Cholesterol steroid hormones and

Steroids are a femily of lipid molecules that includes cholesterol, steroid hormones, and bile salts. These amphipathic molecules (containing both hydrophobic and hydrophilic regions) are derived from two-carbon acetyl-CoA... 

Steroid A hp having a four-ring structure, includes cholesterol, steroid hormones, and vitamin D. 

Acetyl-CoA is the source of the carbon atoms for the synthesis of cholesterol, steroid hormones, and fatty acids. Various enzymes catalyze biological versions of aldol reactions and Claisen condensations during the syntheses of these compounds. 

A good example of the occurrence of these types of ring systems is in the steroids, all of which contain a carbon skeleton consisting of 3 six-membered rings and 1 five-membered ring connected as shown here. This ring system is present in both animal and plant steroids. Steroids are present in human metabolism as cholesterol, steroid hormones, and bile acids (Section 26.4). 

Carbonyl condensations are among the most widely used reactions in the biological world for the assembly of new carbon-carbon bonds. One source of carbon atoms for the synthesis of biomolecules is acetyl-CoA, a thioester of acetic acid and the thiol group of coenzyme A (Problem 25.34). In this section, we examine the series of reactions by which the carbon skeleton of acetic acid is converted to isopentenyl pyrophosphate, a key intermediate in the synthesis of terpenes, cholesterol, steroid hormones, and bile acids. Note that in the discussion that follows, we will not be concerned with the mechanism by which each of these enzyme-catalyzed reactions occurs. Rather, our concern is in recognizing the types of reactions that take place. 

It detoxicates and excretes many substances including bilirubin, cholesterol, steroid hormones and drugs. 

In the early 1930 s, when the prime research aim was the commercial synthesis of the sex hormones (whose structures had just been elucidated), the principal raw material available was cholesterol extracted from the spinal cord or brain of cattle or from sheep wool grease. This sterol (as its 3-acetate 5,6-dibromide) was subjected to a rather drastic chromic acid oxidation, which produced a variety of acidic, ketonic and hydroxylated products derived mainly by attack on the alkyl side-chain. The principal ketonic material, 3j -hydroxyandrost-5-en-17-one, was obtained in yields of only about 7% another useful ketone, 3 -hydroxypregn-5-en-20-one (pregnenolone) was obtained in much lower yield. The chief acidic product was 3j -hydroxy-androst-5-ene-17j -carboxylic acid. All three of these materials were then further converted by various chemical transformations into steroid hormones and synthetic analogs ... 

Cholesterol is a widely distributed sterol found free or esterified to fatty acids. It is an important intermediate in the biosynthesis of steroid hormones and the principal component of cell plasma membranes and the membranes of intracellular organelles. 

Cholesterol is the parent of all of the steroid hormones and bile acids in the... 

Cytochrome P450 pathways in human brain cholesterol, steroid hormones, vitamin D, and bile... 

Many steroids have an alcoholic hydroxyl attached to the ring system, and are known as sterols. The most common sterol is cholesterol, which occurs in most animal tissues. There are many different steroid hormones, and cholesterol is the precursor for all of them. Cholesterol is also the precursor of vitamin D. 

Cholesterol is an important structural component of cellular membranes, where it plays a role in modulating membrane fluidity and phase transitions, and, together with sphingomyelin, forms lipid rafts or caveolae, which are sites where proteins involved in diverse signaling pathways become concentrated. Furthermore, cholesterol is a precursor of oxysterols, steroid hormones, and bile acids. 

Cholesterol is doubtless the most publicized lipid, notorious because of the strong correlation between high levels of cholesterol in the blood and the incidence of human cardiovascular diseases. Less well advertised is cholesterol s crucial role as a component of cellular membranes and as a precursor of steroid hormones and bile acids. Cholesterol is an essential molecule in many animals, including humans, but is not required in the mammalian diet—all cells can synthesize it from simple precursors. 

Steroids are tetracyclic hydrocarbons that are much more common in eukaryotes than in prokaryotes. Cholesterol, the most prominent member of the steroid family, is an important component of many eukaryotic membranes (see chapter 17). In addition, it is the precursor of the other two major classes of steroids The steroid hormones and the bile acids (fig- 20.1). 

Most hormones fall into three classes Polypeptides, steroids, and amino acid derivatives. Polypeptide hormones are synthesized from large precursors. Steroid hormones are derivatives of cholesterol. Thyroid hormones and epinephrine are amino acid derivatives. 

The final chapter in part 5, chapter 20, Metabolism of Cholesterol, deals with the synthesis of cholesterol and some of its derivatives, the steroid hormones and the bile acids. This chapter considers the structure, function and metabolism of these molecules. Also, the health-related concerns associated with cholesterol excess are addressed. 

Cholesterol is a component of cell membranes and is the precursor of steroid hormones and the bile salts. 

The foregoing discussion has attempted to trace the ways in which cholesterol, derived from plasma lipoproteins, is converted into the various steroid hormones and how these are secreted back into the blood. Of necessity, many details have had to be omitted but it is hoped that this up-date has shown the complexities of steroid biosynthetic pathways and that earlier classical ideas have had to be modified as greater knowledge of intermediates, isoenzymes and multiple forms of cyt P-450s has become available. Perspectives for future studies are indeed exciting. 

Cholesterol (Figure 6.12) is a very common starting point for making many interesting compounds essential to our bodies.3 These include bile acids (which help dissolve cholesterol in ingested food), steroid hormones and vitamin D (note some... 

HMG-CoA reductase is the rate-limiting step of cholesterol biosynthesis, and is subject to complex regulatory controls. A relatively constant level of cholesterol in the body (150-200 mg/dl) is maintained primarily by controlling the level of de novo synthesis. The level of cholesterol synthesis is regulated in part by the dietary intake of cholesterol. Cholesterol from both diet and synthesis is utilised in the formation of membranes and in the synthesis of the steroid hormones and bile acids. The greatest proportion of cholesterol is used in bile acid synthesis. 

Mammalian nuclear receptors for steroid hormones (SHRs) are of great importance in physiology and medicine, because they control not only developmental pathways but also regulate central physiological and metabolic functions in the adult organism. Steroid hormones and vitamin D are derivatives of cholesterol. Structures of vitamin D3 and of... 

Lipids represent a diverse group of compounds, which are readily soluble in organic solvents such as chloroform or toluene and are essential for the structure and function of ah hving ceUs. Lipids include oils, fatty acids, waxes, steroids (e.g., cholesterol and steroid hormones), and other related compounds. Specihc classes of lipids are the primary structural components of membranes, provide sources of metabolic fuel, and function as bioactive signaling molecules. 

The broad definition of lipids allows the inclusion of cholesterol, a molecule which contains many hydrocarbon groups and is not too soluble in water. Cholesterol contains a skeleton sterol ring (fig. 4.1), and is the precursor of other sterol-pontaining molecules, including the steroid hormones and bile acids (H-11). Cholesterol is ingested in the diet but may also be synthesized. All the carbons of cholesterol come from acetyl CoA. Acetyl CoA units... 

As cholesterol is used in cell membranes and in the formation of the steroid hormones, and the liver is the most important source of cholesterol synthesis, the liver exports much choiesterol in lipoprotein trams that carry cholesterol (as well as triglycerides) to other areas of the body, where it is used in further syntheses. The liver also exports choiesterol and bile salts into the bile. 

Hydroxylation reactions play a very important role in the synthesis of cholesterol from squalene and in the conversion of cholesterol into steroid hormones and hile salts. All these hydroxylations require NADPH and O. The oxygen atom of... 

Figure 26.24. Biosynthetic Relations of Classes of Steroid Hormones and Cholesterol.

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