In this article we will discuss about:- 1. Historical Review of Riboflavin 2. Chemistry of Riboflavin 3. Properties 4. Distribution 5. Functions 6. Deficiency 7. Daily Requirement.
Historical Review of Riboflavin:
(Vitamin B2 Lactoflavin)
In 1932 Warburg and Christian were able to isolate yellow enzyme from yeast. The yellow enzyme was named riboflavin. In 1935 it was synthesised.
Chemistry of Riboflavin:
A Flavin derivative riboflavin is an orange-yellow compound containing D-ribitol (α-ribose alcohol) and a heterocyclic substance—isoalloxazine (flavin). The one-carbon of the ribityl group is attached at position 9 of isoalloxazine. In the living cell it combines with phosphoric acid and a specific protein radicle. As prosthetic group of a number of enzymes, riboflavin acts either as a mono- or dinucleotide.
Flavin mononucleotide (FMN) is formed as:
It has been artificially synthesised in 1935. Normally riboflavin is phosphorylated in the intestinal mucosa and adrenal cortical hormone plays a major regulatory role in this mechanism.
Properties of Riboflavin:
It is orange-yellow crystals, soluble in water to a limited degree and insoluble in fat-solvents. It is heat-stable in neutral and acid media, but sensitive to light, unstable in alkaline solutions, stands ordinary cooking and canning. Aqueous solutions are unstable to visible and ultra-violet light. On exposure to light the ribityl residue splits off, forming a compound—lumiflavin in alkaline solution and lumichrome in acid or neutral solution, and this reaction is irreversible.
However, riboflavin readily undergoes reversible reduction by thiosulphate, H2S and H2 to colourless compound—leucoriboflavin, which is the functional basis related to catalysing cellular oxidation-reduction reactions. This vitamin produces a yellow solution having a yellow-green fluorescence.
Distribution of Riboflavin:
i. Animal Sources:
Milk (lactoflavin), liver, kidney, muscle, eggs.
ii. Vegetable Sources:
Whole grain, green leafy vegetables. It has also been synthetically prepared.
Functions of Riboflavin:
i. Essential for growth.
ii. Both FMN and FAD in combination with protein (apoenzyme) play major role in a number of enzyme system and the enzyme containing this vitamin called flavoprotein.
Riboflavin present in various enzyme systems in its two forms:
a. In the form of riboflavin phosphate (FMN = Flavin mononucleotide). FMN is a constituent of yellow enzyme of Warburg and Christian, cytochrome c reductase, l-amino acid dehydrogenase.
b. In the form of FAD (flavin adenine dinucleotide). A great number of enzymes like xanthine oxidase, diaphorase, a synthetic enzyme of Warburg and Christian, fumaric dehydrogenase, liver aldehyde oxidase, Haas enzymes, glycine oxidase, etc., contain FAD as prosthetic group and the enzyme xanthine oxidase also contains iron and molybdenum.
iii. Riboflavin is related to the metabolic process of proteins.
iv. It is also an integral part of the prosthetic group of acyl-CoA dehydrogenase—the enzyme which mediates the first oxidative step in the oxidation of fatty acids.
v. A suggestion has been made that riboflavin plays some part in cornea and deficiency produces corneal vascularisation. But this is still controversial.
Deficiency of Riboflavin:
The tissues which have originated from ectoderm (e.g., nervous system, skin, and eye) are first affected.
a. Cheilotic fissures at the corner of the mouth and ulcers on the lips (Fig. 11.5),
b. Angular stomatitis,
c. Glossitis (magenta tongue).
Keratitis, corneal opacities, vascularisation of cornea and photophobia.
Loss of hair and dry and scaly skin.
iv. Arrested growth occurs.
Daily Requirement of Riboflavin:
1.5 to 1.8 mgm in adults. The daily requirement of riboflavin can be calculated in term of protein intake, i.e., 0.025 mgm of riboflavin per 1 gm of protein.