HE structures of vitamin B12 and many related analogs are now known, yet the metabolic pathways requiring the vitamin have still to be characterized. Deficiency of the vitamin expresses itself with eloquence in man, producing a megaloblastic anemia and related blood changes, neurological lesions, and evidence of depressed nuclear maturation in the cells of the upper gastrointestinal tract and possibly in the liver. In lower mammals and in fowls, decreased rate of growth is the only constant expression of deficiency, and no changes in the blood or nervous systems are evident. This species difference is unexplained. In certain micro-organisms vitamin B12 requirements have been extensively studied, both in culture and in cell-free systems.
Many functions have been ascribed to the vitamin, but which are direct and fundamental and which are secondary is not at present clear. Attempts to relate structure to function have been generally unconvincing. It is