Lecturer of medical biochemistry Folic acid & Vitamin B12 Dr Eman Khairy Lecturer of medical biochemistry & molecular biology

Understand that vitamins are crucial nutrients that are important to health. Know that folic acid and vitamin B12 are the hematopoietic vitamins. Explain how vitamin B12 is important in the activation of folic acid. Clarify why deficiencies of both of them produce the same type of anemia. 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 Fat soluble A, E, K & D Water soluble C & B complex

Water soluble vitamins B Family Vitamin C Ascorbic acid

B Family Hematopoietic Energy Related Folic Acid Vitamin B12 Others Thiamin (B1) Biotin Riboflavin (B2) Niacin (B3) Pantothenic acid Others Pyridoxine (B6)

Folic Acid (Vitamin B9)

Structure of Folic Acid Human can not synthesize folic acid. so it must supply in diet.

SOURCES Liver, yeast and green leafy vegetables are the major sources. Bacterial flora in intestine synthesizes folic acid.

ABSORPTION A conjnugase enzyme (-glutamyl hydrolase) (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.

Requirements 400 g/day to be increased during pregnancy and lactation 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

FUNCTIONS OF H4- FOLATE: The active form of folic acid, tetrahydrofolic acid, acts as a carrier of one carbon groups (except CO2 which is carried by biotin) methyl (-CH3), methylene (-CH2-), methenyl (-CH=), formyl (-CHO), formimino (CH=NH) groups. The one C is carried on N5 and N10.

FUNCTIONS OF H4- FOLATE: I. Synthesis of purines (It provides C2 and C8 of purine ring) and the synthesis of dTMP. 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.

Synthesis of purine nucleotide C8 of purines by methenyl H4F C2 of purines by formyl H4F methenyl H4F

synthesis of dTMP from dUMP using Methylene group (-CH2) Thymidylate synthase H

II. Formation of serine & methionine Homocysteine methyl transferase

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.

Folate Analogs (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 -Used as antibiotic.

Causes : Folate deficiency (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: 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.

inhibition of DNA synthesis which affects rapidly dividing cells as 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.

due to impaired DNA synthesis in GIT mucosal lining. 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.

Antipernicious Anemia Cobalamin (Vitamin B12) Antipernicious Anemia  

Sources of B12: 1-Exogenous source: * The daily requirement of B12 is 1—2 μg/day. *vitamin B12 is found only in animal sources eg. liver, meat , eggs , and fish.

2-Endogenous source:. Intestinal flora synthesize B12 in the colon 2-Endogenous source: * Intestinal flora synthesize B12 in the colon. * Absorption occurs in the ileum.

The vitamin is composed of 2 halves: 1) Corrin ring: tetrapyrrol ring containing a single cobalt atom in the center. 2) A nucleotide (benzimidazol and ribose 3-phosphate)

Forms (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.

Liver stores provide up to 6 years supply of B12 ABSORPTION In the ileum vitamin B12 should be bound to a glycoprotein secreted by parietal cells of the stomach, the intrinsic factor. 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). Liver stores provide up to 6 years supply of B12

Function of Vit. B12: Two coenzymes 1. Methylcobalamin

2. 5’-deoxyadenosyl-cobalamin

DEFICIENCY OF B12 pernicious anemia. 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.  

DEFICIENCY OF B12 Causes of deficiency: 1- Vegetarians. 2- Malabsorption ( in cases of atrophic gastritis, achlorohydria , gastrectomy, elderly people ) → decreased secretion of intrinsic factor→ B12 malabsorption.

pernicious anemia: Manifestations 1) Megaloblastic Anaemia (macrocytic hyperchromic anemia): 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 Methyl transferase

2) Neurological manifestations: Progressive demyelination of nervous system  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.

NOTE Pernicious anemia is treated by cyanocobalamin intramuscular injection Not by oral supplementation because of lack of intrinsic factor

NOTE 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.   40