Vitamin riboflavin

Riboflavin (vitamin B2 6.18) consists of an isoalloxazine ring linked to an alcohol derived from ribose. The ribose side chain of riboflavin can be modified by the formation of a phosphoester (forming flavin mononucleotide, FMN, 6.19). FMN can be joined to adenine monophosphate to form flavin adenine dinucleotide (FAD, 6.20). FMN and FAD act as co-enzymes by accepting or donating two hydrogen atoms and thus are involved in redox reactions. Flavoprotein enzymes are involved in many metabolic pathways. Riboflavin is a yellow-green fluorescent compound and, in addition to its role as a vitamin, it is responsible for the colour of milk serum (Chapter 11). 

Milk is a good source of riboflavin whole milk contains about 0.17 mg per 100 g. Most (65-95%) of the riboflavin in milk is present in the free form the remainder is present as FMN or FAD. Milk also contains small amounts (about 11% of total flavins) of a related compound, 10-(2 -hydroxyethyl) flavin, which acts as an antivitamin. The concentration of this compound must be considered when evaluating the riboflavin activity in milk. The concentration of riboflavin in milk is influenced by the breed of 

COW (milk from Jersey and Guernsey cows contains more riboflavin than Holstein milk). Summer milk generally contains slighly higher levels of riboflavin than winter milk. Interspecies variations in concentration are also apparent. Raw sheep s milk contains about 0.32 mg per 100 g while the mean value for pasteurized goats milk (0.13 mg per 100 g) is lower human milk contains 0.03 mg per 100 g. Dairy products also contain significant amounts 

Riboflavin (vitamin B ) plays an essential role in cellular metabolism, being the precursor of the co-enzymes flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD) that both act as hydrogen carriers in biological redox reactions involving enzymes such as NADH dehydrogenase. Once riboflavin is absorbed in the human body, the synthesis of these flavin co-enzymes is controlled by thyroid hormones that regulate the activities of the flavin biosynthetic enzymes (Rucker et al. 2001). 

The recommended riboflavin requirements for humans vary with respect to sex, age, and physiological state (such is the case during pregnancy and lactation). Normal adults need to consume between 0.9 and 1.6 mg of this vitamin on a daily basis since the human body does not have deposits of riboflavin and an excess of vitamin intake is eliminated in urine (Institute of Medicine 1998). Although present in a wide variety of foods, riboflavin deficiency (ariboflavinosis) still occurs in both developing and industrialized countries (O Brien et al. 2001 Blanck et al. 2002). Even though severe cases of ariboflavinosis are not common in most societies, subclinical manifestations are frequent and these are only detectable by measuring the blood vitamin concentrations. 

Dairy products are not considered a good source of this essential vitamin since milk contains approximately 1.2 mg of riboflavin per liter, meaning that an average adult would need to consume about one liter per day to meet the daily requirement. This level of milk consumption is not normal amongst residents of industrialized countries such as the USA, where the daily pa- capita consumption of fresh milk is approximately 200 ml (Putman and AUshouse 2003). Thus, increasing the levels of riboflavin in milk is an interesting proposal to prevent deficiencies in populations where dairy consumption is low. 

Riboflavin biosynthesis has been described both in Gram (-I-) and Gram (-) bacteria, with more detail in Bacillus (B.) subtilis (Perkins and Pero 2002) and Escherichia coli (Bacher et al. 1996). 

Microbial biosynthesis of riboflavin from the precursor guanosine triphosphate (GTP) and D-ribulose 5-phosphate occurs through seven enzymatic steps, which have been reviewed previously (Bacher et al. 2000). 

The role of a pyridinium-4-aldehyde in enzyme-catalysed amino acid decarboxylation 32.2.3 Riboflavin (Vitamin B2) 

Riboflavin is incorporated into another complex co-enzyme, flavin adenine dinucleotide (FAD). This is involved in enzyme-catalysed reductions of carbon-carbon double bonds, and the reverse. By accepting two hydrogens, the co-enzyme is converted into a dihydro derivative (FADH2), the driving force for this being the relief of the unfavoured interaction between the polarised, opposed C=N bonds. 

Thiamin pyrophosphate acts as a coenzyme in several biochemical processes and, in each case, its mode of action depends on the intermediacy of a C-2-deprotonated species - an ylide (24.1.2.1 and 24.10). For example, in the later stages of alcoholic fermentation, which converts glucose into ethanol and carbon dioxide, the enzyme pyravate decarboxylase catalyses the conversion of pyruvate into ethanal 

The role of a thiazolium ylide in the enzyme-catalysed decarboxylation of pyruvate 

Aromatic thiophenes play no part in animal metabolism, however aromatic thiophenes do occur in some plants, in association with polyacetylenes with which they are biogenetically linked. Biotin (vitamin H), is a tetrahydrothiophene. 

Sources are fortihed cereals and grains, lean meat and poultry, milk and other dairy products, raw mushrooms. 

The FAD-requiring enzymes in mammalian systems include the D- and L-amino acid oxidases, mono- and diamine oxidases, glucose oxidase, succinate dehydrogenase, a-glycerophosphate dehydrogenase, and glutathione reductase. FMN is a cofactor for renal L-amino acid oxidase, NADH reductase, and a-hydroxy acid oxidase. In succinate dehydrogenase, FAD is linked to a histidyl residue in liver mitochondrial monoamine oxidase, to a cysteinyl residue. In other cases, the attachment is nonco-valent but the dissociation constant is very low. 

Use of oral contraceptives may increase the dietary requirement for riboflavin. Riboflavin status can be evaluated from the activity of erythrocyte glutathione reductase, an FAD-requiring enzyme, before and after addition of exogenous FAD. A low initial activity or a marked stimulation by FAD (or both) is indicative of ariboflavi-nosis. 

Flavin mononucleotide (FMN) (also called riboflavin phosphate) 

Dietary riboflavin is present mostly as a phosphate, which is rapidly hydrolyzed before absorption in the duodenum.In humans, the rapid, saturable absorption of riboflavin following an oral dose suggests that it is transported by a carrier-mediated pathway located predominantly in duodenal enterocytes. The process may be sodium-dependent. Bile salts enhance absorption of riboflavin. Fecal riboflavin is derived from the intestinal mucosa and the intestinal flora. This is the predominant excretory route for the vitamin. 

Signs of riboflavin deficiency include cheilosis, angular stomatitis,magenta tongue,and localized seborrheic der-matitis.Some of these conditions may be due to concurrent deficiency of other B-complex vitamins, since it is diffi- 

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Biotransformations are carried out by either whole cells (microbial, plant, or animal) or by isolated enzymes. Both methods have advantages and disadvantages. In general, multistep transformations, such as hydroxylations of steroids, or the synthesis of amino acids, riboflavin, vitamins, and alkaloids that require the presence of several enzymes and cofactors are carried out by whole cells. Simple one- or two-step transformations, on the other hand, are usually carried out by isolated enzymes. Compared to fermentations, enzymatic reactions have a number of advantages including simple instmmentation reduced side reactions, easy control, and product isolation. 

Riboflavin (vitamin Bg) Nicotinamide adenine dinncleotide phosphate (NADP+) Flavin adenine dinncleotide (FAD)... 

Important organic applications are to the determination of quinine and the vitamins riboflavin (vitamin B2) and thiamine (vitamin Bj). Riboflavin fluoresces in aqueous solution thiamine must first be oxidised with alkaline hexacyanoferrate(III) solution to thiochrome, which gives a blue fluorescence in butanol solution. Under standard conditions, the net fluorescence of the thiochrome produced by oxidation of the vitamin Bj is directly proportional to its concentration over a given range. The fluorescence can be measured either by reference to a standard quinine solution in a null-point instrument or directly in a spectrofluorimeter.27... 

Riboflavin (vitamin Eremothecium ashbyii Ashbya gossypii Treatment of vitamin B2 deficiency disease... 

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. 

Milk is an excellent source of calcium, phosphorus, riboflavin (vitamin B2), thiamine (vitamin Bl) and vitamin B12, and a valuable source of folate, niacin, magnesium and zinc (Food Standards Agency, 2002). In particular, dairy products are an important source of calcium, which is vital for maintaining optimal bone health in humans (Prentice, 2004). The vitamins and minerals it provides are all bioavailable (i.e. available for absorption and use by the body) and thus milk consumption in humans increases the chances of achieving nutritional recommendations for daily vitamins and mineral intake (Bellew et al., 2000). 

Group-transfer reactions often involve vitamins3, which humans need to have in then-diet, since we are incapable of realizing their synthesis. These include nicotinamide (derived from the vitamin nicotinic acid) and riboflavin (vitamin B2) derivatives, required for electron transfer reactions, biotin for the transfer of C02, pantothenate for acyl group transfer, thiamine (vitamin as thiamine pyrophosphate) for transfer of aldehyde groups and folic acid (as tetrahydrofolate) for exchange of one-carbon fragments. Lipoic acid (not a vitamin) is both an acyl and an electron carrier. In addition, vitamins such as pyridoxine (vitamin B6, as pyridoxal phosphate), vitamin B12 and vitamin C (ascorbic acid) participate as cofactors in an important number of metabolic reactions. 

Riboflavin (vitamin B2) was chosen as the number one index vitamin because among the so-called water-soluble vitamins, it is the least soluble in water. If riboflavin is demonstrated to dissolve within the specified time, it is assumed that all other water-soluble vitamins will have also... 

The SP procedure of water-soluble vitamins from multivitamin tablets is particularly challenging due to the diverse analytes of varied hydrophobicities and pfC. Water-soluble vitamins (WSVs) include ascorbic acid (vitamin C), niacin, niacinamide, pyridoxine (vitamin B ), thiamine (vitamin Bj), folic acid, riboflavin (vitamin B2) and others. While most WSVs are highly water soluble, riboflavin is quite hydrophobic and insoluble in water. Folic acid is acidic while pyridoxine and thiamine are basic. In addition, ascorbic acid is light sensitive and easily oxidized. The extraction strategy employed was a two-step approach using mixed solvents of different polarity and acidity as follows ... 

Riboflavin (vitamin B2) is a component of flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD), coenzymes that play a major role in oxidation-reduction reactions (see Section 15.1.1). Many key enzymes involved in metabolic pathways are actually covalently bound to riboflavin, and are thus termed flavoproteins. 

Riboflavin (vitamin Bj) is chemically specified as a 7,8-dimethyl-10-(T-D-ribityl) isoalloxazine (Eignre 19.22). It is a precnrsor of certain essential coenzymes, such as flavin mononucleotide (FMN) and flavin-adenine dinucleotide (FAD) in these forms vitamin Bj is involved in redox reactions, such as hydroxylations, oxidative carboxylations, dioxygenations, and the reduction of oxygen to hydrogen peroxide. It is also involved in the biosynthesis of niacin-containing coenzymes from tryptophan. 

The group of B vitamins consists of thiamine or aneurine (vitamin B i), riboflavin (vitamin B2), nico-... 

Riboflavin (vitamin B2) is found in liver, milk, meat, green vegetables, cereals and mushrooms. It is active in the form of two coenzymes, flavin mononucleotide and flavin adenine dinucleotide. As a coenzyme for proton transfer in the respiratory chain it is indispensable for energy-release from carbohydrates, lipids and proteins. Riboflavin deficiency only occurs in combination with deficiencies of other members of the vitamin B family. The symptoms of such deficiency consist of angular stomatitis, lesions of the cornea, dermatoses and normochromic normocytic anaemia. 

Riboflavin vitamin Bf) deficiency results in local seborrheic dermatitis that may be limited to the face and scrotum. Other symptoms of ariboflavinosis include angular stomatitis, cheilitis, and glossitis. Specific ocular... 

The stability of some vitamins is influenced by aw. In general, the stability of retinol (vitamin A), thiamin (vitamin Bj) and riboflavin (vitamin B2) decreases with increasing aw. At low av (below 0.40), metal ions do not have a catalytic effect on the destruction of ascorbic acid. The rate of loss of ascorbic acid increases exponentially as aw increases. The photodegradation of riboflavin (Chapter 6) is also accelerated by increasing aw. 

More than one century ago a yellow, fluorescent pigment was isolated from whey by Blyth In the subsequent years yellow pigments were extracted from various biological materials. Depending either on the source of isolation or the physical appearance, these natural products were named e.g. lactochrome , lycochrome , ovoflavin , lactoflavin , hepatoflavin , or verdoflavin . Later, it became evident that all these compounds are riboflavin (vitamin B2). 

Some analytes, such as riboflavin (vitamin B2)16 and polycyclic aromatic compounds (an important class of carcinogens), are naturally fluorescent and can be analyzed directly. Most compounds are not luminescent. However, coupling to a fluorescent moiety provides a route to sensitive analyses. Fluorescein is a strongly fluorescent compound that can be coupled to many molecules for analytical purposes. Fluorescent labeling of fingerprints is a powerful tool in forensic analysis.17 Sensor molecules whose luminescence responds selectively to a variety of simple cations and anions are available.18 Ca2+ can be measured from the fluorescence of a complex it forms with a derivative of fluorescein called calcein. 

Riboflavin (vitamin B2) 6,7-dimethyl-9-(D-l-ribityl)isoalloxazine (63), was discovered as a coloring matter in milk in 1879, but its importance was not then realized. Deficiency causes lesions of the eye and of the angle of the mouth. Riboflavin is phosphorylated by adenosine triphosphate (ATP) to give riboflavin 5 -phosphate (flavinadenine mononucleotide, FMN) and then flavinadenine dinucleotide (FAD) (64 R = riboflavin). These function as prosthetic groups in a number of flavoproteins which are dehydrogenation catalysts by virtue of the oxidation-reduction properties of the isoalloxazine system. 

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