Bile Acids

The most abundant derivatives of cholesterol are bile acids. Cholesterol is important in metabolism and is biosynthesized as well as consumed in the diet. It is the precursor for vitamin D (cholecalci-ferol) and for the adrenocortical hormones, such as estrogen, androgens, and progesterone. These compounds require very small amounts of the cholesterol precursor. The sterol nucleus cannot be broken by mammalian enzymes, except for the formation of cholecalciferol. Bile acids constitute approximately 50% of the excretion products of cholesterol metabolism and perform essential functions in digestion and absorption of dietary lipids. They are synthesized in the liver (Figure 11) and exist in metabolism as conjugates with taurine and glycine (Figure 12). As with other lipids in metabolism, the amphipathic properties of the compounds characterize their functions. The planar sterol moiety with an acid group at the 24 carbon is capable of separating water and lipid interfaces and is important in facilitating interaction between lipids and enzymes in digestion. The even greater contrast of polar and neutral within the same molecule is exemplified by the conjugated bile acids. They are also active in digestion and exist in all tissues, so they may have additional functions as amphipathic facilitators between enzymes and lipids.

See also: Cholesterol: Sources, Absorption, Function and Metabolism; Factors Determining Blood Levels. Fats and Oils. Fatty Acids: Metabolism; Monounsaturated; Omega-3 Polyunsaturated; Omega-6 Polyunsaturated; Saturated; Trans Fatty Acids. Fertility. Hyperlipidemia: Overview; Nutritional Management. Lipids: Composition and Role of Phospholipids.

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