Pseudovitamin B₁₂

Pseudovitamin B₁₂ is a structural analog of cobalamin, a natural corrinoid with a structure similar to the vitamin B₁₂ group of vitamers. It has no vitamin activity in humans, but can act as a cofactor in some microbial enzymes.^[1][2] Pseudovitamin B₁₂ is the majority corrinoid in spirulina, an algal dietary supplement sometimes erroneously claimed as having this vitamin activity.^[3]

Pseudovitamin B₁₂ is a coordination complex of cobalt, which occupies the center of a corrin ligand and is further bound to an adenosine-containing sidechain. The sixth ("upper") ligand for the metal is alternatively cyano, methyl, hydroxo, or a second adenosyl group.^[4][5] All these analogs are biologically inactive in humans.^[1]

Compared to cobalamin (vitamin B₁₂), pseudovitamin B₁₂ has the "lower" ligand, 5,6-dimethylbenzimidazole (DMB), replaced with adenine.^[6]

Most cyanobacteria, including Spirulina, and some algae, such as Porphyra tenera (used to make a dried seaweed food called nori in Japan), have been found to contain mostly pseudovitamin B₁₂ instead of biologically active B₁₂.^[3][7] These pseudo-vitamin compounds can be found in some types of shellfish,^[1] in edible insects,^[8] and at times as metabolic breakdown products of cyanocobalamin added to dietary supplements and fortified foods.^[2][9]

Pseudovitamin B₁₂ can act as a coenzyme in a similar way to normal vitamin B₁₂ when a microbiological assay with Lactobacillus delbrueckii subsp. lactis is used, as that bacteria can utilize the pseudovitamin despite it being unavailable to humans. To get a reliable reading of B₁₂ content, more advanced techniques are available. One such technique involves pre-separation by silica gel and then assessment with B₁₂-dependent E. coli bacteria.^[1]

Pseudovitamin B₁₂ is the main corrin cofactor produced by Clostridium cochlearium,^[4] Limosilactobacillus reuteri, and the methanogenic methanococcales and Methanoplanus^[10] under anaerobic conditions, and by the cyanobacteria Nostoc commune and Aphanizomenon flos-aquae.^[11][12]

Despite production of this compound in groups as distantly related as lactic acid bacteria^[13] and cyanobacteria, DMB is preferred over adenine by the vast majority of versions of CobT, the enzyme responsible for making the active phosphoribosylated lower sidechain of cobalamin.^[6]

Salmonella enterica is able to make either B₁₂ or pseudovitamin B₁₂ depending on the availability of DMB. Its enzymes prefer DMB, but it remains able to grow when DMB is unavailable and pseudo-B₁₂ has to be made instead.^[11]

In organisms that produce pseudovitamin B₁₂, it takes the same role as vitamin B₁₂ does in humans: as a corrin cofactor that facilitates the function of enzymes.^[11] Pseudovitamin B₁₂ is also functional in some non-corrin-producing relatives of organisms that produce pseudovitamin B₁₂. This includes Lactobacillus delbrueckii subsp. lactis (LLD), which is in the same family as Limosilactobacillus reuteri.^[1] LLD is also able to use factor S and factor A (see § Other human-inactive cobamides below).^[14]

Cobalamide-dependent growth behavior of Sinorhizobium meliloti largely correlates with the cofactor binding selectivity of its methylmalonyl-CoA mutase (MCM). Among adenyl-cobamides, paseudovitamin B₁₂ does not bind to its MCM, factor A (see below) does slightly, and vitamin B₁₂ binds well.^[15]

Human apo-methionine synthase (MS) is able to be activated by methyl-pseudovitamin B₁₂ in vitro (in solution). Apo-MS is extremely unselective of cofactors: It is activated by all tested cobamides (vitamin B₁₂). It appears to only require the central cobalt atom to have an oxidation state of +2.^[16] Hydroxo-pseudovitamin B₁₂ is able to activate the MS in COS-7 cells, but unlike hydroxo-vitamin B₁₂, it does not increase the translation of MS (hydroxo-B₁₂ achieves this by binding to the internal ribosome entry site of MS mRNA).^[5]

Human methylmalonyl-CoA mutase (MCM) normally relies on the adenosyl form of vitamin B₁₂. It binds and works with some vitamin B₁₂ analogs in vitro (in solution), but not purinyl ones such as pseudovitamin B₁₂.^[17] Adenyl-pseudovitamin B₁₂ does not function as a cofactor or inhibitor of MCM in COS-7 cells.^[5]

Human (mamallian in general) intrinsic factor binds pseudovitamin B₁₂ with 500-fold lower affinity than to its usual target, vitamin B₁₂. This prevents mammals from absorbing trace amounts of pseudovitamin B₁₂ from food.^[11] Factor III (see below) has a remarkable 80% affinity relative to vitamin B₁₂.^[18]

Another protein involved in the absorption of vitamin B₁₂ is haptocorrin. Human haptocorrin binds pseudovitamin B₁₂ with the same affinity as vitamin B₁₂. It is the least selective protein among all three human vitamin B₁₂-binding proteins.^[18]

Transcobalamin II (TCN2) is responsible for carrying vitamin B₁₂ around in blood and into cells. It is relatively unselective, with pseudovitamin B₁₂ showing 80% relative affinity.^[18] There is a concern that excess pseudovitamin B₁₂ may compete with vitamin B₁₂ for available TCN2. In COS-7 cells, 10 nM of pseudovitamin B₁₂ in the culture medium inhibits the uptake of 1 nM vitamin B₁₂. Pseudovitamin B₁₂ is not able to show inhibition when vitamin B₁₂ is more abundant..^[5]

Factor S (2-methylmercaptoadenyl cobamide), which differs from pseudo-B₁₂ by the addition of a methylmecropto (-S-CH₃) group, is found alongside pseudovitamin B₁₂ in crickets. It is presumed to have been made by the gut bacteria of crickets.^[19] It is also the predominant corrinoid in human feces. It is also found in escargot.^[20]

Factor A (2-methyladenyl cobamide), which differs from pseudo-B₁₂ by the addition of a methyl (-CH₃) group, is found in crickets.^[14]

Factor IIIm (methoxybenzimidazolyl cobamide) is found in escargot. It differs from vitamin B₁₂ by the removal of a methyl (-CH₃) group and the replacement of a methyl group with a methoxy (-O-CH₃) group.^[20] Factor III (5-hydroxybenzimidazolyl-cobamide), which differs from factor IIIm by the replacement of methoxy with hydroxy (-OH), is common in methanogenic bacteria.^[10]

Thermal decomposition of all six of these corrinoids results in the same material, since the sidechain cleaves at the phosphate ester which attaches them to the main heterocycle.^[14]

A related concept is antivitamin B₁₂ – compounds (often synthetic) that not only have no vitamin action, but also actively interfere with the activity of true vitamin B₁₂. The design of these compounds mainly involves the replacement of the metal ion with rhodium, nickel, or zinc; the attachment may have an inactive ligand such as 4-ethylphenyl. These compounds may be used to analyze B₁₂ utilization pathways or to attack B₁₂-dependent pathogens.^[21]