In the medical literature, the term vitamin B12 includes all vitamin-active cobalamins (Cbl) whose basic structure consists of a nearly flat corrin ring system, a porphyrin-like compound with four reduced pyrrole rings (A, B, C, D) and a central cobalt atom. The central cobalt atom is tightly bound to the four nitrogen atoms of the pyrrole rings and alpha-axially to the nitrogen of 5,6-dimethylbenzimidazole, which is crucial for the vitamin function of cobalamins.Beta-axially, the cobalt atom may be substituted with various residues, such as:
- Cyanide (CN-) – cyanocobalamin (vitamin B12).
- A hydroxy group (OH-) – hydroxocobalamin (vitamin B12a)
- Water (H2O) – aquocobalamin (vitamin B12b)
- Nitrogen dioxide (NO2) – nitrocobalamin (vitamin B12c)
- A methyl group (CH3) – methylcobalamin (coenzyme)
- 5′-deoxyadenosyl – 5′-deoxyadenosylcobalamin (adenosylcobalamin, coenzyme).
Of the listed derivatives (derivatives), only cyanocobalamin, which is produced synthetically, and hydroxocobalamin, which is the physiological depot form, play a therapeutic role. These are converted in the organism to the physiologically active forms methylcobalamin and adenosylcobalamin [1, 2, 6, 8, 11-14].
Synthesis
Vitamin B12 synthesis is very complex and occurs exclusively in specific microorganisms. Thus, species-specifically-in different animal species, enteric synthesis (formation by the intestinal flora) contributes more or less to meeting vitamin B12 requirements. While in herbivores (herbivores) the enteric synthesis – or gastrointestinal synthesis in ruminants (formation by the rumen or intestinal flora) – is completely sufficient, carnivores (carnivores) are able to cover their requirements not only through the synthesis by the intestinal flora, but also through the vitamin B12 supply with meat.For humans, the vitamin B12 formed by the large intestine flora cannot be used sufficiently. For this reason, humans depend on the additional intake of the B vitamin with food. The daily vitamin B12 requirement is 3 to 4 µg per day, with reserves sufficient for 1-2 years.
Absorption
In foods, vitamin B12 is present bound to proteins or in free form. Bound dietary cobalamin is released from its protein binding in the stomach by gastric acid and pepsin (digestive enzyme) and is largely attached to glycoproteins called haptocorrins (HC) or R-binder proteins secreted (secreted) by salivary glands and gastric mucosal cells. In the case of freely available dietary cobalamin, attachment to HC already occurs in saliva [1, 2, 5, 7, 8-10, 12-14]. The Cbl-HC complex enters the upper segment of the small intestine where, under the action of trypsin (digestive enzyme) and an alkaline pH, cleavage of the complex and binding of vitamin B12 to a glycoprotein called intrinsic factor (IF) formed by the occupant cells of the gastric mucosa occurs [1, 2, 5, 7, 8, 9, 12-14]. The Cbl-IF complex is transported to the distal ileum (lower segment of the small intestine), where it is taken up into the mucosal cells in an energy-dependent manner via calcium-dependent endocytosis (membrane transport). This process occurs through specific receptors (binding sites) and proteins including cubilin (CUBN) and megalin (LRP-2), as well as amnionless (AMN) and receptor-associated protein (RAP), which are localized as a complex in the microvilli membranes of ileal enterocytes (epithelial cells of the lower small intestine). Intracellularly (inside the cell), dissociation (disassembly) of the Cbl-IF receptor complex occurs in the endosomes (membrane vesicles) by lowering the pH using proton adenosine triphosphate (ATP)ases (ATP-cleaving enzymes). While the dissociated cubilin-megalin compound returns to the apical cell membrane (facing the inside of the intestine) via vesicles, the endosomes mature into lysosomes (cell organelles) in which the release of cobalamin from its compound is accelerated by further lowering of pH.This is followed by the binding of free vitamin B12 to the transport protein transcobalamin-II (TC-II) in secretory vesicles, which release the Cbl-TCII complex or holotranscobalamin-II (HoloTC) into the blood via the basolateral membrane (facing away from the intestine). IF-mediated vitamin B12 absorption is only a maximum of 1.5-2.0 µg per meal because the incorporation capacity (uptake capacity) of the ileal mucosa (mucosa of the lower small intestine) for the Cbl-IF complex is limited (restricted). Approximately 1% of dietary cobalamin enters the bloodstream through the gastrointestinal tract (GI tract) or mucosa without prior binding to IF by a nonspecific mechanism. With oral vitamin B12 intake above a physiological intake level of approximately 10 µg, IF-independent, passive cobalamin absorption becomes increasingly important. For example, after oral administration of 1,000 µg of vitamin B12, only 1.5 µg (14 %) of the total absorbed cobalamin amount of 10.5 µg is IF-dependent and already 9 µg (86 %) is absorbed IF-independently via passive diffusion. However, the passive resorption pathway is not nearly as effective compared with the energy-dependent transport mechanism, which is why the total amount absorbed increases in absolute terms with increasing cobalamin dose but decreases in relative terms [1-3, 8, 12, 13].
Transport and cellular uptake
The Cbl-TCII complex enters the bloodstream via the portal circulation and from there to target tissues. Cellular uptake of HoloTC occurs by megalin (LRP-2)- and TC-II receptor-mediated endocytosis (membrane transport) in the presence of calcium ions. Intracellularly, TC-II is proteolytically (enzymatically) degraded in the lysosomes (cell organelles) and vitamin B12 is released into the cytosol in the form of hydroxocobalamin with a trivalent cobalt atom (OH-Cbl3+). With cleavage of the OH group, reduction of Cbl3+ to Cbl2+ occurs. On the one hand, this is methylated by S-adenosylmethionine (SAM, universal methyl group donor) and bound as methylcobalamin to apo-methionine synthase (enzyme that regenerates methionine from homocysteine), leading to its enzymatic activation. On the other hand, Cbl2+ enters the mitochondrion (“energy powerhouse” of the cell), where it is reduced to Cbl1+ and converted to adenosylcobalamin by adenosyl transfer of ATP (universal energy carrier) with cleavage of triphosphate. This is followed by the binding of adenosylcobalamin to the apoenzymes L-methylmalonyl-coenzyme A (CoA) mutase (enzyme that converts L-methylmalonyl-CoA to succinyl-CoA during the degradation of propionic acid) and L-leucine mutase (enzyme that initiates the degradation of the amino acid leucine by the reversible conversion of alpha-leucine to 3-aminoisocapronate (beta-leucine)), thereby catalytically activating them.
Distribution in the body
TC-II contains 6-20% of the vitamin B12 circulating in plasma and is the metabolically active vitamin B12 fraction. It has a relatively short biological half-life of one to two hours. For this reason, HoloTC rapidly falls below normal levels in the event of insufficient vitamin B12 absorption and is suitable for early diagnosis of vitamin B12 deficiency.Bound to haptocorrin, also known as TC-I, is 80-90 % of plasma cobalamin – holohaptocorrin. Unlike TC-II, this does not contribute to the supply of vitamin B12 to peripheral cells, but transports excess cobalamin peripherally back to the liver and is therefore the metabolically less active fraction. Since TC-I has a biological half-life of nine to ten days, it falls off slowly when vitamin B12 supply is inadequate, making it a late indicator of vitamin B12 deficiency.TC-III is the R-binder protein of granulocytes (a group of white blood cells) and is an exceedingly small fraction. It resembles TC-I in its metabolic function.The main storage organ for vitamin B12 is the liver, where about 60% of the body’s cobalamin is deposited. About 30 % of the B vitamin is stored in the skeletal muscles. The remainder is in other tissues such as the heart and brain. The total body stock is 2-5 mg.Vitamin B12 is the only water-soluble vitamin that is stored in appreciable amounts. The relatively high body stocks and low turnover rate (turnover rate) of vitamin B12 (2 µg/day) are the reason that vitamin B12 deficiency does not become clinically apparent for years. For this reason, strict vegetarians develop vitamin B12 deficiency symptoms only after 5-6 years despite a low-cobalamin diet.However, in patients with disease or surgical removal of the stomach or terminal ileum (lower segment of the small intestine), vitamin B12 deficiency may occur after as little as 2-3 years because neither dietary cobalamin can be reabsorbed nor vitamin B12 excreted biliary (via bile) [1-3, 7, 10, 12, 13].
Excretion
Because of an effective enterohepatic circuit (liver–gut circuit), the 3-8 µg of cobalamin excreted daily in bile is reabsorbed in the terminal ileum (lower portion of the small intestine).Vitamin B12 excretion by the kidneys is very low at normal intakes and is 0.143% per day at an average daily intake of 3-8 µg of vitamin B12. With increasing dose, the proportion of absorbed vitamin B12 in the urine increases significantly by exceeding the retention capacity. After 1,000 µg of cyanocobalamin administered, 94% (9.06 µg) of the absorbed 9.6 µg of vitamin B12 is still retained and 6% (0.54 µg) is eliminated renally (via the kidneys). With increasing oral dose, the fraction of vitamin B12 absorbed by the total body decreases from 94 to 47%, and the renally eliminated fraction increases correspondingly from 6 to 53%.
