The role of vitamins in metabolism

1. The role of vitamins in metabolism Dr. Samah Kotb Nasr Eldeen

2. 3-Niacin

4. Function • Niacin, also called nicotinic acid, is involved in oxidation-reduction reactions as coenzymes • Niacin is involved in the synthetic pathway of adenosine tri-phosphate (ATP) and in ADP-ribose transfer reactions. • The two niacin coenzymes involved in these processes are nicotinamide-adenine dinucleotide (NAD) and nicotinamide-adenine dinucleotide phosphate (NADP). • NAD is a substrate for the enzyme poly (ADP-ribose) polymerase (PARP) which is involved in DNA repair. • Tryptophan is a precursor to niacin.

5. Clinical conditions • Niacin deficiency leads to a disorder called pellagra. • Pellagra is characterized by the 3 "D"s: dermatitis, diarrhea, and dementia. • Skin exposed to sunlight develops dark, scaly dermatitis. There is a characteristic "necklace" lesion on the neck. Absorption and excretion • Niacin is absorbed into the intestine by diffusion. Niacin metabolites are excreted in the urine.

6. Intestinal inflammation leads to diarrhea. Long-term niacin deficiency leads to central nervous system dysfunction manifested as confusion, apathy, disorientation and neuritis. • Individuals may be predisposed to niacin deficiency if they are also consuming low levels of vitamin B6, copper, and riboflavin. • Niacin deficiency can also lead to a disease called Hartnup's syndrome. • Hartnup disease is an inherited nutritional disorder with primary symptoms including a red, scaly rash and sensitivity to sunlight.

7. Food Sources • Meat is an excellent source of niacin. Legumes, milk, eggs, and yeast are good sources. Grains are commonly enriched with niacin. Milk and eggs are actually low in niacin, but high in tryptophan, a precursor to niacin. • Good sources: liver, heart, kidney, chicken, beef, fish (tuna, salmon), avocados, dates, tomatoes, leafy vegetables, broccoli, carrots, sweet potatoes, asparagus, nuts, legumes, mushrooms, and brewer's yeast.

8. Supplementation • Niacin is used for the treatment of hyperlipidemia. Niacin decreases very low-density lipoprotein (VLDL) and increases high density lipoprotein (HDL) leading to a regression in artherosclerotic plaques.

9. 4-Pyridoxine

10. Vitamin B6 is a collective term for pyridoxine, pyridoxal, and pyridox-amine, all derivatives of pyridine. They differ only in the nature of the functional group attached to the ring

11. Function • Pyridoxine, or vitamin B6, is a coenzyme in reactions of amino acid, carbohydrate, and fat metabolism. • Vitamin B6 is actually a term used for a group of vitamins with similar functions: pyridoxine, pyridoxal, and pyridoxamine. All are precursors to pyridoxine coenzyme pyridoxalphosphate (PLP). • This coenzyme is involved in reactions involving many systems within the body. PLP has a role in gluconeogenesis through transaminase reactions. • Pyridoxine occurs primarily in plants, whereas pyridoxal and pyridoxamine are found in foods obtained from animals.

12. The conversion of tryptophan to niacin utilizes an enzyme that requires PLP. In red blood cells PLP is a coenzyme for transaminases. PLP is also involved in the synthesis of several neurotransmitters, such as serotonin, taurine, dopamine, and norepinephrine. • Intake of vitamin B6 has been associated with immune function. Absorption and excretion • Pyridoxine is absorbed passively into the small intestine ( jejunum). • The majority of B6 is stored in the muscle.

13. Clinical conditions • Vitamin B6 metabolism is affected by an alteration in tryptophan metabolism or plasma PLP concentration. • Many conditions result in altered tryptophan metabolism. These include asthma, breast cancer, diabetes, and rheumatoid arthritis. • PLP plasma concentration is decreased in asthma, alcoholism, breast cancer, diabetes, renal disease, sickle-cell anemia, and smoking. • Deficiency may cause anemia, peripheral neuropathy • Megadoses of vitamin B6, one gram or more per day, have been reported to cause nerve damage. • Requirements of vitamin B6 are increased by high protein intake and possibly with exercise.

14. Food Sources • Good sources of vitamin B6 are meats, grains, nuts, bananas and vegetables. When milk is dried it loses about half of its B6. Freezing and canning can also reduce content. Supplementation • Vitamin B6 has been used for Down's syndrome, autism, gestational diabetes, diabetic neuropathy, and depression.