Cyanocobalamin: Functions & Deficiency | Vitamin B | Organisms

In this article we will discuss about:- 1. Historical Review of Cyanocobalamin 2. Chemical Structure of Cyanocobalamin 3. Properties and Distribution 4. Functions 5. Deficiency 6. Daily Requirement.

Historical Review of Cyanocobalamin:

(Vitamin B₁₂, Cobamide) (Antipernicious Anaemia Factor, Extrinsic Factor of Castle)

In 1948 Smith and Parker isolated cyanocobalamin from the liver. In 1955 its complex structure was determined.

Chemical Structure of Cyanocobalamin:

The central portion of the molecule consists of four reduced and extensively substituted pyrrol rings surrounding a single cobalt atom. The central structure is referred to as a corrin ring and is similar to that of the poryphyrins except that two pyrrol rings are joined directly.

Below the ring system it contains the base dimethyl-benziminazole linked with ribose and phosphoric acid, so that one end is connected to the central cobalt atom and the other through ribose phosphate and aminopropanol to a side chain on ring IV of the tetrapyrrole nucleus.

Different cobala-min compounds possessing common prosthetic group have vitamin B₁₂ activity. A cyanide group (-CN) co-ordinately bound to cobalt atom may be removed giving the compound cobalamin. Addition of cyanide gives cyanocobalamin, substitution of cyanide with hydroxyl group (-OH) gives hydroxycobalamin, B_12a and with nitro group-nitrocobalamin.

All of these have similar biologic action although hydroxycobalamin is more active enzymatically. In vitamin B₁₂ containing coenzymes, cobamides, and the cyanide (cyano) group is replaced by an adenine deoxy nucleoside. Co is present in this prosthetic group in a similar way as Fe in haemoglobin. The empirical formula of vitamin B₁₂has been suggested as C₆₃H₈₈O₁₄N₁₄PCo.

Properties and Distribution of Cyanocobalamin:

Red crystalline substance, soluble in water. Contains metalloporphyrin nucleus and cobalt (4.5%). Molecular weight is 1,500 approximately.

Distribution:

Absent in plants, but present in almost all animal tissues. Rich sources are liver (1 unit of liver extract contains approximately 1 micro gram of B₁₂), kidney, egg, beef extract, milk etc. It is synthesized by microfloral fermentation in the alimentary tract of ruminants. This vitamin is found in the fungus Streptomvces griseus. Hence vitamin B₁₂ is obtained as a by-product in the manufacture of Streptomycin.

Absorption and Excretion:

Absorption takes place from the ileum. Intrinsic factor (IF) present in normal gastric juice is essential for the proper absorption of B₁₂. It is stored in the liver. From the liver it passes to the bone- marrow and helps in the maturation of red cells. The absorption and excretion have been studied by using Vitamin B₁₂, containing radioactive cobalt. Vitamin B₁₂ is excreted in the faeces and urine.

Functions of Cyanocobalamin:

i. Essential for the formation and maturation of red blood cells. Recent idea is that Vitamin B₁₂ is Castle’s extrinsic factor, which combines with the intrinsic factor present in normal gastric juice and is absorbed. The intrinsic factor has not been identified as yet but is a non-dialysable, thermolabile substance possibly a mucoprotein.

ii. It plays an essential role in the synthesis of nucleic acid. Deficiency causes a disturbance of the deoxyribose nucleic acid metabolism.

iii. Increases the white cell count and the platelets through its action in the bone-marrow.

iv. Concerned with the maintenance of normal health and activity of certain parts of the nervous system. Vitamin B₁₂ not only aires the pernicious anaemia but also the neurological manifestations of pernicious anaemia e.g., subacute combined degeneration of the spinal cord. [Folic acid cures only the pernicious anaemia]

v. As it helps in the growth of micro-organism called the Lactobacillus lactis so this vitamin can be assayed by studying the growth of this micro-organism.

vi. Biosynthesis of methyl group is increased by vitamin B₁₂ and it is said that vitamin B₁₂ plays an enzymatic role in one-carbon metabolism and synthesis.

vii. This vitamin has also a role in transmethylation process.

viii. Formation of lipid from carbohydrate is influenced by vitamin B₁₂.

ix. A coenzyme of vitamin B₁₂ helps in isomerization (e.g., that of glutamic acid to β-methyl aspartate). This is clearly observed in the isomeric reaction where methyl malonyl CoA is converted to succinyl CoA.

x. Hyperglycaemia due to vitamin B₁₂ deficiency can be corrected after administration of Vitamin B₁₂ or glutathione (GSH).

Deficiency of Cyanocobalamin:

Pernicious macrocytic or megaloblastic anaemia is produced. Brings about hyperglycaemia, slow growth as well as nervousness and irritability are observed in pigs and rats.

Deficiency might occur as a result of:

(a) Lack in diet,

(b) Lack of intrinsic factor as in pernicious anaemia,

(d) Infection with micro-organisms which have affinity for vitamin B₁₂ and thus destroy it in the intestinal tract, and

(e) Other important features in vitamin B₁₂ deficiency in man are mucosal atrophy and inflammation of tongue, mouth, etc. degenerative lesion of the posterior and lateral columns of the spinal cord.

Daily Requirement of Cyanocobalamin:

The requirement is very small. A daily intake of 1.0 µ gm is sufficient to prevent deficiency. However, it is not known accurately as yet.