Steroids Cholesterol, Bile Salts, and Steroid Hormones

6 Steroids Cholesterol, Bile Salts, and Steroid Hormones 

Cholesterol, which is one of the most important and abnndant steroids in the body, is a sterol because it contains an oxygen atom as a hydroxyl gronp (—OH) on carbon 3. Like many steroids, cholesterol has a double bond between carbon 5 and carbon 6, methyl gronps at carbon 10 and carbon 13, and a carbon chain at carbon 17. In other steroids, the oxygen atom forms a carbonyl group (C=0) at carbon 3. 

Cholesterol is a component of cellular membranes, myelin sheath, and brain and nerve tissue. It is also found in the liver and bile salts large quantities of it are found in the skin, and some of it becomes vitamin D when the skin is exposed to direct sunlight. In the adrenal gland, cholesterol is used to synthesize steroid hormones. The liver synthesizes sufficient cholesterol for the body from fats, carbohydrates, and proteins. Additional cholesterol is obtained from meat, milk, and eggs in the diet. There is no cholesterol in vegetable and plant products. 

FIGURE 15.7 Excess cholesterol forms plaque that can block an artery, resulting in a heart attack, (a) A cross section of a normal, open artery shows no buildup of plaque, (b) A cross section of an artery that is almost completely clogged by atherosclerotic plaque. What property of cholesterol would cause it to form deposits along the coronary arteries  

Refer to the structure of cholesterol for each of the following questions  

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Waxes and Triacylglycerols Chemical Properties of Triacylglycerols Phospholipids Steroids Cholesterol, Bile Salts, and Steroid Hormones Cell Membranes... 

The third class of lipids is steroids. Included in this category of lipids are cholesterol, bile salts, and sex hormones. Steroid structures contain fused rings consisting of three six-carbon rings and a five-carbon ring ... 

XI. Cholesterol, Bile Salt, and Steroid Hormone Metabolism... 

XI. Cholesterol, bile salt, and steroid hormone metabolism 214... 

Most of the lipids found in the body fall into the categories of fatty acids and triacylglycerols glycerophospholipids and sphin-golipids eicosanoids cholesterol, bile salts, and steroid hormones and fat-soluble vitamins. These lipids have very diverse chemical structures and functions. However, they are related by a common property their relative insolublity in water. 

The naturally occurring steroids include the bile salts and various hormones. Steroids are also present as membrane constituents. They are derived from cholesterol, which is present in appreciable amounts in normal diets. 

We turn now to the biosynthesis of lipid structures. We begin with a discussion of the biosynthesis of fatty acids, stressing the basic pathways, additional means of elongation, mechanisms for the introduction of double bonds, and regulation of fatty acid synthesis. Sections then follow on the biosynthesis of glyc-erophospholipids, sphingolipids, eicosanoids, and cholesterol. The transport of lipids through the body in lipoprotein complexes is described, and the chapter closes with discussions of the biosynthesis of bile salts and steroid hormones. 

Important Derivatives of Cholesterol Inelude Bile Salts and Steroid Hormones... 

The class of lipids includes cholesterol, the bile salts and steroid hormones (which are derived from cholesterol), the fat-soluble vitamins, and compounds such as the prostaglandins. 

Bile salts and steroid hormones are derived from cholesterol (see Figure 6-18). 

Cholesterol is one of the most highly recognized molecules in human biology, in part because of a direct relationship between its concentrations in blood and tissues and the development of atherosclerotic vascular disease Cholesterol, which is transported in the blood in Upoproteins because of its absolute insolubility in water, serves as a stabilizing component of cell membranes and as a precursor of the bile salts and steroid hormones. Precursors of cholesterol are converted to ubiquinone, dolichol, and, in the skin, to cholecalciferol, the active form of vitamin D. As a major component of blood lipoproteins, cholesterol can appear in its free, unesterifiedform in the outer shell of these macromolecules and as cholesterol esters in the lipoprotein core. 

Cholesterol, the most abundant steroid, is an important constituent of cell membranes and the starting material for biosynthesis of other steroids. This includes the sex hormones, such as progesterone, estradiol, testosterone, and androsterone, adrenocortical hormones such as aldosterone and cortisone, bile salts, and vitamin D. 

Steroids are componnds that have four rings fnsed together in a specific way. The most abundant steroid in humans is cholesterol, which serves as a starting material for other important steroids snch as bile salts, adrenocor-ticoid hormones, and sex hormones. 

Cholesterol is readily synthesized from acetyl-CoA (page 231) and is the parent substance for the other natural steroids these include cholic acid which is a constituent of the bile salts, the sex hormones and the adrenocortical hormones. In addition, a derivative of cholesterol, namely 7-dehydrocholesterol, is present in the skin and this, on irradiation with ultraviolet light, is converted to vitamin D which is a steroid derivative (page 156). 

Biosynthesis represents the major metabolic fate of cholesterol, accounting for more than half of the 800 mg/day of cholesterol that the average adult uses up in metabolic processes. By comparison, steroid hormone biosynthesis consumes only about 50 mg of cholesterol per day. Much more that 400 mg of bile salts is required and secreted into the intestine per day, and this is achieved by recycling them. Most of the bile salts secreted into the upper region of the small intestine are absorbed along with the dietary lipids that they emulsified at the lower end of the small intestine. They are... 

Cholesterol is a soft waxy substance that is a steroidal alcohol or sterol. It is the most abundant steroid in the human body and is a component of every cell. Cholesterol is essential to life and most animals and many plants contain this compound. Cholesterol biosynthesis occurs primarily in the liver, but it may be produced in other organs. A number of other substances are synthesized from cholesterol including vitamin D, steroid hormones (including the sex hormones), and bile salts. Cholesterol resides mainly in cell membranes. 

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