Riboflavin Flashcards
what are the properties of riboflavin?
- yellow/orange crystal
- Heat STABLE
- water soluble
- unstable in UV light and alkaline solutions
yellow sugar
What is the active form of riboflavin?
Flavin mononucleotide (FMN) - riboflavin+phosphate
Flavin adenine dinucleotide (FAD) cofactors
- riboflavin + ADP
How are FMN and FAD formed?
riboflavin converted to FMN by flavokinase using ATP, and FMN –>FAD by FAD synthase (uses ATP by using the AMP and releasing PPi)
What hormone regulates the conversion of free form of B2 to FMN and FAD?
Thyroid hormone (increased flavokinase activity)
What is the mode of action of the cofactors?
FMN and FAD can carry hydrogens (accept 2 Hs from substrates).
Where is the reactive region of the cofactors?
its on the 2 diagonal nitrogens on the isoalloxazine
What processes is riboflavin important in?
- Energy production* (CAC, ETC, B-OX)
- Regulation of redox status
- Detoxification
- Conversion of vitamins (B6 and folate) into their coenzyme form.
- Synthesis of niacin from tryptophan
How is B2 involved in the ETC?
FMN accepts electrons and Hs from NADH in complex 1 and FAD accepts electrons and Hs in complex 2 from succinate.
How is B2 involved in B-ox?
FAD acts as an acceptor in the RLS with acyl-coa dehydrogenase
Briefly explain how FAD is involved in redox status regulation?
Once glutathione is oxidized by glutathione peroxidase (selenium), it must be converted back to 2 reduced glutathione which involves glutathione reductase (requires FAD to accepts electrons and Hs).
- Important for 2 GSH replenishment
What are the steps in riboflavin absorption?
- FMN/FAD-protein broken down by gastric acid and intestinal proteases
- FMN + FAD dephosphorylated to riboflavin by FMN-phosphatase and FAD-pyrophosphatase.
- Riboflavin taken up by carrier-mediated transport (RFT1/3) + passive diffusion.
- Metabolic trapping by flavokinase and FAD-synthetase.
- Sent to the liver for processing
The body stores relatively ______ amounts of B2 and is highest in the ________ (1/3), as well as kidney and heart.
What is the main storage form of B2?
Low
Liver
FAD.
Describe the excretion of B2
Reflects dietary intake since little storage.
Excess = rapidly excreted in urine (peak at 2 hours post ingestion).
What are some FOOD sources of B2 and what form is it usually consumed as?
Dairy, eggs, meat (kidney + liver), some green leafy veggies, enriched flour.
Mainly in coenzyme form (FAD**)
What factors affect bioavailability?
Related to digestibility (dairy/meat>plants)
Alcohol impairs
In general, what are the 2 sources of flavins that the gut is exposed to?
- Diet
- Flavins synthesized by bacteria in large intestine (unknown if available to host).
What are the requirements for B2 and what does it depend on?
Depends on energy intake and other nutrients.
Around 1 mg/d for adults.
NO UL - since no reported cases of toxicity (may be due to low sol. and promt excretion).
How can riboflavin status be determined?
- Urinary excretion (limited accuracy)
- Serum concentration (variable + indicates only current intake)
How is the FAD effect used to assess B2 status?
With the erythrocyte glutathione reductase activity coefficient (EGRAC) where FAD is added to isolated blood and the activity of glutathione reductase is assessed. If the marker is over 1.4 then that indicates deficiency. Means that blood is not saturated enough with FAD.
What illness can arise from B2 deficiency and what are the signs/symptoms?
Ariboflavinosis
- stunted growth
- lack of E
- skin lesions/dermatitis
-corneal vascularization
- angular stomatitis
Complicated by B6 deficiency (similar symptoms)
* urinary riboflavin excretion negligible (reutilization by the body).
When can the lesions of deficiency be seen in regards to amount of intake?
When its <0.35mg/1000kcal
What 3 nutrients is B2 deficiency associated with and why?
- Iron - absorption + mobilization requires Fe3+ to Fe2+ (flavins are most efficient reductants)
- B6 - enzyme that converts B6 to active form requires FMN
- Folate - final step in conversion of oxidized folate is catalyzed by flavin-dependent enzyme.
What are 3 populations at risk for B2 deficiency?
- Low intake (deficiency is endemic where diets lack diary products + meat) - elderly
- pregnancy
- Disease states (thyroid, diabetes, hyperbilirubinemia)
