Vitamin B12, serum or plasma

Vitamin B12 (cobalamin) in serum is bound to two proteins: transcobalamin (TC) and haptocorrin (HC). The transcobalamin-vitamin B12 complex is called holotranscobalamin (HoloTC). HoloTC contains the biologically available cobalamin as only HoloTC promotes the uptake of cobalamin by all cells via specific receptors. Methods based on specific anti-TC antibodies have been available and confirm the usefulness of HoloTC for diagnosing B12 deficiency. As expected, HoloTC levels are low in patients with biochemical signs of vitamin B12 deficiency. Notably, low values have been reported in vegetarians, vegans, and in populations with a low intake of vitamin B12. In addition, low levels of HoloTC (but not vitamin B12) in serum were reported in patients with Alzheimer’s disease compared to levels of HoloTC in a healthy control group.