Biological Chemistry - Berezov T. T., Korovkin B. F. 1998

Lipid Metabolism
Adipose tissue and its role in lipid metabolism

The total amount of adipose tissue in an average-weight adult male is approximately 20 kg, while in obese individuals it can be tens of kilograms greater. Adipose tissue, consisting primarily of fat Cells, or adipocytes, is distributed throughout the body: subcutaneously, in the Abdominal cavity, and forming protective fat layers around individual Organs. About 65% of the mass of adipose tissue accounts for stored triglycerides, which constitutes roughly 95% of all triglycerides in the body.

It is well established that the primary source of Fatty acids used as "fuel" is the reserve fat contained within adipose tissue. It is generally accepted that triglycerides in fat depots play a role in Lipid METABOLISM similar to that of Glycogen in the Liver for Carbohydrate Metabolism, while Higher Fatty Acids are functionally analogous in their energy yield to glucose generated during glycogen phosphorolysis. During physical exertion and other physiological states requiring increased Energy Expenditure, the utilization of adipose tissue triglycerides as an energy reserve rises accordingly.

Triglyceride lipolysis in adipose tissue*. Only free, i.e., unesterified, fatty acids can serve as an energy source. Therefore, triglycerides are first hydrolyzed by specific tissue Enzymes—lipases—into glycerol and free fatty acids. The latter can pass from fat depots into Blood Plasma (mobilization of higher fatty acids), after which they are utilized by body Tissues and organs as an energy substrate.

Adipose tissue contains several lipases, of which the most important are triglyceride lipase (the so-called hormone-sensitive lipase), diglyceride lipase, and monoglyceride lipase. The activity of the latter two enzymes is 10–100 times higher than that of the first. Triglyceride lipase is activated by A number of Hormones (e.g., adrenaline, noradrenaline, Glucagon, etc.), whereas diglyceride lipase and monoglyceride lipase are insensitive to their action. Triglyceride lipase Functions as a regulatory enzyme.

It has been established that hormone-sensitive lipase (triglyceride lipase) exists in adipose tissue in an inactive form, and its activation by hormones proceeds via a complex cascade pathway involving at least two enzymatic systems. The process begins with the Interaction of a hormone with a cellular receptor, resulting in a Modification of the receptor Structure (the hormone itself does not enter The Cell), and this activated receptor subsequently stimulates adenylate cyclase (EC 4.6.1.1). The latter, as is known, catalyzes The formation of cyclic adenosine monophosphate (cAMP) from adenosine triphosphate (ATP):

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The generated cAMP activates the enzyme protein kinase (EC 2.7.1.37), which converts inactive triglyceride lipase into its active form via phosphorylation (Fig. 11.1). Active triglyceride lipase breaks down triglycerides into diglycerides and fatty acids. Subsequently, the action of di- and monoglyceride lipases yields the End products of lipolysis—glycerol and free fatty acids—which enter the bloodstream.

The rate of triglyceride lipolysis is not constant; it is subject to the regulatory influence of various factors, among which neurohormonal factors are of particular importance (Table 11.1).

Bound to blood plasma albumins as a complex, free Fatty acids are transported by the bloodstream to organs and tissues, where the complex dissociates and the fatty acids either undergo ß-Oxidation or are partially utilized for the synthesis of triglycerides, Glycerophospholipids, sphingophospholipids, and Other Compounds, as well as for Cholesterol Esterification.

* In fat cells (adipocytes), alongside active triglyceride lipolysis, several other metabolic processes take place, including Glycolysis, glucose oxidation via the Pentose Phosphate Pathway, The Tricarboxylic Acid Cycle, fatty acid ß-oxidation, fatty acid synthesis, fatty acid re-esterification (triglyceride formation), and fatty acid mobilization (release).

Fig. 11.1. Lipolytic cascade (according to Steinberg).

TG — triglycerides; DG — diglycerides; MG — monoglycerides; GL — glycerol; FA — fatty acids.

Table 11.1. Effect of various factors on triglyceride lipolysis in adipose tissue [Klimov A.N., Nikulcheva N.G., 1995]