Folic Acid and vitamin B12

Folic Acid and vitamin B12 ILOs: by the end of this lecture, you will be able to: 1. Understand that vitamins are crucial nutrients that are important to health. 2. Know that folic acid and vitamin B12 are the hematopoietic vitamins. 3. Explain how vitamin B12 is important in the activation of folic acid. 4. Clarify why deficiencies of both of them produce the same type of anemia. 5. Understand why B12 deficiency results in neurological manifestations. Vitamins are organic compounds occurring in small quantities in different natural foods. They are essential factors that must be supplied in the diet. They are not synthesized by humans. They are needed in small quantities (trace amounts) for normal growth, differentiation and maintenance of normal cellular function. Deficiency of vitamins result in various metabolic diseases. Vitamins are divided into 1-Fat soluble A, E, K & D 2-Water soluble C & B complex Vitamin B complex includes: Thiamin (B1) Biotin Riboflavin (B2) Niacin (B3) Pantothenic acid Pyridoxine (B6) Folic Acid Vitamin B12 Folic Acid (Vitamin B9- folacin) Structure Folic acid or folate consists of the base pteridine attached to one molecule of each p-aminobenzoic acid (PABA), and glutamic acid. 1

Human cannot synthesize folic acid. Animals are not capable of synthesizing PABA or attaching glutamic to pteridine. So it must supply in diet. Sources (1) Liver, yeast and green leafy vegetables are the major sources. It is present as polyglutamate conjugate consisting of linked polypeptide chain of 2-7 glutamate residues. (2) Bacterial flora in intestine synthesizes folic acid. Absorption A conjnugase enzyme (gamma-glutamyl hydrolase) which presents in the brush border of the intestine acts on the ingested polyglutamate form of folic acid in food. It cleaves glutamate residues leaving a single one glutamate. The resulting monoglutamate form is absorbed into mucosal cell by a specific carrier. In the tissues, the monoglutamate form is converted to the polyglutamate form which is the functional form of tetrahydrofolate. Requirements 400 g/day to be increased during pregnancy and lactation Activation of Folate to THF The biologically active form is the tetrahydrofolate. It is the carrier of activated one carbon units that are essential for the synthesis of choline, serine, glycine, methionine and purines. The one C is carried on N5 and N10. (Except CO2 which is carried by biotin) methyl (-CH3), methylene (-CH2-), methenyl (-CH=), formyl (-CHO), formimino (CH=NH) groups. Functions of H4- folate I. Synthesis of purines (It provides C2 and C8 of purine ring) and the synthesis of dtmp. 2

So, it is important for DNA synthesis (replication) especially in rapidly dividing cells e.g. developing RBCs in bone marrow, surface epithelial cells of mouth, GIT, urinary and respiratory tract. In purine nucleotide, C8 of purines added by methenyl H4F and C2 of purines by formyl H4F also, synthesis of dtmp from dump by using Methylene group (-CH2) II. Formation of serine & methionine Folate Analogues Methotrexate It is potent inhibitor of human dihydrofolate reductase; so prevent folate activation to H4F. As H4F used in purines & thymine synthesis, folate analogues are applied therapeutically as blockers of tumor growth. 3

Trimethoprim They are competitive inhibitor of bacterial dihydrofolate reductase. -Therefore, it inhibits DNA synthesis in bacteria. -It is applied therapeuticaly as antibiotic. Sulfonamide -It is PABA analogue -It inhibits synthesis of folic acid only in bacteria and used as antibiotic. Folate deficiency Causes: (1) Increase rate of cell division e.g. pregnancy, hemolytic anemia. (2) Long term use of drugs: oral contraception, estrogens, alcohol, and anticonvulsants (decrease absorption and increase catabolism of folate). (3) Administration of methotrexate (inhibitor of dihydrofolate reductase) as anticancer chemotherapeutic. (4) Vitamin B12 deficiency (Folate trap) Manifestations: 1) Megaloblastic anemia (macrocytic hyperchromic anemia) due to impaired DNA synthesis. It is called megaloblastic because there is accumulation of large, immature red cell precursors, known as megaloblasts, in the bone marrow and the blood. 2) Glossitis and gastrointestinal disturbances due to impaired DNA synthesis in GIT mucosal lining. 3) Neural tube defects (Spina bifida and anencephaly) in the fetus. Occurs in the first week s pregnancy. Folic acid supplementation during the first trimester significantly reduces the defects Why patients with folate malabsorption suffer from megaloblastic anemia? As, the one C. unit carried by THF is required for synthesis of purines and pyrimidines. So, folate deficiency leads to inhibition of DNA synthesis which affects rapidly dividing cells as bone marrow cells. 4

Cobalamin (Vitamin B12) - Antipernicious Anemia Sources of B12: 1-Exogenous source: * The daily requirement of B12 is 1 2 μg/day. * Vitamin B12 is exclusively formed by microorganisms. It is absent from plant sources. vitamin B12 is found only in animal sources eg. Liver, meat, eggs, and fish. 2-Endogenous source: * Intestinal flora synthesize B12 in the colon. Structure The vitamin is composed of 2 halves: 1) Corrin ring: tetrapyrrol ring containing a single cobalt atom in the center. Cobalt is essential for activity, it is the cause of the red color. 2) A nucleotide (benzimidazol and ribose 3-phosphate) Forms Many groups can be attached to cobalt as a) Methyl (methyl cobalamin), the major circulating form of B12. (b) 5-deoxyadenosine (5-deoxy adenosyl cobolamin), the major storage form. (c) Hydroxy (hydroxycobalamin): the most active form. (d) Cyanide (cyanocobalamin): inactive form. Absorption Occurs in the ileum, Intestinal absorption needs a highly specific glycoprotein receptor called intrinsic factor secreted by parietal cells of the stomach. All forms of cobalamin are transported in plasma as Transcobalamin II-vitamin B12 complex which delivers cobalamin to liver and other tissues. In the liver, cobalamin is stored attached to a protein (Transcobalamin I). 5

It is the only water soluble vitamin that is stored in the liver. Liver stores provide up to 6 years supply of B12. Metabolic Functions of Vit. B12: Two coenzymes -Deoxyadenosylcobolamin is the coenzyme for conversion of methylmalonyl CoA to succinyl CoA which is a member of the citric acid cycle. -Methylcobolamin is the coenzyme in the combined conversion of (1) homocysteine to methionine, and (2) methyltetrahydrofolate to tetrahydrofolate. Deficiency of B12 Vitamin B12 is wide spread in animal food and liver stores provide up to 6-year supply, so deficiency is very rare and usually occurs in patients who fail to absorb the vitamin from the intestine, resulting in pernicious anemia. Causes of deficiency: 1- Vegetarians. 2- Malabsorption (in cases of atrophic gastritis, achlorohydria, gastrectomy, elderly people) decreased secretion of intrinsic factor B12 malabsorption. Manifestations (I) Pernicious anemia: 1) Megaloblastic Anaemia (macrocytic hyperchromic anemia): 6

It reflects the effect of vitamin B12 on folate metabolism (folate trap, or methyl trap). It is due to failure of transfer of CH3 from folic acid to B12. Folate will be trapped as methyltetrahydrofolate (folate trap) leads to impaired purine and pyrimidine synthesis (DNA synthesis) and prevention of cell division with the accumulation of immature erythrocytes in the circulation. 2) Neurological manifestations: Progressive demyelination of nervous system with sensory and motor symptoms. It is due to relative deficiency of methionine which causes defective methylation of phosphatidyl ethanolamine to phosphatidyl choline with subsequent demyelination of nerves. (II) Glossitis and lesions in mucous surfaces (III) Methylmalonic Acidurea. (IV) Homocysteinemia and Homocysteinuria Homocysteinemia is related to cardiac disease. NOTES Pernicious anemia is treated by cyanocobalamin intramuscular injection Not by oral supplementation because of lack of intrinsic factor. If folic acid is administered in case of B12 deficiency, it will improve only the megaloblastic anemia but not the neurological manifestations. Thus, therapy of megaloblastic anemia is often initiated with folic acid and vitamin B12 until the cause of the anemia can be determined. 7