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. Pseudovitamin B₁₂ is the majority corrinoid in spirulina, an algal dietary supplement sometimes erroneously claimed as having this vitamin activity.

Chemical structure

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. All these analogs are biologically inactive in humans.

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

Occurrence

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. These pseudo-vitamin compounds can be found in some types of shellfish, in edible insects, and at times as metabolic breakdown products of cyanocobalamin added to dietary supplements and fortified foods.

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.

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

Despite production of this compound in groups as distantly related as lactic acid bacteria 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.

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.

Activity as enzyme cofactor

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. 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. LLD is also able to use factor S and factor A (see below).

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.

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. 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).

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. Adenyl-pseudovitamin B₁₂ does not function as a cofactor or inhibitor of MCM in COS-7 cells.

Interaction with vitamin B₁₂-binding proteins and transporters

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. Factor III (see below) has a remarkable 80% affinity relative to vitamin B₁₂.

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.

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. 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..

Other vitamin B₁₂ analogs

Other human-inactive cobamides

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. It is also the predominant corrinoid in human feces. It is also found in escargot.

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

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. Factor III (5-hydroxybenzimidazolyl-cobamide), which differs from factor IIIm by the replacement of methoxy with hydroxy (-OH), is common in methanogenic bacteria.

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.

Antivitamin B₁₂

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.

References