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NUTRIENTS

Following ingestion, flavin coenzymes are released from noncovalent attachment to proteins by gastric acid- ification and subsequent proteolysis. Nonspecific py- rophosphatases and phosphatases act on coenzyme forms to release riboflavin. Covalently bound flavin coenzymes make up about 5 percent to 10 percent of the riboflavin naturally occurring in foods, and the 8-(amino acid)- riboflavins obtained from their digestion cannot by used for resynthesis of coenzymes. Free riboflavin is actively taken up from the small intestine. Riboflavin and small amounts of riboflavin catabolites are excreted in urine.

Riboflavin is required for synthesis of flavin mono- nucleotide (FMN), which is riboflavin 5-phosphate, and flavin-adenine dinucleotide (FAD). Fully reduced forms of these coenzymes are indicated by FMNH2 FADH2

and

. Riboflavin coenzymes are involved in oxidation- reduction reactions in which the ring portion of the coen- zyme undergoes sequential addition or loss of hydrogens and electrons. Flavoproteins function in either one- or two-electron transfer reactions.

The flavin coenyzmes, FAD and FMN, function in- dispensably in oxidation-reduction reactions involved in the catabolism of glucose, fatty acids, ketone bodies, and amino acids, as well as in energy production via the res- piratory chain and in reductive biosynthetic reactions.

Inadequate dietary intake of riboflavin can result in stunting of growth, a variety of lesions involving the skin and the epithelium of the gastrointestinal tract, anemia, and neuropathy. Riboflavin has a low toxicity, perhaps because of its low solubility or ready excretion in the urine. No tolerable upper intake level has been estab- lished because of a lack of suitable data.

Thiamin

Thiamin, also known as vitamin B1 , is 3-(2-methyl-

4-aminopyrimidinyl)methyl-4-methyl-5-(-hydroxy- ethyl)thiazole. Excellent sources of thiamin include un- refined cereal germs and whole grains, meats (especially pork), nuts, and legumes. Enriched flours and grain prod- ucts in the United States contain thiamin, as well as niacin, riboflavin, iron, and folic acid.

The RDAs for thiamin are 1.2 mg of thiamin for men and 1.1 mg for women (Institute of Medicine, 1998). Typical intakes of thiamin in the United States average 1.2 to 2.0 mg per day for adults (Institute of Medicine, 1998). The recommended 1.2 mg of thiamin per day is provided by a 31 wheat bread, 12 roasted peanuts.

⁄2 ⁄3

Thiamin is released from its phosphate ester forms in which it is found in most natural foods by the action of pyrophosphatases and phosphatases in the small intes- tine. Free thiamin is absorbed by an active transport process that is probably carrier mediated. Trapping of thi- amin as thiamin pyrophosphate in the mucosal cells ap- pears to facilitate the uptake by metabolic trapping. Excess thiamin is excreted in the urine as various metabolites.

582

Raw fish may contain microbial thiaminases, which hydrolyze and, thus, destroy thiamin in the gastro- intestinal tract. Certain thiamin antagonists that are found in coffee, tea, rice bran, and heme-containing an- imal products can impair thiamin uptake or utilization. Chronic alcoholism results in impaired thiamin absorp- tion, which may be secondary to a folate deficiency. Thiamin requirements also appear to be elevated in individuals with high caloric intakes, especially when calories are derived primarily from carbohydrates, in re- nal patients undergoing long-term dialysis, in patients fed intravenously for long periods, and in patients with chronic febrile infections.

Thiamin is required for synthesis of thiamin py- rophosphate (TPP), which is also known as thiamin diphosphate (TDP); this may be the sole coenzyme form of thiamin. However, monophosphate and triphosphate esters occur naturally, and thiamin triphosphate has been implicated in nerve function. TPP functions in two gen- eral types of reactions in which TPP functions as a Mg2

-

coordinated coenzyme for “active aldehyde transfers.” First, TTP is a coenzyme for the oxidative decarboxy- lation of -keto acids (catalyzed by the pyruvate, - ketoglutarate, and branched-chain keto acid dehydroge- nase complexes). Second, TPP is required as a coenzyme for transketolase, which catalyzes sugar rearrangements in the pentose phosphate pathway of glucose metabolism.

Thiamin deficiency, or beriberi, affects the nervous and cardiovascular systems. Clinical symptoms include mental confusion, anorexia, muscular weakness, ataxia, peripheral paralysis, paralysis of the motor nerves of the eye, edema, muscle wasting, tachycardia, and an enlarged heart. In Western countries, symptomatic thiamin defi- ciency is usually observed only in association with alco- holism.

No toxic effects of thiamin administered by mouth have been reported in humans, and thiamin is readily cleared by the kidneys. Injection of doses of thiamin that are more than 200 times those required for optimal nu- trition produces a variety of pharmacological effects and can even induce death because of depression of the res- piratory center. No tolerable upper intake level has been established for thiamin because of a lack of sufficient data.

Vitamin B12

Vitamin B12

-ounce pork chop, 20 slices of whole cups of pecan halves, or 17 ounces of

, or cobalamin, consists of a central cobalt atom coordinately linked to the four pyrrole nitrogens of a heme-like planar corrin ring structure. The 5th

coordi-

nate bond of cobalt is to one of the nitrogens in a phos- phoribo-5,6-dimethylbenzimidazolyl side group of the corrin ring structure, and the 6th

coordinate bond of

cobalt can be occupied by a number of ligands. In vita-

min B12

preparations, this ligand is typically a cyano group that is formed by trace amounts of cyanide during purification of the vitamin from natural sources.

Vitamin B12

is synthesized by some anaerobic mi- croorganisms and by some algae, such as seaweed. Most

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