Riboflavin

Question 1

Properties of Riboflavin

Answer 1

• From flavus = yellow and ribose = sugar
• yellow/ orange crystal
• heat stable
• water soluble (reasonably)
• unstable in UV light & alkaline solutions

Question 2

Structure of riboflavin

Answer 2

• ribityl side chain
• isoalloxazine

structure but not active form

Question 3

What is the active form of riboflavin?

Answer 3

Active form is a co-factor
* FMN = flavin mononucleotide (riboflavin-5-phosphate) which as an additional phosphate group
* FAD = flavin adenine dinucleotide (riboflavin-5-adenosyl-diphosphate) which the phosphate group like FMN but also an AMP added

Question 4

How is riboflavin converted to FMN and FAD?

Answer 4

• flavokinase: riboflavin → FMN (needs Zn2+ and an ATP → ADP)
• FAD synthetase: FMN → FAD (needs Mg2+ and an an ATP → PPi)

Question 5

What regulated flavokinase and FAD synthetase?

Answer 5

conversion of free form to FMN & FAD regulated by thyroid hormone
* ↑ flavokinase activity
* So hypothyroidism would cause a functional deficiency in riboflavin

Question 6

Mode of action for riboflavin

Answer 6

FMN and FAD can accept 2 H-atoms from substrates (or donate) at the double bonds of the nitrogens.
* cofactor for > 100 flavoproteins
* oxidized = no H+
* reduced = 2 H+

Question 7

Metabolic role of riboflavin

Answer 7

• Energy production by carrying reducing equivalents into mito. resp. chain (TCA cycle, Electron transport chain, β-oxidation of fatty acids)
• Regulation of redox status
• Detoxification (FMOs)
• Conversion of vitamins (B6 and folate) into their coenzyme form
• Synthesis of niacin (B3) from tryptophan

Question 8

Role of riboflavin in the CAC

Answer 8

• FAD is a co-factor in the PDH complex (E3)
• FAD is co-factor for succinate dehydrogenase
• ???Also a co-factor for a-ketoglutarate dehydrogenase???

Question 9

FAD role in PDH

Answer 9

Is a cofactor for E3 subunit (dihydrolipoyl dehdyrogenase) accepting 2 H+ from E2 and donating the 2 H+ to NAD+ (niacin) to form NADH+H

Question 10

FAD as a co-factor in succinate dehydrogenase

Answer 10

Succinate dehydrogenase is an enzyme in the CAC but it is also complex II in the ETC. so conversion of succinate to fumarate requires FAD to accept 2 H+ to form FADH2. Taking the 2 H+ also means it takes 2 e- so it then transfer the 2e- to an Fe-S cluster which continues down the chain and releases the H+ to become FAD again.

Question 11

Riboflavin in the ETC

Answer 11

• FAD is co-factor for complex II (succinate dehydrogenase)
• FMN is a co-factor in complex I

Question 12

FMN role in complex I of the ETC

Answer 12

FMN accepts 2 H+ from NADH+H and 2e- which then passes them on to Fe-S cluster where they continue down the chain and it releases the 2 H+. This complex also pumps H+ the outside

Question 13

riboflavin role in β-oxidation of fatty acids

Answer 13

FAD is a co-factor for acyl-CoA dehydrogenase, the first step enzyme of β oxidation which oxidizes the fatty acid, reducing FAD to FADH2

Question 14

riboflavin role in redox status regulation

Answer 14

riboflavin is a co-factor for glutathione reductase so it accepts 2 H+ from NADH+H and then donates the 2 H+ to GSSG to convert it back to GSH

Question 15

How is riboflavin absorbed?

Answer 15

• Free FMN and FAD from being protein bound via gastric acid and proteases in stomach/intestine
• FMN & FAD are big so broken down the free riboflavin via FMN-phosphatase & FAD-pyrophosphatase
• With low concentration of fiboflavin goes through transporters RFT1&2 but with high concentration some can diffuse.
• In enterocyte some ribofalvin might convert to FAD or FMN but most is transported out to portal vein as free riboflavin or FMN bound to proteins.
• Liver will take some to be used, but otherwise sent to other tissue.

Question 16

How does riboflavin stay in cells?

Answer 16

present as FAD and FMN in cells and stays within via metabolic trapping

Question 17

What are the important enzymes to know for absorption?

Answer 17

• proteases
• FMN-phosphatase
• FAD-pyrophosphatase
• RFT 1&2

Question 18

riboflavin storage

Answer 18

little storage
* highest concentrations in liver (1/3 total body flavins, stored as FAD), kidney & heart

Question 19

excretion of riboflavin

Answer 19

little storage therefore urinary excretion of flavins reflects dietary intake
* Excess riboflavin is rapidly excreted in urine (peak at 2 hrs after ingestion

Question 20

Sources of riboflavin

Answer 20

Gut is exposed to flavins from 2 sources
* diet
* bacteria in LI

Question 21

diet sources of riboflavin

Answer 21

Mainly in coenzyme form (> 2/3 as FAD)
(in US 1⁄3 of RDA)
* dairy and eggs (in USA 1/3 of RDA)
* meat (liver, kidney)
* some green leafy vegetables
* enriched flour & breakfast cereals

Question 22

bioavailability of riboflavin

Answer 22

related to digestibility of food
* diary & meat > plants
* Impaired by excess alcohol

Question 23

riboflavin from gut bacteria

Answer 23

• unknown whether this is available to host tissues (Besides colonic epithelial cells)
• synthesis increases with vegetarian diet vs. meat-based diet

Question 24

riboflavin DRIs

Answer 24

M=1.3 mg/d and F=1.1mg/d
* increases with pregnancy and lactation
* depends on energy intake & other nutrients (including protein)
* UL is not set because no reported cases of riboflavin toxicity (may be due to low solubility & prompt excretion)

Question 25

measuring riboflavin status

Answer 25

**urinary excretion** * 24-hour excretion of <10% ingested may reflect inadequacy * limited accuracy however **Serum concentration** * variable & indicates only current intake **FAD effect!!**

Question 26

What is the FAD effect used to measure riboflavin status?

Answer 26

Measure enzyme glutathione reductase in RBCs which contains FAD, then add more FAD and if activaty goes up substantially then person is likely deficient * **EGRAC**: Erythrocyte glutathione reductase activity coefficient * **FAD effect:** If act. ↑ with added FAD, blood is not saturated enough with FAD

Question 27

What are the EGRAC values?

Answer 27

%FAD effect (AC): * Adequate: <20%(1-1.20) * Marginal: 20-40%(1.20-1.40) * Deficient: >40%(>1.40)

Question 28

What is a condition wtih riboflavin deficiency?

Answer 28

Ariboflavinosis which is a severe case * mostly lesions * Lesions of deficiency seen with intakes of 0.35 mg/1000 kcal

Question 29

signa and symptoms of ariboflavinosis

Answer 29

* stunte growth * lack of energy * cheilosis & glossitis * angular stomatitis * skin lesions/ dermatitis * corneal vascularization

Question 30

But does ribofalvin deficiency usually mean?

Answer 30

Usually riboflavin deficiency is not by itself and is complicated by B6 deficiency (similar symptoms) * Urinary riboflavin excretion negligible (reutilization by body)

Question 31

How is riboflavin associated with iron?

Answer 31

iron absorption & mobilization require reduction of Fe3+ to Fe2+ and flavins are most efficient reductants

Question 32

How is riboflavin associated with vitamin B6?

Answer 32

enzyme that converts to B6 to active form requires FMN * may also affect other kinases & phosphatases involved with B6 metabolism

Question 33

How is riboflavin associated with folate?

Answer 33

final step in conversion of oxidized folate is catalyzed by a flavin-dependent enzyme

Question 34

populations at risk for riboflavin deficiency

Answer 34

* low intake (elderly and common in countries with less dairy and meat consumptions) * pregnancy * disease states (thyroid, diabetes, hyperbilirubinemia)