Folic acid (B9)

Metabolism of Folic Acid (B9)

1. Folic acid (B9] Gandham . Rajeev Department of Biochemistry, Akash Institute of Medical Sciences & Research Centre, Devanahalli , Bangalore, Karnataka, India. E-Mail: [email protected]

2. Folic acid ( B9 )

3. Chemistry The word folic acid is derived from latin word Folium means leaf & it is also isolated from the leafy vegetable spinach Folic acid mainly consists of three components Pteridine ring PABA (p-amino benzoic acid) Glutamic acid residue (1 to 7 residues) Hence it is known as Pteroyl-glutamic acid

4. Active form Tetrahydrofolate (THF or FH 4 ) is the active form of folic acid 5 10

5. N H H H N I H H 2 N N N H I N - CH 2 – NH- O II - C H I - N - CH –COO - I CH 2 I CH 2 I COO - 8 7 6 5 Folic Acid Dihydrofolate reductase 2NADPH + 2H + 2NADP 5,6,7,8 – Tetrahydrofolic acid (THF)

6. Metabolism Absorption: Formation of monoglutamate form: Most of the dietary folic acid exists as polyglutamate with 3-7 glutamate residues It is not absorbed in the intestine The glutamate side chains are cleaved by the enzyme folate conjugase or polylpolyglutamate hydrolase

7. Only monoglutamyl form of folic acid is absorbed from the intestine The enzyme folate conjugase is present in duodenum & jejunum Mucosal uptake & metabolism in mucosal cell Folate monoglutamate is taken up by the mucosal cell In the mucosal cell, folate monoglutamate is reduced to tetrahydrofolate & methylated to form N 5 methyl tetrahydrofolate (in circulation)

8. N 5 methyl tetrahydrofolate enters the circulation Storage: Inside the cells, tetrahydrofolates are found as polyglumates (with 5-6 amino acid residues) Which are biologically most potent Polyglutamate is the storage form of folic acid It is mainly stored in the liver (10-20 mg)

9. Biochemical functions Folic acid is not biologically active The active coenzyme forms of folic acid are Tetrahydrofolic acid (FH 4 ) N 5 methyl tetrahydrofolic acid (N 5 FH 4 ) N 5 ,N 10 methylene tetrahydrofolic acid N 10 formyl tetrahydrofolate(N 10 formyl FH 4 ) N 5 formimino tetrahydrofolate (N 5 formimino FH 4 )

10. N 5 formyl tetrahydrofolate (N 5 formyl FH 4 ) N 5,10 methenyl tetrahydrofolate N 5,10 methenyl FH 4 ) N 5 formyl THF -CHO N 10 formyl THF -CHO N 5 formimino THF -CH=NH N 5 ,N 10 methenyl THF = CH N 5 ,N 10 methylene THF =CH 2 N 5 methyl THF - CH 3

11. The coenzymes of folic acid are actively involved in the one carbon metabolism THF acts as an acceptor or donor of one carbon units (formyl, methyl etc.) in reactions involving amino acid & nucleotide metabolism The one carbon units bind with THF at position N5 or N10 or on both N5 &N10 of pteroyl structure

12. ONE CARBON METABOLISM Amino acid metabolism is important for transfer or exchange of one carbon units The following one carbon fragments are involved in biological reactions Methyl (-CH 3 ) Hydroxymethyl (-CH 2 OH) Methylene (=CH 2 ) Methenyl (-CH=) Formyl (-CH=O) Formimino (-CH=NH)

13. THF is a versatile coenzyme actively participates in one carbon metabolism Transfer of methyl groups from S- adenosylmethionine B 12 is also involved The one carbon units covalently binds with THF at position N5 or N10 or on both N5 &N10 of pteroyl structure of folate

14. Generation of one carbon units Many compounds particularly amino acids act as donors of one carbon units The formate is released from glycine & tryptophan metabolism combines with THF to form N10 – formyl THF Histidine contributes formimino fragment to produce N5 – formimino THF Serine is converted to glycine , N5,N10 methylene THF is formed This is most common entry of 1C units into one carbon pool

15. Choline contributes to the formation of N5 methyl THF Different derivatives of THF carrying one carbon units are interconvertible , & this is metabolically significant for the continuity of one carbon pool Utilization of one carbon units Utilized for synthesis of wide variety of compounds

16. These includes Purines Formylmithionine tRNA (initiation of protein synthesis) Glycine Pyrimidine nucleotide etc Role of methionine & vitamin B12 Methyl group is an important one carbon unit Methionine is active donor of methyl groups in transmethylation reactions

17. After the release of methyl group, methionine is converted to homocysteine For regeneration of methionine , homocysteine & N5-methyl THF are required & this reaction is dependent on Vitamin B12 The one carbon pool, under the control of THF, is linked with methionine metabolism through Vitamin B12

18. Glycine Tryptophan Formate N 10 -Formyl THF Purine (C2) Histidine FIGLU N 5 -FormiminoTHF N 5 ,N 10 -Methenyl THF Serine N 5 ,N 10 -Methylene THF Choline Betaine N 5 Methyl THF B12 Methionine S- Adenosyl Methionine Homocysteine CH 3 - Transmethylation Formylmethionine Purines (C8) Serine Thymidylate Major sources Major Products THF THF THF THF One Carbon Metabolism

19. Dietary sources Rich sources are green leafy vegetables such as spinach, cauliflower Poor sources are liver, kidney, milk, fruits

20. rda Men -100 µg/day Women -100 µg/day Pregnancy -400 µg/day Lactation -150 µg/day

21. deficiency Dietary deficiency is the most common cause of folic acid Dietary deficiencies are caused by Inadequate intake seen in alcoholics Overcooking of food resulting in loss of folic acid activity Impaired absorption due to small intestinal diseases, Drugs interfere with folic acid absorption- sulfamethaxazole

22. Increased demand of folic acid seen in pregnancy Hemolytic anemia Hence folic acid preparations are prescribed in pregnancy &hemolytic anemia Other causes Loss of folic acid seen in patients undergoing dialysis Impaired synthesis of active form seen in patients receiving folic acid antagonists such as methotrexate

23. Clinical features Megaloblastic anemia characterized by hyperchromic macrocytic anemia (due to maturation bloked ) Magaloblastic changes are seen in bone marrow & mucosa Patients look pale Glossitis

24. Laboratory findings Peripheral smear shows macrocytic hyperchromic anemia Hypersegmentation of neutrophils is common

25. Bone marrow shows megaloblastic changes characterized by abnormally large size of erythroid cells with cytoplasmic maturation but impaired nuclear maturation due to defective DNA synthesis Defective red cell production

26. Biochemical findings Low plasma folic acid levels (<3ng/ml) Low red cell folic acid levels (<150 ng /ml) Normal plasma Vitamin B12 levels Increased plasma LDH levels

27. FIGLU excretion test:- Folic acid deficiency is associated with increased excretion of formiminoglutamate (FIGLU) in urine Due to impaired conversion of FIGLU to glutamate in a reaction requiring FH 4 Histidine FIGLU Formimino FH 4 Histidine Glutamate FH 4 FIGLU Formimino FH 4 Glutamate FH 4 Urine Folic acid deficiency

28. Folic acid deficiency & neural tube defects Folic acid supplementation during pregnancy helps to prevent neural tube defects Mainly involved in brain & spinal cord Science, folic acid involved in nucleic acid & amino acid metabolism Deficiency results in impaired & aberrant neural development

29. Folic acid deficiency & homocysteinemia Homocysteine is a risk factor for CHD Folic acid is required for conversion of homocysteine to methionine Deficiency is associated with increased plasma levels of Homocysteine Folic acid suplementation decreases plasma homocysteine levels Homocysteine levels are also increased in Vitamin B12 & Pyridoxine deficiency

30. Folic acid antagonists Aminopterin & Amethopterin ( methotrexate ) Aminopterin & Amethopterin ( methotrexate ) competitevely inhibit the enzyme dihydrofolate reductase in humans It impaires the formation of active form of tetrahydrofolate from dihydrofolate Significance:- During the conversion of deoxyuridylate to deoxythymidylate , dihydrofalate is formed, utilizes N 5,10 methylene FH4

31. Deoxythymidylate is required for DNA synthesis Folic acid antagonists will block DNA synthesis & inhibit cell division Clinical uses:- Aminopterin & Amethopterin ( methotrexate ) inhibit DNA synthesis in cancer cells Used in treatment of cancer Particularly leukemia & choriocarcinoma

32. trimethoprim It is a folic acid antagonist & it inhibits the bacterial dihydrofolate reductase Thus impairs the deoxythymidylate synthesis leading to decreased synthesis of DNA It is mainly used in bacterial infections

33. References Harper’s Biochemistry 25 th Edition. Fundamentals of Clinical Chemistry by Tietz. Text Book of Medical Biochemistry-A R Aroor. Text Book of Biochemistry-DM Vasudevan Text Book of Biochemistry-MN Chatterjea Text Book of Biochemistry-Dr.U.Satyanarana