The serum contains Vitamin-B12 which bounds to two proteins, viz. transcobalamin (TC) and haptocorrin (HC). This creates complexes named holotranscobalamin (HoloTC). It contains biologically available cobalamin as the uptake of the compound by cells through a specific receptor is possible due to the HoloTC. On the other hand, nearly 80% of the HC carried by the cobalamin is inert due to the absence of the cellular receptors. Though the receptors are present in the liver, the HoloTC have a short circulating half-life when compared to the holohaptocorrin (HoloHC). The decrease of HoloTC is the first symptom of cobalamin deficiency.
