outflow decreases. However, the release of glycerol continues, demonstrating that the effect of glucose is not mediated by reducing the rate of lipolysis. The effect is due to the increased provision of glycerol-3-phosphate, which enhances esterification of FFA. Glucose can take several pathways in adipose tissue, including oxidation to CO2 via acetyl-CoA and the citric acid cycle, oxidation in the pentose phosphate pathway, conversion to long-chain fatty acids (via acyl-CoA), and formation of acylglycerol via glycerol-3-phosphate (see Figure 1). When glucose utilization is high, a larger proportion of the uptake is oxidized to CO2 and converted to fatty acids. However, as total glucose utilization decreases, the greater proportion of the glucose is directed to the production of glycerol-3-phosphate for the esterification of acyl-CoA to form triacylglycerol, which helps to minimize the efflux of FFA.

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مواضيع ذات صلة

Brown Adipose Tissue Promotes Thermogenesis

Hormones Regulate Fat Mobilization

Classification of Proteins

Precipitation Reactions of Proteins

Study of Protein Structure

Imbalance in the Rate of Triacylglycerol Formation & Export Causes Fatty Liver

The Liver Plays A Central Role in Lipid Transport & Metabolism

HDL Takes Part in Both Lipoprotein Triacylglycerol & Cholesterol Metabolism

Tertiary Structure of Proteins

Secondary Structure of Proteins

Primary Structure of Proteins

Chylomicrons & Very-low Density Lipoproteins are Rapidly Catabolized