VITAMIN B12

also called Cyanocobalamin, a complex water-soluble organic compound that is essential to a number of microorganisms and animals, including humans. Vitamin B12, known as the antipernicious-anemia factor, is also known as the extrinsic factora substance from outside the bodythat aids in the development of red blood cells in higher animals. The vitamin, which is unique in that it contains a metallic ion, cobalt, has a complex chemical structure. Vitamin B12 occurs in several forms, called cobalamins; cyanocobalamin is the principal one used in vitamin supplements and pharmaceuticals. In the 1930s the American physician W.B. Castle isolated an intrinsic factor in normal gastric secretion that was absent in the stomachs of persons suffering from pernicious anemia, an acute and sometimes fatal disorder of the red blood cells. Castle postulated that an extrinsic factor that alleviated or prevented pernicious anemia was to be found in animal liver, because improvement had been noted when diets of these patients included large amounts of liver. In 194849 simultaneous studies by the chemists Karl Folkers in the United States and Alexander Todd (later created Baron Todd) in England isolated and identified vitamin B12 as the pure antianemia factor present in liver. The human daily requirement for vitamin B12 is 3 micrograms; good dietary sources are eggs, meat, and dairy products. So far as is known, vitamin B12 is not present in higher plants. It is required in the diet of all higher animals that have been studied. It is synthesized by bacteria, fungi, and algae, and the ultimate source of the vitamin in liver and other animal materials generally appears to be microorganisms of various kinds. Microorganisms that synthesize vitamin B12 occur in the rumen (the first stomach chamber) of cows and sheep. From the rumen it is transferred to the muscle and other tissues, which humans eat. Several kinds of bacteria unable to make the substance require minute amounts for growth. Vitamin B12 is involved in cellular metabolism in two active coenzyme formsmethylcobalamin and 5-deoxyadenosylcobalamin. Vitamin B12 cooperates with folate in the synthesis of DNA. A deficiency of either compound leads to disordered production of DNA and, hence, to the impaired division of red blood cells that is the cause of pernicious anemia (q.v.). Vitamin B12 also has a separate biochemical role, unrelated to folate, in the synthesis of fatty acids in the myelin sheath that surrounds nerve cells.