Cholesterol hormones

Steroids are the only common lipids without fatty acids. Chemically, they consist of a fused carbon ring structure consisting of four rings, three rings of which are six membered and one ring of which is five membered, as shown in Figure 16.5. Some steroids, such as cortisone, function as hormones. Cholesterol is an important component of membranes in animal cells. 

Circulating hormones, cholesterol, blood sugar Rat, dog Parenteral, oral Serum concentration... 

Steroid hormones Cholesterol is the precursor of the five major classes of steroid hormones (Table 1). The synthesis of steroid hormones is initiated by the removal of a six-carbon unit from carbon 20 of the cholesterol side chain to form pregnenolone, the common precursor of all steroid hormones (Fig. 5). A series of reactions catalyzed by cytochrome P450 modify pregnenolone to give rise to the individual hormones (Fig. 5). 

Cholesterol is synthesized from acetyl-CoA in a multistep pathway that occurs primarily in the liver. Small amounts of cholesterol are used to synthesize biologically powerful steroid hormones. Cholesterol is degraded primarily by conversion to the bile salts, which facilitate the emulsification and absorption of dietary fat. 

Vitamin D Bile acids Steroid hormones Cholesterol esters Modified proteins (hedgehog)... 

See also Steroid Metabolism, Steroid Hormones, Cholesterol Biosynthesis, Cholesterol... 

Cholesterol, with a C27 carbon skeleton (Figure 6.1), is a sterol characteristic for higher animals. It is a steroid that is present in all animal tissues as a major structural component of cellular membranes. It is the precursor of bile acids, provitamin B, and the steroid hormones. Cholesterol can be present in the free form or esterified at the hydroxyl group with fatty acids of various chain length and saturation. Cholesterol also occurs in plants, usually in very small quantities, and marine algae. 

Hepatocyte cells in the liver have been likened to ants in a colony (Tiffany-Castiglioni, 2004). Most ants are workers who perform mnltiple tasks, almost as interchangeable units. Hepatocytes perform multiple tasks as interchangeable nnits. Unlike the cells of the small intestine that do many things with very specialized cells, the Uver does many things (such as absorb nutrients and ammonia from the blood, store vitamins, release glncose, make blood proteins, hormones, cholesterol, and bile, and detoxify poisons) with jnst one kind of hepatocyte cell. This is just another example of the parallelism that exists among BU. 

Compounds with steroid structures, such as adrenocortical hormones, sex hormones, cholesterol and bile acids, occupy a central position... 

Any discussion of the biochemistry of steroids must, of necessity, center upon the many aspects of the biochemistry of cholesterol. By nature of its prevalence in the mammalian body and its central position as a precursor of bile acids and steroid hormones, cholesterol is, biochemically, the most important steroid. Because of its wide distribution and its involvement in atherosclerotic heart disease and its sequelae, cholesterol has been the most widely studied of all biologically occurring steroids. The ensuing discussion will be concerned primarily with this sterol. 

Cholesterol is the major sterol in the body and acts as both an important component of cell membranes (controlling the fluidity of the phospholipid bilayer) and also as the precursor for the synthesis of the steroid hormones. Cholesterol can be obtained from the diet in meat and dairy products, but is synthesized mainly in the liver, and then transported to other tissues of the body as LDL complexes in the plasma. All tissues are capable of cholesterol synthesis to some extent, but the liver is the only tissue for the conversion of excess cholesterol to bile salts, for excretion from the body in the bile. 

---