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NAD or nicotinamide adenine dinucleotide (Coenzyme 1) Figure 18.12

Niacinamide is incorporated into molecules of coenzyme I.

In developed nations, beriberi mainly occurs in people with chronic alcoholism who have substituted alcohol for foods. Moreover, since thiamine is required for the metabolic oxidation of alcohol, people with alcoholism are particularly likely to develop a thiamine deficiency.

2. Riboflavin, or vitamin B2. Riboflavin is a yellowish brown crystalline substance that is relatively stable to the effects of heat, acids, and oxidation but is destroyed by exposure to bases and ultraviolet light. This vitamin is part of several enzymes and coenzymes that are known as flavoproteins. One such coenzyme, FAD, is an electron carrier in the citric acid cycle and electron transport chain of aerobic respiration. Flavoproteins are essential for the oxidation of glucose and fatty acids and for cellular growth. The absorption of riboflavin is regulated by an active transport system that controls the amount entering the intestinal mucosa. Riboflavin is carried in the blood combined with blood proteins called albumins. Excess riboflavin in the blood is excreted in the urine, and any that remains unabsorbed in the intestine is lost in the feces.

The amount of riboflavin the body requires varies with caloric intake. About 0.6 mg of riboflavin per 1,000 calories is sufficient to meet daily cellular requirements.

Riboflavin is widely distributed in foods, and rich sources include meats and dairy products. Leafy green vegetables, whole-grain cereals, and enriched cereals provide lesser amounts. Vitamin B2 deficiency produces dermatitis and blurred vision.

3. Niacin. Niacin, which is also known as nicotinic acid, occurs in plant tissues and is stable in the presence of heat, acids, and bases. After ingestion, it is converted to a physiologically active form called niacinamide (fig. 18.11). Niacinamide is the form of niacin that is present in foods of animal origin.

Niacin functions as part of two coenzymes (coenzyme I, also called NAD [fig. 18.12], and coenzyme II, called NADP) that play essential roles in the oxidation of glucose, acting as electron carriers in glycolysis, the citric acid cycle, and the electron transport chain, as well as in the synthesis of proteins and fats. These coenzymes are also required for the synthesis of the sugars (ribose and deoxyribose) that are part of nucleic acids.

Niacin in large doses is prescribed as a drug to lower serum cholesterol.

Niacin is readily absorbed from foods, and human cells synthesize it from the essential amino acid tryptophan. Consequently, the daily requirement for niacin varies with tryptophan intake. Nutritionists recommend a daily niacin (or niacin equivalent) intake of 6.6 mg per 1,000 calories.

Rich sources of niacin (and tryptophan) include liver, lean meats, peanut butter, and legumes. Milk is a poor source of niacin but a good source of tryptophan.

Historically, niacin deficiencies have been associated with diets largely consisting of corn and corn products, which are very low in niacin and lack adequate tryptophan. Such a deficiency causes a disease called pellagra that produces dermatitis, inflammation of the digestive tract, diarrhea, and mental disorders.

Although pellagra is relatively rare today, it was a serious problem in the rural South of the United States in the early 1900s. Pellagra is less common in cultures that extensively use corn treated with lime (CaCO3) to release niacin from protein binding in corn. It sometimes occurs in people with chronic alcoholism who have substituted alcohol for foods.

4. Pantothenic acid, or vitamin B5. Pantothenic acid is a yellowish oil that is destroyed by heat, acids, and bases. It functions as part of a complex molecule called coenzyme A, which, in turn, reacts with intermediate products of carbohydrate and fat metabolism to become acetyl coenzyme A, which enters the citric acid cycle. Pantothenic acid is therefore essential to cellular energy release.

A daily adult intake of 4-7 mg of pantothenic acid is adequate. Most diets provide sufficient amounts, since deficiencies are rare, and no clearly defined set of deficiency symptoms is known. Good sources of pantothenic acid include meats, whole-grain cereals, legumes, milk, fruits, and vegetables.

5. Vitamin B6. Vitamin B6 is a group of three compounds that are chemically similar, as figure 18.13 shows. They are called pyridoxine, pyridoxal, and pyridoxamine. These compounds have similar actions and are fairly stable in the presence of heat and acids. Oxidation or exposure to bases or ultraviolet light destroys them. The vitamin B6 compounds function as coenzymes that

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