Riboflavin - Flavin adenine dinucleotide (FAD); flavin mononucleotide (FMN)
Adequate Intake ?:
Electron (hydrogen) transfer reactions for nutrient metabolism and energy production. Ariboflavinosis -- Cheilosis, glossitis, angular stomatitis, edema oral cavity, dermatitis. Major Food Sources: Meats, eggs, yougurt, cheese, milk. RDA: 1.3 mg
Typically highest in animal sourced foods including liver, meat, eggs, dairy.
Other good sources include nuts, pulses, dark green leafy vegetables.
Milling of white rice and cereals results in 2/3rd loss of riboflavin.
Two main active forms in animals and plants
Riboflavin-adenine dinucleotide (FAD)
Impaired mitochondrial metabolism suspected in patients with migraines: Riboflavin catalyzes the activity of flavoenzymes in
the mitochondrial respiratory chain.
Ariboflavinosis: main deficiency
The signs and symptoms include skin disorders, hyperemia (excess blood) and edema of the mouth and throat, angular stomatitis (lesions at the corners of the mouth), cheilosis (swollen, cracked lips), hair loss, and reproductive problems.
Rarely occurs in isolation as b-vitamins are usually available in
Many foods rich in thiamin are also rich in riboflavin.
History & Discovery:
In 1879, Alexander Wynter Blyth, a English chemist, described water-soluble material from milk that glowed with a yellow-green fluorescence when exposed to light. He called lactochrome, ‘lacto’ (milk) and ‘chrome’ meaning color because of the yellow pigment.
In 1933-1935, Kuhn and Karrer, two Austrian scientists and Nobel Prize winners, isolated lactoflavin from milk and found the same chacteristics described by Blyth. They described the structure of riboflavin.
Riboflavin occurs in most foods as protein complexes of the co-enzyme forms FMN and FAD, uptake depends on hydrolytic conversion to free riboflavin.
FAD -> FAD pyrophosphatase -> FMN -> FMN phosphatase -> Riboflavin
This occurs through protelytic activity in the lumen by intestinal brush border phophatases, which liberate riboflavin to its free form where it can be transported across the intestinal brush borders (intestinal villi) using riboflavin vitamin transporters (RFVTs).
Jejunum is primary site of absorption.
RVFT3 carries riboflavin across the brush border membrane.
RFVT1 and RFVT2 carries riboflavin across the basolateral membrane of the intestinal cell.
RFVT2 is present in high amounts in tissues and transports riboflavin into tissues.
Absorption and Storage:
Usually bound to proteins such as albumin
Primary uptake into tissue is through RFVT2 as free riboflavin.
Following uptake riboflavin is converted to FMN through riboflavin kinase in an ATP dependent process.
Kinases catalyze the transfer of a phosphate from ATP to a specified molecule.
Then FMN is converted to FAD by FAD synthase.
Kinases catalyze the linking together of two molecules, without ATP.
Oxidative decarboxylation of pyruvate and α-ketoglutarate:
FAD serves as an electron carrier (high redox potential) meaning it takes the electrons from pyruvate and α-ketoglutarate and is transformed into FADH2.
Succinate dehydrogenase: removes electrons from succinic acid to form fumarate, and forms FADH2. The electrons are then passed into the electron transport chain by coenzyme Q10 (CoQ10).
Glutathione reductase is "recharged" by FAD
Riboflavin: anti-oxidant roles
Thioredoxins are ubiquitous antioxidant enzymes that play important roles in many health-related cellular processes including protection from cancer and viral diseases.
FAD transfers reducing equivalents from NADPH through its bound FAD to reduce disulfide bonds within the oxidized form of thioredoxin to bring it back to a reduced state. Reduced thioredoxin has a radical-scavening activity.
Reduction of ribonucleotides to deoxyribonucleotides by ribonucleotide reductase
Riboflavin: Co-factor for other vitamins
Pyridoxine (PN) -> phosphatase -> Pyridoxine-PO4 (PNP) -> oxidase (FMN) -> Pyridoxal-PO4 (PLP) = primary form of Vit B6
Pyridoxal (PL) -> phosphatase -> Pyridoxal-PO4 (PLP) = primary form of Vit B6
Pyridoxamine (PM) -> phosphatase -> Pyridoxamine-PO4 (PMP) -> oxidase (FMN) -> Pyridoxal-PO4 (PLP) = primary form of Vit B6
FAD-dependent -> Retinal aldehyde
Tryptophan can be converted to niacin using FAD and B6
Riboflavin involved in the breakdown of neurotransmitters like dopamine
Deficiency Diseases, Detection, Cures:
Cheilosis and/or glossitis
Nausea and vomiting and/or diarrhea
Pigmentation changes, erythema
Nervous system - general effects