Haptocorrin (HC) (also known as transcobalamin-1 (TC-1), or cobalophilin) is a transcobalamin glycoprotein that in humans is encoded by the gene. It is essential to protect the acid-sensitive vitamin B<sub>12</sub> from degradation while in the stomach. It is also present in the serum where it binds most circulating vitamin B<sub>12</sub>, rendering it unavailable for uptake by cells (this is conjectured to be a circulating storage function).
HC is produced by the salivary glands of the oral cavity in response to ingestion of food. Vitamin B<sub>12</sub> is highly structural susceptible to denaturation by the acidic environment of the stomach. Haptocorrin has a high affinity for the molecular structure of vitamin B<sub>12</sub> forming a haptocorrinâÂÂB<sub>12</sub> complex that is impervious to stomach acid, enabling it to reach the more alkaline duodenum intact. In the duodenum, pancreatic proteases (a component of pancreatic juice) cleave haptocorrin, releasing vitamin B<sub>12</sub>. Intrinsic factor (IF) that is secreted by parietal cells of the stomach now binds B<sub>12</sub> released from haptocorrin to subsequently enable cubilin receptors of the ileum to take up B<sub>12</sub>âÂÂIF complexes by endocytosis-mediated absorption before B<sub>12</sub> is finally released into circulation. Without IF, only 1% of vitamin B<sub>12</sub> is ultimately absorbed.
HC is present in blood serum where it binds 80-90% of circulating B<sub>12</sub>, rendering it unavailable for cellular uptake by transcobalamin II. This is conjectured to be a circulating storage function.
Several serious, even life-threatening diseases cause elevated serum HC, measured as abnormally high serum vitamin B<sub>12</sub> while at the same time manifesting as a vitamin deficiency because of insufficient vitamin bound to transcobalamin II.