Cobalamin transporter protein (TC (Transcobalamin))

Cobalamin transporter proteins are required for the absorption, plasma transport, and tissue delivery of vitamin B12 (cobalamin) in mammals. Three main proteins facilitate this process: haptocorrin (which binds B12 in the saliva and stomach), intrinsic factor (which binds B12 and is required for its absorption in the intestine), and transcobalamin (which transports B12 in the bloodstream to tissues via receptor-mediated uptake). Each transporter binds cobalamin with high specificity and protects it from degradation. Cellular uptake of B12 requires a sequence of binding and release from these transporters, culminating in receptor-mediated endocytosis via the transcobalamin–cobalamin complex and its specific cell surface receptor. Genetic or acquired defects anywhere along this pathway can result in vitamin B12 deficiency, leading to hematologic and neurologic disorders. The proteins themselves are not direct therapeutic targets, but are key to the mechanism of action and pharmacokinetics of vitamin B12 preparations. The ambiguity in the generic term “cobalamin transporter protein” necessitates clarification in scientific and clinical discussions to specify whether transcobalamin, intrinsic factor, haptocorrin, or the relevant receptor is meant.

Other names

Transcobalamin (TC, TC II)Transcobalamin IIVitamin B12 binding proteinCobalamin-binding protein (generic)Cbl transporter (generic)Haptocorrin (HC; if intended)Intrinsic factor (IF; if intended)TC II receptor (TCblR; for the receptor protein, if relevant)Cubilin (receptor for IF-Cbl complex)

Drugs (vitamin B12 formulations) interact by binding to transcobalamin, haptocorrin, or intrinsic factor for distribution and cellular uptake. Intrinsic factor–cobalamin complex binds cubilin for intestinal absorption; transcobalamin–cobalamin complex binds to transcobalamin receptor (CD320/TCblR) for cellular uptake.