Principle function of cyanocobalamine is

The principle function of cyanocobalamine is coenzyme for one carbon transfer in nucleic acid metabolism. When the methyl group replaces the adenosyl group, it is known as methylcobalamin. This is the major form seen in blood circulation as well as in the cytoplasm of cells. Tetrahydrofolate is mostly trapped as N5-methyl THF in which form it is present in the circulation. Vitamin B12 is needed for the conversion of N5-methyl THF to THF, in a reaction wherein homocysteine is conveed to methionine. This step is essential for the liberation of free THF and for its repeated use in one-carbon metabolism. In B12 deficiency, conversion of N5-methyl THF to THF is blocked Ref: satyanarayana 4th ed page 151