Cyanocobalamin is synthesized by certain micro-organisms in the intestine lumen. Cyanocobalamin is not synthesized by animal or plants. Liver, kidney, heart, egg yolk and milk are rich sources of vitamin B12. Commercial source of it is from streptomyces greseus.
The daily nutritional requirements are 3—5 tg, which must be obtained from animal by-products in the diet. Parietal cells of the stomach secrete intrinsic factor (a specific glycoprotein) which forms a complex with vitamin B12 (extrinsic factor).
This complex is absorbed in the distal ileum by a highly specific receptor-mediated transport system. In blood, B12 is transported with a specific plasma globulin (transcobalamin). Some of it is excreted in bile every day, but most of it is reabsorbed (enterohepatic circulation).
Excess amount remains in free form and excreted in urine. It is available as injectable as well as for oral administration. Its absorption by oral route is not reliable in absence of or deficiency of intrinsic factor.
Cyanocobalamin is given by deep intramuscular or deep subcutaneous injection. It is available in combination with other minerals and vits for oral administration. In human beings, two main biochemical reactions require vitamin B12:
• The conversion of methyl-tetrahydrofolate (FH4), inactive form of FH4, to active formyl-FH4 which after polyglutamation, is a cofactor in the synthesis of purines and pyrimidines.
• Isomerisation of methyl malonyl-CoA to succinyl CoA.
Hydroxocobalamin is now the form of vitamin B12 of choice. Since it is more protein bound, it is retained for longer time in the body.
A single dose of Cyanocobalamin can maintain plasma vits concentration in the normal range for up to 3 months. Cyanocobalamin may, however, produce antibodies to the vitamin complex.
Dose: Cyanocobalamin up to 30 jig is administered by intramuscular route.
Hydroxocobalamin is given in a dose of 1 mg initially which may be repeated every 2—3 days.
Liver preparations containing activity are also available.
Therapeutic uses of Cyanocobalamin:
• Vitamin B12 deficiency
• Pernicious anaemia
• In degeneration of spinal cord
• Prophylactically after surgical removal of stomach (site of production of intrinsic factor)or the terminal ileum (site of vitamin B12 absorption).