Riboflavin vitamin metabolic function

The water-soluble vitamins generally function as cofactors for metabolism enzymes such as those involved in the production of energy from carbohydrates and fats. Their members consist of vitamin C and vitamin B complex which include thiamine, riboflavin (vitamin B2), nicotinic acid, pyridoxine, pantothenic acid, folic acid, cobalamin (vitamin B12), inositol, and biotin. A number of recent publications have demonstrated that vitamin carriers can transport various types of water-soluble vitamins, but the carrier-mediated systems seem negligible for the membrane transport of fat-soluble vitamins such as vitamin A, D, E, and K. 

Pantothenic acid and biotin were thus found to be growth factors for yeast. Like riboflavin these molecules are incorporated into larger molecules in order to exert their essential metabolic function. Unlike the other vitamins there has been no evidence of pathological signs in man which can be attributed to dietary deficiencies in biotin or pantothenic acid. 

Several of the B vitamins function as coenzymes or as precursors of coenzymes some of these have been mentioned previously. Nicotinamide adenine dinucleotide (NAD) which, in conjunction with the enzyme alcohol dehydrogenase, oxidizes ethanol to ethanal (Section 15-6C), also is the oxidant in the citric acid cycle (Section 20-10B). The precursor to NAD is the B vitamin, niacin or nicotinic acid (Section 23-2). Riboflavin (vitamin B2) is a precursor of flavin adenine nucleotide FAD, a coenzyme in redox processes rather like NAD (Section 15-6C). Another example of a coenzyme is pyri-doxal (vitamin B6), mentioned in connection with the deamination and decarboxylation of amino acids (Section 25-5C). Yet another is coenzyme A (CoASH), which is essential for metabolism and biosynthesis (Sections 18-8F, 20-10B, and 30-5A). 

In higher mammals, riboflavin is absorbed readily from the intestines and distributed to all tis.sues. It is the precursor in the biosynthesis of the cocnzyme.s flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD). The metabolic functions of this vitamin involve these Iwocoenzymes. which participate in numerous vital oxidation-reduction proces.ses. FMN (riboflavin 5 -phosphate) is produced from the vitamin and ATP by flavokinasc catalysis. This step con be inhibited by phcnothiazincs and the tricyclic antidepressants. FAD originates from an FMN and ATP reaction that involves reversible dinucicotide formation catalyzed by flavin nucleotide pyrophosphorylase. The.se coenzymes function in combination with several enzymes as coenzyme-en-zyme complexes, often characterized as, flavoproteins. 

One has only to think of the extraordinarily varied metabolic functions of thiamine, riboflavin, pantothenic acid, pyridoxine, and biotin to realize that it is most unlikely that ascorbic acid could possibly replace every one of these. Moreover, one would have to postulate a quite different mechanism for the large number of other substances, such as sorbitol, sorbose, arabitol, and starch, which spare B vitamins even more readily than ascorbic acid, but which do not have its redox properties. 

Water-Soluble Vitamins. Vitamin G (ascorbic acid) functions in the formation of collagen, wound healing, metabolic functions, and other roles. Foods high in vitamin G include citrus fruits, strawberries, cantaloupe, and cruciferous vegetables. B vitamins are important in energy metabolism. Thiamin (Bj) is called the antineuritic vitamin. Riboflavin (B ), rarely deficient in the diet, is found most abundantly in milk and dairy products. Niacin (Bj) is prevalent in meats, poultry, fish, peanut butter, and other foods. Other major B vitamins include folic acid (B ), B, and Bj2-... 

Vitamins are a well-known group of compounds that are essential for human health. Water-soluble vitamins include folate (vitamin B9) to create DNA. Folate also plays an important role in preventing birth defects during early pregnancy. Thiamine is the first vitamin of the B-complex (vitamin Bl) that researchers discovered. It allows the body to break down alcohol and metabolize carbohydrates and amino acids. Like many other B vitamins, riboflavin (vitamin B2) helps the body to metabolize carbohydrates, proteins, and fat. Niacin (vitamin B3) protects the health of skin cells and keeps the digestive system functioning properly. Pantothenic acid (vitamin B5) and biotin allow the body to obtain energy from macronutrients such as carbohydrates, proteins, and fats. Vitamin B6 (pyridoxine) acts as a coenzyme, which means it helps chemical reactions to take place. It also plays a vital role in the creation of nonessential amino acids. 

Riboflavin or B2 functions as part of enzymes called flavoproteins (FMN and FAD) critically important in respiration and cell metabolism. It plays a major role with thiamin and niacin in oxidation-reduction reactions. The deficiency of this vitamin is characterized by photophobia, angular lip stomatitis, dermatitis, and swelling of the tongue. 

Rice bran is the richest natural source of B-complex vitamins. Considerable amounts of thiamin (Bl), riboflavin (B2), niacin (B3), pantothenic acid (B5) and pyridoxin (B6) are available in rice bran (Table 17.1). Thiamin (Bl) is central to carbohydrate metabolism and kreb s cycle function. Niacin (B3) also plays a key role in carbohydrate metabolism for the synthesis of GTF (Glucose Tolerance Factor). As a pre-cursor to NAD (nicotinamide adenine dinucleotide-oxidized form), it is an important metabolite concerned with intracellular energy production. It prevents the depletion of NAD in the pancreatic beta cells. It also promotes healthy cholesterol levels not only by decreasing LDL-C but also by improving HDL-C. It is the safest nutritional approach to normalizing cholesterol levels. Pyridoxine (B6) helps to regulate blood glucose levels, prevents peripheral neuropathy in diabetics and improves the immune function. 

Vitamins are chemically unrelated organic compounds that cannot be synthesized by humans and, therefore, must must be supplied by the diet. Nine vitamins (folic acid, cobalamin, ascorbic acid, pyridoxine, thiamine, niacin, riboflavin, biotin, and pantothenic acid) are classified as water-soluble, whereas four vitamins (vitamins A, D, K, and E) are termed fat-soluble (Figure 28.1). Vitamins are required to perform specific cellular functions, for example, many of the water-soluble vitamins are precursors of coenzymes for the enzymes of intermediary metabolism. In contrast to the water-soluble vitamins, only one fat soluble vitamin (vitamin K) has a coenzyme function. These vitamins are released, absorbed, and transported with the fat of the diet. They are not readily excreted in the urine, and significant quantities are stored in Die liver and adipose tissue. In fact, consumption of vitamins A and D in exoess of the recommended dietary allowances can lead to accumulation of toxic quantities of these compounds. 

Subsequently, the functions of the vitamin were better established and requirements for the vitamin were set. Riboflavin is an Integral part of two coenzymes, flavin-5 -phosphate (FMN) and flavin adenine dinucleotide (FAD), which function in oxidation/reductlon reactions. Indeed, riboflavin is an enzyme cofactor which is necessary in metabolic processes in which oxidation of glucose or fatty acid is used for production of adenosine triphosphate (ATP) as well as in reactions in which oxidation of amino acids is accomplished. The minimum requirement for riboflavin has been established as that amount which actually prevents the signs of deficiency. A range of intakes varying from 0.55 to 0.75 mg/day of riboflavin has been established as the minimum amount which is required to prevent appearance of deficiency signs. 

Most vitamins function either as a hormone/ chemical messenger (cholecalciferol), structural component in some metabolic process (pantothenic acid), or a coenzyme (phytonadi-one, thiamine, riboflavin, niacin, pyridoxine, biotin, folic acid, cyanocobalamin). At least one vitamin has more than one biochemical role. Vitamin A as an aldehyde (retinal) is a structural component of the visual pigment rhodopsin and, in its acid form (retinoic acid), is a regulator of cell differentiation. The precise biochemical functions of ascorbic acid and a-tocopherol still are not well defined. 

It has been shown by the author that examination of the products excreted after administration of tryptophan to vitamin-deficient animals can give valuable information on the function of that vitamin in tryptophan metabolism (142, 171, 173). When tryptophan is given to the riboflavin-deficient rat there is a large excretion of those substances which lie to the left of line BB in diagram 19 (142, 582). This clearly indicates that this is the step at which riboflavin functions, and this is strongly supported by the fact that riboflavin deficiency can reduce up to ten-fold the conversion of tryptophan to quinolinic acid, whereas similar conversion of hydroxykynurenine is unaffected (385). On the other hand, the excretory pattern... 

Riboflavin (REY-bo-FLAY-vin), commonly known as vitamin B2, is an orange-yellow crystalline solvent with a bitter taste. It is relatively stable when exposed to heat, but tends to decompose in the presence of light for extended periods of time. Riboflavin is used in the body for a variety of functions, including the metabolism of carbohydrates for the production of energy and the production of red blood cells. 

The 1930s were a golden age for the discoveries of structures and functions of other vitamins. In 1935, the laboratories of both Kuhn and Karrer reported synthesis of vitamin B2 (riboflavin, see the strucmre below). Two years earlier Warburg found a yellow oxidative enzyme in bottom yeasts and Kuhn identified it as vitamin B2. Its REDOX role in the metabolism of carbohydrates, fats, and proteins would soon be under-... 

Most media contain water-soluble B vitamins. Common to many formulations are vitamins Bi (thiamine), B2 (riboflavin), B3 (niacinamide), Bj (pantothenic acid). Be (pyridoxine), and Bg (folic acid). Biotin (vitamin H), cyanocobalamin (vitamin B]2 ), and ascorbic acid (vitamin C) are also common vitamin components. Although choline and inositol are classically grouped with vitamin components, in cell culture they function as metabolic substrates rather than as catalysts. 

From a nutritional standpoint, it is significant that five of the B-complex vitamins (riboflavin, nicotinamide, thiamine, vitamin Be, and pantothenic acid) have been shown to be constituents of the coenzymes. The nutritional requirement of these vitamins is explained on the basis of their coenzyme function. In all cases the coenzyme form appears to be the sole bound form of the vitamin, and this then becomes the only metabolically active form for these particular vitamins. 

The vitamin B complex contains a number of factors which are closely associated in their distribution in nature and have related functions in intermediate metabolism. Of the eleven factors which are available in pure form, five have been shown to be constituents of coenzymes, namely, thiamine, riboflavin, niacinamide, pyridoxine, and pantothenic acid. It seems likely that other B vitamins may be found to function in a similar manner. Two members of the B complex, choline and inositol, appear to have lipotropic activity, and two others, folic acid and vitamin B12, have antianemic properties. Deficiency of vitamins of the B complex is one of the most frequently encountered syndromes of malnutrition in man. 

Riboflavin, also called vitamin B2, is stmcturally composed of an isoafloxazine ring with a ribityl side chain at the nitrogen at position 10. This vitamin functions metabol-icafly as the essoitial component of two flavin coenzymes, flavin adenine dinucleotide (FAD) and flavin mononucleotide (FMN), complexed with proteins, which act as intmnediaries in transfers of electrons in biological oxidation-reduction reactions. Both FAD and FMN function as coenzymes for flavoproteins of flavoenzymes. Flavoproteins are essoitial for the metabolism of carbohydrates, amino acids, and lipids and for pyridoxine and folate conversion to their respective coenzyme forms. 

---