FOLIC ACID (B9]

Gandham. Rajeev

Department of Biochemistry,Akash Institute of Medical Sciences & Research Centre,Devanahalli, Bangalore, Karnataka, India.

E-Mail: gandhamrajeev33@gmail.com

Folic acid ( B9 )

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

ACTIVE FORM

Tetrahydrofolate (THF or FH4) is the active

form of folic acid

5

10

NH

H

HNIH

H2N

N

N

HIN

- CH2 – NH-

O II- C

H I - N - CH –COO-

ICH2

ICH2

ICOO-

87

65

Folic Acid

Dihydrofolate reductase

2NADPH + 2H+

2NADP

5,6,7,8 – Tetrahydrofolic acid (THF)

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

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)

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)

Biochemical functions

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)

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

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

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 (-CH3)

Hydroxymethyl (-CH2OH)

Methylene (=CH2)

Methenyl (-CH=)

Formyl (-CH=O)

Formimino (-CH=NH)

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

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

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

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

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

GlycineTryptophan

Formate N10-Formyl THF

Purine (C2)

Histidine

FIGLU N5-FormiminoTHF N5,N10-Methenyl THF

Serine N5,N10-Methylene THF

CholineBetaine

N5 Methyl THF

B12

Methionine

S-Adenosyl Methionine

Homocysteine

CH3-

Transmethylation

Formylmethionine

Purines (C8)

Serine

Thymidylate

Major sourcesMajor Products

THF

THF

THF

THF

One Carbon Metabolism

DIETARY SOURCES

Rich sources are green leafy vegetables

such as spinach, cauliflower

Poor sources are liver, kidney, milk,

fruits

RDA

Men -100 µg/day

Women -100 µg/day

Pregnancy -400 µg/day

Lactation -150 µg/day

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

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

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

LABORATORY FINDINGS

Peripheral smear shows macrocytic

hyperchromic anemia

Hypersegmentation of neutrophils is

common

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

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

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

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

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

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 N5,10 methylene FH4

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

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

REFERENCES

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