Phar 722 Pharmacy Practice III Vitamins- Riboflavin (B 2 ) Spring 2006

Riboflavin (B 2 ) Study Guide The applicable study guide items in the Vitamin Introduction History Structure including commercial forms of the vitamin Conversion to cofactor forms Function of the cofactor including the specific types of reactions Deficiency condition

History Shortly after the discovery of thiamine from yeast concentrates, the presence of a second nutritional factor in such materials was suggested. This second factor also was reported to have a pellagra-preventative activity since it alleviated a deficiency-induced dermatitis in rats. It was called Vitamin B2 in England and Vitamin G in the United States.

Chemistry Riboflavin has a characteristic flavin ring system which gives it a unique spectroscopic and instability properties. Two commercial forms. –Riboflavin, itself, is poorly water soluble (1 gm/10,000 ml). –Riboflavin phosphate’s solubility is 0.1 gm/ml

Solubility: 1 gm/10,000 ml; A commercial form Solubility: 0.1 gm/ml; A commercial form

Riboflavin Uptake and Metabolism Because of its poor water soluble, uptake of riboflavin is slow since the vitamin should be in solution in order to enter the intestinal mucosa cell. Therefore, it is recommended that supplemental administration of riboflavin be done with food in order to delay intestinal emptying. In the mucosa cell, a flavokinase phosphorylates the terminal alcohol with a phosphate from an ATP forming riboflavin phosphate. –The latter, as the sodium salt, also is the water soluble (0.1 gm/1 ml) commercial form of the vitamin. The conversion of riboflavin phosphate (FMN) to the more common cofactor, flavin adenine dinucleotide (FAD) requires the conversion of ATP to AMP and the formation of an anhydride linkage. –The pyrophosphate will be cleaved providing enough energy for this synthesis to go to completion. While it is not clear, this step probably occurs in the tissues requiring FAD. While flavin chemistry has been studied extensively, there has been little research of riboflavin biochemistry.

Biochemical Functions-1 Riboflavin, as FMN/FMNH2 or FAD/FADH2, is the coenzyme/cofactor of flavin enzymes. –It is required for many oxidation-reduction reactions generally (but not always) of carbon-carbon bonds. –Even though there is no defined deficiency syndrome, this vitamin is required for several life-supporting oxidation-reduction biochemical reactions. Some examples are on the next slides.

Biochemical Functions-2 Oxidation-reduction of carbon-carbon bonds.

Biochemical Functions-3 Xanthine Oxidase

Biochemical Functions-4 Electron (respiratory) transport chain

Biochemical Functions-5 Oxidation-Reduction of Thiol-Disulfide Systems Examples: glutathione reductases and lipoic acid in oxidative decarboxylations of α- ketoacids (pyruvate, α- ketoglutarate)

Biochemical Functions-6 Monoamine oxidase (MAO) Examples: Metabolism of dopamine, norepinephrine, epinephrine and serotonin

Biochemical Functions-7 Biochemical Transformations of Other Vitamins –Folic Acid –Pyridoxine NOTE: This illustrates how one vitamin is dependent on there being an adequate concentration of another vitamin. A deficiency of one vitamin may induce a deficiency of another vitamin.

Riboflavin Deficiency Ariboflavinosis –Clinical riboflavin deficiency. –There is no deficiency syndrome in humans associated with this vitamin. –Usually associated with deficiencies of other “B vitamins”. Symptoms –Sore throat –Redness and swelling of the lining of the mouth and throat –Cracks or sores on the outside of the lips (cheliosis) and at the corners of the mouth (angular stomatitis) –Moist, scaly skin inflammation (seborrheic dermatitis) –Vascularization of the cornea There is no good assay that correlates a riboflavin deficiency with specific biochemical function. It appears that humans do store appreciable amounts (possibly over six months) of the vitamin.

Hypervitaminosis Riboflavin Because of its poor solubility, it probably would be difficult to obtain a toxic dose of this vitamin. –Water soluble riboflavin phosphate may be another matter. Nevertheless, no significant toxicities have been reported. Estimated toxic dose: 1,000 mg (1 gm) –There is no UL.

Dosage Forms Most of the commercial forms of the vitamin are synthetic. Stability –Riboflavin is one of the most unstable of the vitamins, particularly in light. Solutions of the vitamin must be protected from light. Riboflavin –This is a neutral molecule. Salt formation is not possible. –Because of its poor water solubility, it is used in dry dosage forms. Riboflavin Phosphate –The sodium salt is very soluble (0.1 gm/1 ml). –It is used in liquid dosage forms.

DRIs-1 AI –Infants mg/day EAR –Children ( years) mg/day –Males ( years)1.1 mg/day –Females ( years)0.9 mg/day –Men ( years)1.1 mg/day –Women ( years)0.9 mg/day –Pregnancy1.2 mg/day –Lactation1.3 mg/day

DRIs-2 RDA –Children ( years) mg/day –Males ( years)1.3 mg/day –Females ( years)1.0 mg/day –Men ( years)1.3 mg/day –Women ( years)1.1 mg/day –Pregnancy1.4 mg/day –Lactation1.6 mg/day UL –None reported

Food Sources Liver Kidney Milk Yeast Plant Animal tissue Animals have to obtain riboflavin from plants or other animals.