Haptocorrin

Haptocorrin (HC) (also known as transcobalamin-1 (TC-1), or cobalophilin) is a transcobalamin glycoprotein that in humans is encoded by the TCN1 gene. It is essential to protect the acid-sensitive vitamin B₁₂ from degradation while in the stomach. It is also present in the serum where it binds most circulating vitamin B₁₂, 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 B₁₂ is highly structural susceptible to denaturation by the acidic environment of the stomach. Haptocorrin has a high affinity for the molecular structure of vitamin B₁₂ forming a haptocorrin–B₁₂ 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 B₁₂.^{[citation needed]} Intrinsic factor (IF) that is secreted by parietal cells of the stomach now binds B₁₂ released from haptocorrin to subsequently enable cubilin receptors of the ileum to take up B₁₂–IF complexes by endocytosis-mediated absorption before B₁₂ is finally released into circulation. Without IF, only 1% of vitamin B₁₂ is ultimately absorbed.

HC is present in blood serum where it binds 80-90% of circulating B₁₂, 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 B₁₂ while at the same time manifesting as a vitamin deficiency because of insufficient vitamin bound to transcobalamin II.