Vitamin B2: Riboflavin

Karilyne Manahan & Alyssa Specht

Chemical Name and Structure

● Riboflavin also known as vitamin B2 and is an essential water-

soluble vitamin

● Chemical name: 7,8-dimethyl-10-ribityl-isoalloxazine1

● Chemical formula: C17H20N4O6 2

● Composed from 3 ring structure call flavin and 5 carbon chain

sugar alcohol named ribitol2

○ Gets name ribo-flavin

Major Coenzymes2

● Two coenzymes derived from B2

○ Flavin mononucleotide (FMN)

○ Flavin adenine dinucleotide (FAD)

● Formed by enzymes and composed of riboflavin and phosphate

group (FMN)

● FAD further adds additional adenosine phosphate group (AMP)

(2)

Chief Functions2

● Coenzymes FMN and FAD involved in several

intermediary reactions involving flavoproteins

● Flavoproteins - enzymes requiring FMN or FAD as

coenzymes

● Specifically in oxidation-reduction (redox) reactions

Redox Reactions2

● Essential for energy production by catabolizing carbohydrates, fats,

and lipids to generate ATP by transferring electrons

● Flavins act as oxidizing agents by accepting 2 hydrogen atoms and

losing 2 elections

● FMN and FAD reduced to FMNH2 and FADH2

Reducing of FMN or FAD2

FAD Reactions - Energy Production2

● Metabolic pathways involved in:

○ Pyruvate Dehydrogenase Complex

○ Fatty Acid Beta-Oxidation

■ acyl-CoA dehydrogenase

○ Citric Acid Cycle

■ α-ketoglutarate dehydrogenase and succinate dehydrogenase

enzyme pathways

○ Complex II of Electron Transport Chain (ETC)

○ FADH2 converted to ATP in ETC (1.5 ATP)

FAD Reactions - Coenzyme Synthesis

● B3 from tryptophan2

○ Kyrureninase monooxygenase

● Antioxidants3

○ Xanthine oxidase to produce uric acid

○ Glutathione reductase to produce glutathione

● Converts folate to active form2

Continue

● aldehydes → carboxylic acids (aldehyde oxidase)2

○ B6 → pyriodoxic acid

○ Retinol → retinoic acid

FMN Reactions2

● Energy production reactions

○ Complex I rxns in ETC

● Synthesis reactions

○ Formation of coenzyme form of B6 (pyridoxal phosphate or PLP)

■ pyridoxine phosphate oxidase

Chief Functions: Disease Prevention and Treatment

● Protective against diseases with root cause from oxidative stress

and inflammation

○ Cardioprotection4

○ Neuroprotection

■ Stoke5 and Migraine Treatment(6,7)

○ Cancer Inhibition(1,2,8)

■ metabolize carcinogens

Metabolic Pathways

Bioavailability

● Riboflavin is not created or stored in the body9 must be consumed

through diet

● Mostly found as FAD form in foods; lesser amount FMN1 and little

“free” riboflavin in foods10

● Must be converted to free riboflavin for absorption if bonded to

proteins or if in FAD or FMN form2

● If bond to histidine or cysteine cannot be converted2

Digestion2

● Starts in stomach

○ Riboflavin-protein compounds → free riboflavin by

hydrochloric acid

● Then small intestine

○ FAD → FMN → free riboflavin at brush border (intestinal

phosphatases)

Absorption ● Once in free form can be absorbed2

● Major absorption site - proximal small intestine2

● Occurs through carrier mediated1 sodium-independent active transport2

● Per meal ~95% of riboflavin is absorbed2

○ Max ~25mg

Metabolism & Transport2

● Quickly upon absorption into intestinal cell riboflavin → FMN

(enzyme flavokinase)

**Requires ATP**

● At serosal membrane

FMN → riboflavin - - > portal vein - - > liver - - > tissues

Metabolism & Transport2

● Flavins in blood mostly found as riboflavin (50%) with some FAD

(40%) and FMN (10%)

● FAD and FMN transported with protein carriers

○ albumin (primarily), fibrinogen, and globulins

● Once transported to tissue absorbed by a carrier-mediated

riboflavin-binding protein

● High concentration enter by diffusion

Metabolism & Transport10

● In tissues: riboflavin → FMN

○ same process as earlier

● Further FMN → FAD (enzyme FAD synthetase)

**Requires ATP**

● Phosphatases in tissue can convert back flavins back to riboflavin

(2)

Excretion

● Excreted through urine2

● Small amounts stored in

○ liver, spleen, sm intestine,kidneys, and heart2

● Can meet body’s needs for 2-6 weeks2

● Protects against toxicity9

Daily Recommended Intake

• Glutathione reductase an adequate indicator of

riboflavin requirements as its processes2

o reduction of glutathione disulfide --> glutathione is

dependent upon FAD and NADPH

• Meeting the DRI is not a large issue in the United States

due to it’s fortification of many grains and cereals(11,12)

Daily Recommended Intake(1,2)

Group Needs (mg/d)

Men 1.3

Women 1.1

Pregnant/Lactating Women 1.4/1.6

Infants 0-6 months .3

Infants 6 months - 1 y .4

Children aged 1-3 y .5

Children aged 4-8 y .6

Deficiency

• Ariboflavinosis (riboflavin disease in isolation) is rare

o often in congruence with other vit./min. deficiencies(1,2)

• Clinical Signs:

o cheilosis, angular stomatitis, oral hyperemia, edema, seborrheic

dermatitis, and neuropathy2

o Protein and DNA damage also possible due to GI phase of Cell cycle

inhibited2

• Anemia, growth retardation, susceptibility to some carcinogens also

possible2

Deficiency• Also related to B12 and folate deficiencies

o Decreased riboflavin = ↓folate = ↓methionine → homocysteine (may ↑ CVD

risk)1

o B12 derivative dependent on flavoproteins

• Current treatment for riboflavin deficiency is 10-20 mg/d supplementation until

symptoms are resolved1

• Other diseases that inc. risk of deficiency:

o thyroid disease, DBM, chronic stress, depression, gastrointestinal

diseases, cataracts(1,2)

Deficiency

• Populations at Risk

o Pregnant/lactating women, infants, school-aged children,

elderly, athletes, vegetarians/vegans, alcoholics, anorexics,

third world country populations1

o Lactating women have estimated 40-90% increased needs12

o Infants treated for hyperbilirubinemia at risk due to

phototherapy treatment1

Toxicity

• The body is protected from riboflavin toxicity due to its

ability to readily excrete excess levels in the urine

• No tolerable upper level intake has been established1

Significant Sources

• Predominantly found as FAD in foods

• Milk and eggs have high amounts of free riboflavin2

o these and other dairy products are primary sources of

riboflavin13

o riboflavin in cow’s milk is 90% free form13

• Grains provide up to 20% daily requirements as whole grains or

fortified products14

• Meat, legumes, and dark leafy vegetables are good sources2

Negative Effects on Riboflavin Content

● Light has the biggest effect on riboflavin - up to 50% of riboflavin

destroyed if held in light for only 2 hours3

● Oxidation15

○ Trolox & ascorbic acid can reduce by antioxidant mechanisms

● Fairly heat resistant, however pasteurization and UHT slightly

lower levels13

Fortification & Additives

● These processes enrich riboflavin content in foods making them

good sources

● Cost-effective way to increase riboflavin in foods → increase

population intakes

● Fortification is highly used in grain products

● Additives, like improved lactic acid bacteria (LAB) strains added to

yogurt to increase riboflavin synthesis16

References1. Powers H. Riboflavin (vitamin B2) and health. AM J Clin Nutr. 2003; 77:1352-60.2. Gropper S, Smith J. Advanced Nutrition and Human Metabolism. 6th ed. Belmont, CA: Wadsworth CENGAGE Learning; 2013. 3. Higdon J, Delage B, McNulty H, McCann A. Riboflavin. [Internet]. Oregon: Linus Pauling Institute. 2002 [2013]. Availible from:

http://lpi.oregonstate.edu/infocenter/vitamins/riboflavin/ 4. Wang G, Li W, Zhao X. Riboflavin alleviates cardiac failure in type I diabetic cardiomyopathy. Heart Int [Internet]. 2011; 6(21): 75-

79. 5. Zhou Y, Zhang X, Su F, Liu X. Importance of riboflavin kinase in the pathogenesis of stroke. CNS Neurosci Ther [Internet]. 2012

Oct 18(10):834-840. Available from Wiley Online Library: http://onlinelibrary.wiley.com/doi/10.1111/j.1755-5949.2012.00379.x/full 6. MacLennan SC, Wade FM, Forrest KM, Ratanayake PD, Fagan E, Antony J. High-dose riboflavin for migraine prophylaxis in

children: a double-blind, randomized, placebo-controlled trial. J Child Neurol 2008;23(11):1300-4. 7. Bruijn J, Duivenvoorden H, Passchier J, Locher H, Dijkstra N, Arts WF. Medium-dose riboflavin as a prophylactic agent in children

with migraine: a preliminary placebo-controlled, randomised, double-blind, cross-over trial. Cephalalgia [Internet]. Mar 26 2010;30(12):1426-34.

8. Sharp L, Carsin A, Cantwell M, Anderson L, Murray L. Intakes of dietary folate and other B vitamins are associated with risks of esophageal adenocarcinoma, barrett’s esophagus, and reflux esophagitis. J Nutr [Internet]. 2013 Dec [cited March 20, 2014] 143(12):1966-1973. Available from: http://nutrition.highwire.org/content/143/12/1966.short

9. Subramanian V, Subramanya S, Ghosal A, Said H. Chronic alcohol feeding inhibits physiological and molecular parameters of intestinal and renal riboflavin transport. American Journal Of Physiology: Cell Physiology [Internet]. 2013, Sep, [cited March 20, 2014];305(5):C539-C546. Available from: Academic Search Complete.

References10. Dey M, Mukherjee D, Dutta M, Mallik S, Ghosh D, Bandyopadhyay D. Flavin mono nucleotide phosphatase from goat heart: A

forgotten enzyme of an important metabolic pathway. Journal Of Cell & Tissue Research [Internet]. 2013, Dec [cited March 20, 2014]; 13(3): 3851-3858. Available from: Academic Search Complete.

11. Feili Lo Y, Pei-Chun L, Yung-Ying C, Jui-Line W, Ning-Sing S. Prevalence of thiamin and riboflavin deficiency among the elderly in Taiwan. Asia Pacific Journal Of Clinical Nutrition [serial on the Internet]. 2005, Sept [cited March 20, 2014];14(3):238-243. Available from: Academic Search Complete.

12. Papathakis P, Pearson K. Food fortification improves the intake of all fortified nutrients, but fails to meet the estimated dietary requirements for vitamins A and B6, riboflavin and zinc, in lactating South African women. Public Health Nutrition [serial on the Internet]. 2012, Oct [cited March 20, 2014];15(10):1810-1817. Available from: Academic Search Complete.

13. Sunaric S, Denic M, Kocic G. Evaluation of riboflavin content in dairy products and non-dairy substitutes. Italian Journal Of Food Science [serial on the Internet]. (2012, Oct), [cited March 20, 2014]; 24(4): 352-357. Available from: Academic Search Complete.

14. Martinez-Villaluenga C, Michalska A, Frias J, Piskula M, Vidal-Valverde C, Zieliński H. Effect of Flour extraction rate and baking on thiamine and riboflavin content and antioxidant capacity of traditional rye bread. Journal Of Food Science [Internet]. (2009, Jan), [cited March 20, 2014];74(1):C49-C55. Available from: Academic Search Complete

15. Hall N, Chapman T, Kim H, Min D. Antioxidant mechanisms of Trolox and ascorbic acid on the oxidation of riboflavin in milk under light. Food Chemistry [Internet]. (2010, Feb), [cited March 20, 2014];118(3):534-539. Available from: Academic Search Complete.

16. Jayashree S, Rajendhran J, Jayaraman K, Kalaichelvan G, Gunasekaran P. Improvement of Riboflavin Production by Lactobacillus fermentum Isolated from Yogurt. Food Biotechnology [Internet]. 2011, July, [cited March 20, 2014]; 25(3): 240-251. Available from: Academic Search Complete.