Riboflavin (3 questions)

Question 1

What is the oxidized CoE. form of riboflavin?

Answer 1

FAD

Question 2

What is the reduced Co.E form of riboflavin?

Answer 2

FADH

Question 3

What are the 2 CoE. forms of riboflavin?

Answer 3

FMN and FAD/FADH

Question 4

RDAs for riboflavin

Answer 4

Males have a slightly HIGHER RDA, starting at ages 14 and up

Question 5

What are the best food sources of riboflavin?

Answer 5

Eggs, kidneys and liver, lean meats, and milk products, green vegetables

Fortified grains and cereals

Question 6

What form is riboflavin in most foods?

Answer 6

FAD

Question 7

What is the form of riboflavin in eggs and milk?

Answer 7

Free riboflavin

Question 8

Similar to XXXXX, riboflavin is soluble in water, meaning….

Answer 8

THIAMIN; about twice as much riboflavin content is lost in cooking water when foods are boiled, and unless the liquid is added back, riboflavin will be lost.

Question 9

Flavokinase

Answer 9

Enzyme converting natural riboflavin —> FMN CoE. form; in the enterocyte

Question 10

FAD Synthetase

Answer 10

Enzyme converting FMN —> FAD

Negative FB here

Unregulated in times of growth

Question 11

How is riboflavin absorbed?

Answer 11

Since it is protein-bound in food, HCl acid and proteases (secreted in the stomach, pancreas, S.I.) cleave off the non-covalent bond.

Question 12

FMN

Answer 12

Riboflavin + phosphate group

Question 13

FAD

Answer 13

Riboflavin + Pyrophosphate + AMP (Adenine Monophosphate group)

Main CoE. form; oxidized form

Question 14

FAD pyrophosphatase

Answer 14

Enzyme secreted from stomach, pancreas, S.I that is required for cleaving the covalent bond for absorption of riboflavin

FAD ———————> FMN
FAD pyrophastase- cleaves off AMP portion

Question 15

FMN phosphatase

Answer 15

For riboflavin absorption

FMN ———————–> Free Riboflavin
FMN phosphatase- cleaves off final phosphate group

Question 16

How is riboflavin absorbed?

Answer 16

It must be in the free riboflavin form, then it will be absorbed in the proximal S.I and COLON, via RFVT3

Question 17

RFVT3

Answer 17

Riboflavin Vitamin Transporter, located in proximal SI and colon

depends on H+ that will be pumped into the enterocytes along with riboflavin

Question 18

(T/F) Riboflavin has poor bioavailability.

Answer 18

FALSE; 95% of riboflavin from food is absorbed, up to 25 mg

Question 19

What happens once riboflavin enters the enterocyte?

Hint: there are two options

Answer 19

1. Free riboflavin can leave the enterocytes and go into portal circulation by way of RIBOFLAVIN VITAMIN TRANSPORT 1 OR 2

OR

2. Free riboflavin can be converted to FMN via FLAVOKINASE (ATP-dependent), then FMN can be converted to FAD via FAD SYNTHETASE (ATP-dependent).
   —–FAD trapped inside the enterocyte is going to be used by the mito. for some of those redux reactions that are similar to niacin and NADH. More later!!!!

Question 20

How is riboflavin transported in the blood?

Answer 20

~50% free riboflavin

~10% FMN

~40% FAD

Question 21

Where is the majority of riboflavin stored?

Answer 21

Liver, kidneys, heart have the greatest concentrations

Question 22

What is the major riboflavin form that is stored in cells? What percentage?

Answer 22

FMN, ~60-90% in cells

FAD is secondary in cells, ~5-20%

Question 23

The body stores enough riboflavin to meet its need for how long?

Answer 23

2-6 weeks in a normal adult.

Note this will vary on activity level, gender, weight

Question 24

Explain how intracellular conversion of FMN to FAD is controlled.

Answer 24

End-product inhibiting FAD synthetase enzyme

Question 25

What accelerates or upregulates the conversion of riboflavin into it’s coenzyme forms?

Answer 25

ACTH, aldosterone, and thyroid hormones

Remember, in times of growth the enzymes are upregulated driving the expression of FAD synthetase

Question 26

What are the general functions of CoE’s of riboflavin?

Answer 26

1. ETC (FADH2 to FAD; dropping off 2 electrons at Complex II)
2. Folate metabolism (FADH2 provides 2 H+ in the conversion of 5,10-methylene THF to 5-methyl THF)
3. FMN (required for pyridoxamine oxidase (which converts PMP and PNP to PLP)
4. Glucose Metabolism (FAD is required for conversion of pyruvate to acetyl-CoA)
5. FA Metabolism (FAD is required for acyl-CoA dehydrogenase in beta-oxidation)
6. Antioxidant System (FAD is required for glutathione reductase)

Question 27

What riboflavin CoE plays a role in the ETC? What does it do?

Answer 27

FADH (reduced form) at Complex II, drops off its 2 electrons, which will be passed down the ETC. The electrons being pumped into the intracellular space builds the IONIC GRADIENT, which is used to power ATP Synthase, producing ATP.

Question 28

What riboflavin CoE. plays an important role in folate metabolism? What does it do?

Answer 28

FADH2 provides two H+ in the conversion of 5,10-methylene THF to 5-methyl THF

FADH2 ((and NADPH)) will act as reducing agents, so they become oxidized, so FAD and NADP are going to be released from this reaction.

Question 29

What riboflavin CoE. plays an important role in vitamin B6 metabolism?

Answer 29

FMN

it is a CoE. for PYRIDOXAMINE OXIDASE ((which drives the reaction toward the formation of PLP (remember??). So PMP and PNP are going to be converted to PLP.

Question 30

How is riboflavin involved with glucose metabolism?

Answer 30

FAD serves as an intermediate electron carrier (along with NADH), being able to pass on electrons and helping with the oxidative decarboxylation of PYRUVATE —-> ACETYL-CoA

Question 31

What riboflavin CoE. form has an important role in FA metabolism? What does it do?

Answer 31

FAD is a coenzyme for Acyl CoA dehydrogenase (used in the first step of beta-oxidation). FAD is reduced to FADH2, which will go on to ETC to generate ATP (in Complex II).

Question 32

Which riboflavin CoE. participates in the indigenous antioxidant system?

Answer 32

FAD functions as a CoE. for GLUTATHIONE REDUCTASE, so that when NADPH transfers, it’s electrons over to FAD, it’s going to be reduced to FADH, and glutathione reductase is going to transfer those 2 electrons to glutathione, thus changing oxidized glutathione to the reduced glutathione form.

From there, the reduced glutathione can participate in these antioxidant activities, so that glutathione peroxidase is going to take the reduced glutathione and transfer the electrons to pro-oxidant compounds, such as hydrogen peroxide, and neutralize those by converting it to water.

Question 33

Riboflavin deficiency

Answer 33

Ariboflavinosis, rarely occurs in isolation.

Characterized by:
——Chelosis (painful lesions on the outside of the lips)
——Glossitis
——-Swollen oral cavity

((Similar to niacin, epithelial cells are affected, specifically in the oral cavity)

Question 34

(T/F) Although not due to a deficiency, mutations in genes of enzymes that require coenzyme forms of riboflavin can effect the binding of FMN/FAD to the enzyme; therefore, impair the biochemical reactions.

Answer 34

TRUE; in some cases, pharmacological doses (up to 200 mg/day) of riboflavin may be beneficial for those with these gene mutations.

Question 35

What is the upper tolerable limit for riboflavin?

Answer 35

None has been established

Question 36

What is the most sensitive method for determining riboflavin status?

HINT: this is done in a research setting

Answer 36

Measure the activity of the FAD-dependent enzyme ERYTHROCYTE GLUTATHIONE REDUCTASE

This allows us to view riboflavin status in isolation.

Clinically, MDs would order a panel to look at water-soluble vitamin status since deficiencies are not seen in isolation.