Haptocorrin

Haptocorrin also known as transcobalamin-1 (TC-1) or cobalophilin is a is_associated_with::protein that in humans is encoded by the TCN1 is_associated_with::gene. The essential function of haptocorrin is protection of the acid-sensitive vitamin B12 while it moves through the stomach.

Function
Haptocorrin (HC), also commonly known as the R-protein, or the R-factor, or previously referred to as transcobalamin I, is a unique is_associated_with::glycoprotein produced by the salivary glands of the oral cavity, in response to ingestion of food. This protein binds strongly to vitamin B12 in what is perhaps an intricate yet necessary evolutionary mechanism to protect this vitamin from the acidic environment of the stomach. Vitamin B12 is an essential water soluble vitamin, the deficiency of which creates anemia (is_associated_with::macrocytic anemia), decreased bone marrow cell production (is_associated_with::anemia, is_associated_with::pancytopenia), neurological problems, as well as metabolic issues (methylmalonyl-CoA acidosis).

Vitamin B12 is therefore an important vitamin for the body to absorb. Despite its vital role however, vitamin B12 is structurally very sensitive to the is_associated_with::hydrochloric acid found in the stomach secretions, and easily denatures in that environment before it has a chance to be absorbed by the small intestine. Found in fresh animal products (such as liver), vitamin B12 attaches haptocorrin, which has a high affinity for its molecular structure. Coupled together vitamin B12 and haptocorrin create a complex. This Haptocorrin-B12 complex is impervious to the insult of the stomach acid, and passes on via the is_associated_with::pylorus to the is_associated_with::duodenum. In the duodenum pancreatic is_associated_with::proteases (a component of is_associated_with::pancreatic juice) cleave haptocorrin, yet again releasing vitamin B12 in its free form.

The same cells in the stomach that produce gastric hydrochloric acid, the is_associated_with::parietal cells, also produce a molecule called the is_associated_with::intrinsic factor (IF), which rebinds the B12 after its release from haptocorrin by digestion, and without which vitamin B12 can not be absorbed. Intrinsic factor (IF) is a glycoprotein, with a MW of 45,000 dalton. In the duodenum, the free vitamin B12 attaches the intrinsic factor (IF) to create a vitamin B12-IF complex. This complex then travels through the small bowel and reaches the terminal tertiary portion of the small intestine, called is_associated_with::ileum. Ileum is the longest of all portions of the small intestine, but has on its surface specialized receptors called is_associated_with::cubilin receptors, that identify the B12-IF complexes and take them up into the circulation via is_associated_with::endocytosis mediated absorption.

In short, the essential function of haptocorrin is protection of the acid-sensitive vitamin B12 while it moves through the stomach. Haptocorrin also circulates and binds approximately 80% of circulating B12, rendering it unavailable for cellular delivery by transcobalamin II