Vitamin Bi2 Vitamers and Nomenclature

Sulfltocobalamin, with a sulfite ligand, occurs in some foods as a result of processing, but is poorly absorbed. 

The cobalt atom is in the Co + oxidation state in hydroxo-, aquo-, methyl-, and cyanocobalamins in the Co+ oxidation state in adenosylcobalamin and, transiently, in the demethylated prosthetic group of methionine synthetase (Section 10.8.1). 

Although cyanocobalamin was the first form in which vitamin B12 was isolated, it is not an important naturally occurring vitamer, but rather an artifact caused by the presence of cyanide in the charcoal used in the extraction procedure. It is more stable to light than the other vitamers, and is commonly used in pharmaceutical preparations. Photolysis of cyanocobalamin in solution leads to the formation of aquocobalamin or hydroxocobalamin, depending on pH. 

Hydroxocobalamin is also used in pharmaceutical preparations and is better retained after parenteral administration than is cyanocobalamin. 

Small amounts of cyanocobalamin are found in the bloodstream (about 2% of total plasma vitamin B12) apparendy as part of the metabolism of cyanide derived from food (and tobacco smoke), but not in erythrocytes or tissues. If it is not converted to aquo- or hydroxocobalamin, cyanocobalamin may have antivitamin action and has been implicated in the neurological damage associated with chronic cyanide intoxication seen in parts ofwest Africa, where the dietary staple, cassava, is rich in cyanogenic glycosides. 

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