Riboflavin absorption

The vitamin is commercially available as riboflavin, riboflavin S-phosphate. and riboflavin S-phosphate sodium. The phosphate esters are used commercially only in multivitamin preparations, and they are hydrolyzed before absorption occurs. Ab.sorption occurs through an active transport. system in which riboflavin is phosphorylated by the intestinal mucosa during absorption. Food and bile enhance absorption. Riboflavin is distributed widely in the body, with limited stores in the liver, spleen, heart, and kidneys. Conversion to FAD occurs primarily in the liver. FMN and FAD circulate primarily protein bound. Only small amounts ( 9%) are excreted in the urine unchanged. Larger amounts can be found after administration of large doses. 

Description of Method. The water-soluble vitamins Bi (thiamine hydrochloride), B2 (riboflavin), B3 (niacinamide), and Be (pyridoxine hydrochloride) may be determined by CZE using a pH 9 sodium tetraborate/sodlum dIhydrogen phosphate buffer or by MEKC using the same buffer with the addition of sodium dodecyl-sulfate. Detection Is by UV absorption at 200 nm. An Internal standard of o-ethoxybenzamide Is used to standardize the method. 

In aqueous solution, riboflavin has absorption at ca 220—225, 226, 371, 444 and 475 nm. Neutral aqueous solutions of riboflavin have a greenish yellow color and an intense yellowish green fluorescence with a maximum at ca 530 nm and a quantum yield of = 0.25 at pH 2.6 (10). Fluorescence disappears upon the addition of acid or alkah. The fluorescence is used in quantitative deterrninations. The optical activity of riboflavin in neutral and acid solutions is [a]=+56.5-59.5° (0.5%, dil HCl). In an alkaline solution, it depends upon the concentration, eg, [a] J =—112-122° (50 mg in 2 mL 0.1 Ai alcohohc NaOH diluted to 10 mL with water). Borate-containing solutions are strongly dextrorotatory, because borate complexes with the ribityl side chain of riboflavin = +340° (pH 12). 

Photochemical decomposition of riboflavin in neutral or acid solution gives lumichrome (3), 7,8-dimethyl all oxazine, which was synthesized and characterized by Karrer and his co-workers in 1934 (11). In alkaline solution, the irradiation product is lumiflavin (4), 7,8,10-trimethyhsoalloxazine its uv—vis absorption spectmm resembles that of riboflavin. It was prepared and characterized in 1933 (5). Another photodecomposition product of riboflavin is 7,8-dimethy1-10-foTmylmethy1isoa11oxazine (12). 

Fig. 2.4 The spectrum of bacterial luminescence measured with B. harveyi luciferase, FMN, tetradecanal and NADH, in 50 mM phosphate buffer, pH 7.0, at 0°C (dashed line from Matheson et al., 1981) and the absorption and fluorescence emission spectra of LumP (solid lines) and Rf-LumP (dotted lines) obtained from P. leiog-natbi, in 25 mM phosphate buffer, pH 7.0, containing 1 mM EDTA and 10 mM 2-mercaptoethanol, at room temperature (from Petushkov et al, 2000, with permission from Elsevier). LumP is a lumazine protein, and Rf-LumP contains riboflavin instead of lumazine in the lumazine protein. Fluorescence emission curves are at the right side of the absorption curves.

Fig. 1. a) UV-Vis absorption and fluorescence emission spectra of riboflavin (RF, 20 pM) and Gum Arabic aqueous solutions at pH 7 (phosphate buffer 100 mM). b) Transient absorption spectra of RF (35 pM) in N2-saturated MeOH-Water (1 1) solution. The insets show the transient decay at 720 nm for the RF species and the Stern-Volmer plot for the quenching of 3RF by GA, eqn 11. 

Melo, T. B. lonescu, M. A. Haggquist, G. W. Naqvi, K. R. (1999). Hydrogen abstraction by triplet flavins. I time-resolved multi-chaimel absorption spectra of flash-irradiated riboflavin solutions in water Spectrochimica Acta, Part A Molecular and Biomolecular Spectroscopy, Vol.55, No.ll, (September 1999), pp. 2299-2307, ISSN 1386-1425. 

The excitation and emission wavelengths used for fluorescence detection in HPLC analysis are 350 and 450 nm for isoxanthopterin and 340 and 450 nm for 2,4,7-trioxopteridine, respectively." The absorption spectra of riboflavin present... 

Since many essential nutrients (e.g., monosaccharides, amino acids, and vitamins) are water-soluble, they have low oil/water partition coefficients, which would suggest poor absorption from the GIT. However, to ensure adequate uptake of these materials from food, the intestine has developed specialized absorption mechanisms that depend on membrane participation and require the compound to have a specific chemical structure. Since these processes are discussed in Chapter 4, we will not dwell on them here. This carrier transport mechanism is illustrated in Fig. 9C. Absorption by a specialized carrier mechanism (from the rat intestine) has been shown to exist for several agents used in cancer chemotherapy (5-fluorouracil and 5-bromouracil) [37,38], which may be considered false nutrients in that their chemical structures are very similar to essential nutrients for which the intestine has a specialized transport mechanism. It would be instructive to examine some studies concerned with riboflavin and ascorbic acid absorption in humans, as these illustrate how one may treat urine data to explore the mechanism of absorption. If a compound is... 

Fig. 10 Urinary excretion of riboflavin (A, B) and ascorbic acid (C, D) in humans as a function of oral dose. Graphs A and C illustrate the nonlinear dependence of absorption on dose, which is suggestive of a saturable specialized absorption process. Graphs B and D represent an alternative graph of the same data and illustrate the reduced absorption efficiency as the dose increases. (Graphs A and C based on data in Ref. 39 and graphs B and D based on data in Ref. 40.)...

Fig. 13 (Top) Influence of food on the absorption of different doses of riboflavin. (Bottom) Influence of food and divided doses on ascorbic acid absorption in three subjects. (Based on data from Refs. 39 and 77.)...

G Levy, MH MacGillivray, JA Procknal. Riboflavin absorption in children with thyroid disorders. Pediatrics 50 896-900, 1972. 

WJ Jusko, N Khanna, G Levy, L Stern, SJ Yaffe. Riboflavin absorption and excretion in the neonate. Pediatrics 45 945-949, 1970. 

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). 

Fig. 7. Change of absorption spectra of flavins upon chemical modification. (7) Dotted line pure pentapeptide from monaminoxidase solid line same, after performic acid oxidation185). (2) Dotted line 8-acetoxy flavin solid line same, after conversion to the anion, as indicated69). (2) Dotted line riboflavin solid line same, after binding to riboflavin binding protein8). For comparison, all spectra have been normalized to the same visible peak height...

Note Data represent the mean S.E. (n = 3). CLapp, apparent membrane permeability clearance ND, not detected. Absorption was evaluated in our laboratory using the closed loop of the rat colon in situ (urethane anesthesia, 1.125 g/4.5 ml/kg, i.p.) in 60 min for riboflavin and 30 min for the others. 

Note Data represent the mean S.E. (n = 3). MW, molecular weight P0/w, octanol-to-water partition coefficient CLapp, apparent membrane permeability clearance SI, midgut area of the small intestine NA, not available or applicable. Absorption was evaluated in our laboratory using the closed loop of the rat intestine in situ (urethane anesthesia, 1.125 g/4.5 ml/kg, i.p.) in 60 min for riboflavin and L-camitine and 30 min for the others. For those that are transported by carriers in part (riboflavin and glycerol in both colon and SI, and L-carnitine, 5-fluorouracil, and cephradine in SI), absorption was evaluated at higher concentrations where the contribution of carrier-mediated transport is negligible. Values of P0/w were obtained from a report by Leo et al. [30] except for that of D-xylose, which was determined in our laboratory. a Data by single-pass perfusion experiments. b Unpublished data from our laboratory. 

In foods vitamin B2 occurs free or combined both as FAD and FMN and complexed with proteins. Riboflavin is widely distributed in foodstnffs, but there are very few rich sources. Only yeast and liver contain more than 2mg/100g. Other good sources are milk, the white of eggs, fish roe, kidney, and leafy vegetables. Since riboflavin is continuously excreted in the urine, deficiency is qnite common when dietary intake is insufficient. The symptoms of deficiency are cracked and red lips, inflammation of the lining of the month and tongue, mouth ulcers, cracks at the comer of the mouth, and sore throat. Overdose of oral intake present low toxicity, probably explained by the limited capacity of the intestinal absorption mechanism [417]. 

Drugs that increase intestinal motility or induce diarrhea may decrease riboflavin absorption. Hyperthyroidism and the administration of thyroxine also reduce riboflavin absorption. 

The 8a-modified flavins show some remarkable physical properties which differ from those of common flavins. Since these properties are very helpful in the identification of modified 8a-substituted flavocoenzymes originating from biological materials the physical properties are presented briefly here. Compared to riboflavin (Scheme 2, (7), R = H) the visible light absorption properties of (2) to... 

Fig. 1. Absorption spectra of neutral and anionic flavin semiquinones. (-) Anionic riboflavin...

Ho, C. Y., Crane, R. T. and Clifford, A. J. 1978. Studies on lymphatic absorption of and the availability of riboflavin from bovine milk xanthine oxidase. J. Nutr. 108, 55-60. 

Figure 15-8 Absorption spectrum of neutral, uncharged riboflavin (A), the riboflavin anion (B), and reduced to the dihydro form (Fig. 15-7) by the action of light in the presence of EDTA (C). A solution of 1.1 x 1CF4 M riboflavin containing 0.01 M EDTA was placed 11.5 cm from a 40-W incandescent lamp for 30 min.

Factors which tend to decrease the availability of riboflavin include (1) cooking, inasmuch as riboflavin is slightly soluble in water (2) in some plant foods, availability is lower than might be expected because of bound forms (3) decreased phosphorylation in intestines prevents absorption ... 

The coenzymes, FMN and FAD, are the physiologically active vitamers. The biochemistry, absorption, metabolism, and physiological functions of riboflavin have been reviewed (80,81). 

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