Digestion hormones Digestive molecules

Dietary triglycerides, 226, 319-320, 366-367 Digestion, see also AbsorpOon Digestive hormones Digestive molecules Digestive system carbohydrates, 10(3-116 control, 58,88 Cephalic phase, 66-68 chemical phase, 69 innerv ation, 58,66-69 dietary bbers. 143 lipids, 93-103... 

Biochemistry is carbonyl chemistiy. Almost all metabolic pathways used by living organisms involve one or more of the four fundamental carbonvl-group reactions we ve seen in Chapters 19 through 23. The digestion and metabolic breakdown of all the major classes of food molecules—fats, carbohydrates, and proteins—take place by nucleophilic addition reactions, nucleophilic acyl substitutions, a substitutions, and carbonyl condensations. Similarly, hormones and other crucial biological molecules are built up from smaller precursors by these same carbonyl-group reactions. 

Albumin 18-20 Maintains plasma oncotic pressure transports small molecules Dehydration, anabolic steroids, insulin, infection Overhydration, edema, kidney insufficiency, nephrotic syndrome, poor dietary intake, impaired digestion, burns, congestive heart failure, cirrhosis, thyro id/adrena / pitu itary hormones, trauma, sepsis... 

Cholesterol also serves as a precursor to other important molecules. Bile acids aid in lipid absorption during digestion. Steroid hormones all derive from cholesterol, including the adrenal hormones that maintain fluid balance Vitamin D, which is an important regulator of calcium status and the male and female sex hormones. Although humans wouldn t survive in one sense or another without cholesterol metabolites, cholesterol brings with it some well-known side effects. Doctors find cholesterol derivatives, being essentially insoluble in water, in the deposits (plaque) that characterize diseased arteries. 

In addition to growth factors, it has been shown that countless other molecules have multiple functions. Cholecystokinin, for example, is a peptide that acts as a hormone in the intestine, where it increases the bile flow during digestion, whereas in the nervous system it behaves as a neurotransmitter. Encephalins are sedatives in the brain, but in the digestive system are hormones that control the mechanical movements of food. Insulin is universally known for lowering the sugar levels in the blood, but it also controls fat metabolism and in other less known ways it affects almost every cell of the body. 

In addition to Its structural role In membranes, discussed In Chapter 5, cholesterol Is the precursor for several Important bloactive molecules. They Include bile acids (see Figure 18-6), which are made In the liver and help emulsify dietary fats for digestion and absorption In the Intestines, steroid hormones produced by endocrine cells (e.g., adrenal gland, ovary, testes), and vitamin D produced In the skin and kidneys. Arthropods need cholesterol or other sterols to produce membranes and ecdysterold hormones, which control development however, they cannot make the precursor sterols themselves and must obtain these compounds In their diet. Another critical function of cholesterol Is Its covalent addition to Hedgehog protein, a key signaling molecule In embryonic development (Chapter 15). 

Steroids are a naturally occurring family of organic molecules of biochemical and medical interest. A great deal of controversy has surrounded various steroids. We worry about the amount of cholesterol in the diet and the possible health effects. We are concerned about the use of anabolic steroids by athletes wishing to build muscle mass and improve their performance. However, members of this family of molecules derived from cholesterol have many important functions in the body. The bile salts that aid in the emulsification and digestion of lipids are steroid molecules, as are the sex hormones testosterone and estrone. 

The digestion of growth hormones with carboxypep-tidase, trypsin, and chymotrypsin yields undialyzable polypeptides that maintain full activity thus suggesting that the large molecule contains multiple active centers. Such a finding does not simplify the identification of the active center indeed, the product of the chymotrypsin and trypsin hydrolysis splits at least 30% of the peptide bonds to yield a complex population of polypeptides that are difficult to separate. Further chemical studies have established that the tyrosine side chain and the s-amino groups of lysine are required for growth hormone activity. 

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