Conversion between forms readily takes place in body tissues.
Cobalamin is synthesized exclusively by bacteria.
Cobalamin is temporarily bound to R-binding proteins, but released by duodenal digestive enzymes. This IF-cobalamin complex binds to receptors on the brush border of the ileal mucosa and is absorbed over a period of several hours. Total body stores of cobalamin measure 2-5 mg. Cobalamin, in the form of adenosylcobalamin, is stored primarily in the liver. Methylcobalamin is an essential part of the folate-cobalamin reaction necessary for DNA synthesis.