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: gandhamrajeev33@gmail.com

4.  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

5.  Tetrahydrofolate (THF or FH4) is the active
form of folic acid
5
10

7. N
H
H
HN
I
H
H2N
N
N
H
I
N
- CH2 – NH-
O
II
- C
H
I
- N - CH –COO-
I
CH2
I
CH2
I
COO-
8
7
6
5
Folic Acid
Dihydrofolate
reductase
2NADPH + 2H+
2NADP
5,6,7,8 – Tetrahydrofolic acid (THF)

8.  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

9.  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
N5 methyl tetrahydrofolate (in circulation)

10.  N5 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)

11.  Folic acid is not biologically active
 The active coenzyme forms of folic acid are
 Tetrahydrofolic acid (FH4)
 N5 methyl tetrahydrofolic acid (N5FH4)
 N5,N10 methylene tetrahydrofolic acid
 N10 formyl tetrahydrofolate(N10 formyl FH4)
 N5 formimino tetrahydrofolate (N5 formimino
FH4)

12.  N5 formyl tetrahydrofolate (N5 formyl FH4)
 N5,10 methenyl tetrahydrofolate N5,10 methenyl
FH4)
 N5 formyl THF -CHO
 N10 formyl THF -CHO
 N5 formimino THF -CH=NH
 N5,N10 methenyl THF= CH
 N5,N10 methylene THF =CH2
 N5 methyl THF - CH3

13.  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

14.  Amino acid metabolism is important for
transfer or exchange of one carbon units
 The following one carbon fragments are
involved in biological reactions
 Methyl (-CH3)
 Hydroxymethyl (-CH2OH)
 Methylene (=CH2)
 Methenyl (-CH=)
 Formyl (-CH=O)
 Formimino (-CH=NH)

15.  THF is a versatile coenzyme actively
participates in one carbon metabolism
 Transfer of methyl groups from S-
adenosylmethionine
 B12 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

16.  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

17.  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

18.  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

19.  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

20. Glycine
Tryptophan
Formate N10-Formyl THF
Purine (C2)
Histidine
FIGLU N5-FormiminoTHF N5,N10-Methenyl THF
Serine N5,N10-Methylene THF
Choline
Betaine
N5 Methyl THF
B12
Methionine
S-Adenosyl Methionine
Homocysteine
CH3-
Transmethylation
Formylmethionine
Purines (C8)
Serine
Thymidylate
Major sources
Major Products
THF
THF
THF
THF
One Carbon Metabolism

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

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

23.  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

24.  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

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

26.  Peripheral smear shows macrocytic
hyperchromic anemia
 Hypersegmentation of neutrophils is common

27.  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

28.  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

29.  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 FH4
Histidine
FIGLU
Formimino FH4
Histidine
Glutamate
FH4
FIGLU
Formimino FH4
Glutamate
FH4
Urine
Folic acid
deficiency

30.  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

34.  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

36.  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 N5,10 methylene FH4

37.  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

38.  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

40.  Harper’s Biochemistry 25th 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