COBALAMINE (B12]

Gandham. Rajeev

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

E-Mail: gandhamrajeev33@gmail.com

Vitamin B12

Cobalamin

Extrinsic factor (EF) of castle

Antipernicious anemia factor

Chemistry:

Vitamin B12 is water soluble, heat stable

and red in colour

It contains 4.35% cobalt by weight

Four pyrrole rings co-ordinated with a

cobalt atom is called as a Corrin ring

Synonyms of Vitamin B12

The corrin ring has four pyrrol units, like

porphyrin

Two of the pyrrole units (A&D) are directly

bound to each other where as the other two

(B & C) are held by methene bridges

The groups namely methyl, acetamide and

propionamide are the substituents on the

pyrrole rings

Cobalt present at the centre of the corrin

ring is bonded to the four pyrrole nitrogens

Cobalt also holds dimethyl benzimidazole

(DMB) containing ribose 5-Phosphate and

amino isopropanol

A nitrogen atom dimethyl benzimidazole is

linked to cobalt

The amide group of aminoisopropanol

binds with D ring of corrin

The cobalt atom also possesses a sixth

substituent group located above the plane

of the corrin ring

A

D

B

C

Aminoisopropanol

Methyl, Adenosine, acetamide, propionamide

Aminoisopropanol

dimethyl benzimidazole

D

BA

C

All the forms of vitamin B12 are called

cobalamins because of the presence of

cobalt atom

Different forms of cobalamins include

Cyanocabalamin:- It refers to the isolated

form of vitamin B12 in which cyanide was

added to promote crystallization of

cobalamin during the isolation process

Hydroxycobalamin:- It was usually present in

the tissues and in the naturally occurring

forms of Vitamin B12

Methylcobalamin :- Methyl group replaces

adenosyl group

Adenosyl cobalamin (Ado-B12):-

When taken up by the cells, these groups

are removed and deoxy adenosyl

cobalamin or Ado-B12 is formed

In the food, vitamin B12 is present as a

complex with proteins

The free form of vitamin B12 is released by

cooking, HCL present in gastric juice and

proteolysis by pepsin in the stomach

Mechanism:-

The absorbance of vitamin B12 requires

intrinsic factor

Intrinsic factor is a glycoprotein secreted by

parietal cells of stomach

metabolism

Vitamin B12 combines with intrinsic factor

The vitamin B12 – intrinsic factor complex

reaches the ileum where it is absorbed

In the ileum, the complex attaches to a

specific receptor and is taken up by the

mucosal cell.

In the mucosal cell, vitamin B12 is released

from its complex and reaches the portal

circulation

In the portal blood, it is transported in

combination with transcobalamin II

Vitamin B12 is presented to cells where it is

taken up by the cells through receptor

mediated endocytosis

Storage:

It is mainly stored in liver, leukocytes and

gastric mucosa

It is stored as complex with transcobalamin-

I&II

Transport

ProteinB12

B12

B12

B12

Protein

IF

IF

IF

Mucosal cellB12

Methyl B12B12

B12

TC 1-B12(90%)

TC 1-B12(90%)

TCII -B12(10%)

TCII -B12(10%)

Tissues

B12

TCII

Methyl B12

Deoxyadenosyl

B12(LIVER)

GIT

Plasma Tissues

About 10 enzymes requiring vitamin B12

have been identified

Most of them are found in bacteria ( mutase,

ribonucleotide reductase, etc.)

There are two reactions in mammals that

dependent on vitamin B12

Biochemical functions

Methyl cobalamin is essential for the

conversion of homocysteine to methionine

and formation of FH4 from methyl FH4

The reaction is catalyzed by homocysteine

methyl transferase

Synthesis of Methionine from homosysteine

Homocysteine

Homocysteine methyl transferase

Methionine

methylcobalamin

THFN5 methyl THF

The circulating methyl FH4 is converted to FH4

FH4 is either used for storage as

folylpolyglutamate form or it is utilized for other

reactions such as formation of methylene FH4

Methyl folate trap:-

In B12 deficiency, impaired conversion of methyl

FH4 to FH4 results in accumulation of methyl

FH4 & is called as methyl folate trap

Methyl folate trap results in decreased availability

of FH4 & FH4 derivatives that are

Significance of the reaction

Needed for purine nucleotide & thymidylate

synthesis

Thus vitamin B12 deficiency, results in

secondary folate deficiency

Role of methyl cobalamin & folate trap

Methyl FH4

FH4

N5,10 methylene FH4dUMP

dTMP

DNA

Homocysteine

Methionine

Methylfolate trapMethyl FH4

Homocysteine methyl transferase

FH4

Serine

Glycine

Thymidylate synthase

B12

Isomerization of methyl CoA to succinyl

CoA:-

The degradation of odd chain fatty acids

and some amino acids (valine, leucine etc)

and pyrimidines (thymine & Uracil) produce

propionyl CoA, an imp. Compound

methylmalonyl CoA

The methyl malonyl CoA mutase converts

methyl malonyl CoA to succinyl CoA in the

presence of Vitamin B12,deoxyadenosyl

cobalamin

In B12 deficiency, methyl malonyl CoA

accumulates and is excreted in urine as

methyl malonic acid

This condition is called as methylmalonic

aciduria, occurs in B12 deficiency.

Demyelination :- Myelination of nerves is

impaired in B12 deficiency due to accumulation

of methylmalonyl CoA

Demyelination is due excessive accumulation

of methylmalonyl CoA

Odd chain FA

Propionyl CoA

Amino acids(Val, Ile, Thr,

Met)Thymine,

uracil

Methyl malonyl CoA

Succinyl CoA

Methyl malonyl CoA mutase

Methyl malonic acid

Excreted in urine

5-Deoxyadenosylcabalamin (of B12)

+ Methylmalonic aciduria

Rich sources of vitamin B12 are meats, egg,

milk, sea foods

B12 is synthesized by microorganisms

Vitamin B12 is not present in Plant sources

Dietary sources

Adults -1 µg/day

Pregnancy & lactation -2 µg/day

Causes:

Inadequate intake-seen in pure vegetarians

and rarely in alcoholism

Impaired absorption

This is mainly caused by lack of intrinsic factor

Lack of intrinsic factor is called as pernicious

RDA

Deficiency

anemia & it is caused by destruction of gastric

mucosa

Impaired absorption is also seen in small

intestinal disorders

Impaired storage and transport:

Inadequate utilization of vitamin occurs because

of liver diseases and abnormalities of transport

proteins

Increased requirements are seen in

hyperthyroidism, infancy & thalassemia

Increased excretion occurs in nephrotic syndrome

Vitamin B12 deficiency is characterized by

Megaloblastic anemia:

Feature of megaloblastic anemia include

pallor,fatigue, glossitis ( beefy red tongue) &

slight yellow discoloration of the conjunctiva

due to increased unconjugated bilirubin

Progression of anemia may result in angina

& congestive cardiac failure

Clinical Features

Gastrointestinal dysfunction:

GIT epithelial cells are undergoing rapid

turnover

& dependent on vitamin B12

B12 deficiency results in weight loss &

diarrhea

Demyelination of nervous tissue

Subacute combined degeneration:

Damage to nervous system is seen in B12

deficiency

There is demyelination affecting cerebral

cortex as well as dorsal column & pyramidal

tract

Symmetrical paresthesia of extremities,

alterations of tendon & deep senses &

reflexes, loss of position sense, unsteadiness

in gait, positive Romberg’s sign & positive

Babinski’s sign are seen

Achlorhydria:

Absence of acid in gastric juice is associated

with B12 deficiency

Romberg’s sign & positive Babinski’s sign

Serum B12: It is quantitated by radio-

immunoassay or by ELISA

Methyl melonic acid is excreted in urine

FIGLU excretion test

Peripheral smear: Peripheral blood & bone

marrow morphology shows magaloblastic

anemia

Homocysteinuria: Excretion of

homocysteine in urine

Assessment of B12 deficiency

Schilling test:

Radioactive labelled (Cobalt-60) vitamin B12

1μ g is given orally

In gastric atrophy cases, there is no

absorption, hence the entire radioactivity is

excreted in faeces & radioactivity is not

observed in liver

If the cause is nutritional deficiency, there

will be increased absorption

Then radioactivity is noted in the liver region,

with very little excretion in feces

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