Vitamin B12 - Mechanism of Action

Mechanism of Action

Vitamin B₁₂ normally plays a significant role in the metabolism of every cell of the body, especially affecting the DNA synthesis and regulation but also fatty acid synthesis and energy production. However, many (though not all) of the effects of functions of B₁₂ can be replaced by sufficient quantities of folic acid (vitamin B₉), since B₁₂ is used to regenerate folate in the body. Most vitamin B₁₂ deficiency symptoms are actually folate deficiency symptoms, since they include all the effects of pernicious anemia and megaloblastosis, which are due to poor synthesis of DNA when the body does not have a proper supply of folic acid for the production of thymine. When sufficient folic acid is available, all known B₁₂ related deficiency syndromes normalize, save those narrowly connected with the vitamin B₁₂-dependent enzymes Methylmalonyl Coenzyme A mutase, and 5-methyltetrahydrofolate-homocysteine methyltransferase (MTR), also known as methionine synthase; and the buildup of their respective substrates (methylmalonic acid, MMA) and homocysteine.

Coenzyme B₁₂'s reactive C-Co bond participates in three main types of enzyme-catalyzed reactions.

1. Isomerases

   Rearrangements in which a hydrogen atom is directly transferred between two adjacent atoms with concomitant exchange of the second substituent, X, which may be a carbon atom with substituents, an oxygen atom of an alcohol, or an amine. These use the adoB₁₂ (adenosylcobalamin) form of the vitamin.

2. Methyltransferases

   Methyl (-CH₃) group transfers between two molecules. These use MeB₁₂ (methylcobalamin) form of the vitamin.

3. Dehalogenases

   Reactions in which a halogen atom is removed from an organic molecule. Enzymes in this class have not been identified in humans.

In humans, two major coenzyme B₁₂-dependent enzyme families corresponding to the first two reaction types, are known. These are typified by the following two enzymes:

1. MUT is an isomerase which uses the AdoB₁₂ form and reaction type 1 to catalyze a carbon skeleton rearrangement (the X group is -COSCoA). MUT's reaction converts MMl-CoA to Su-CoA, an important step in the extraction of energy from proteins and fats (for more see MUT's reaction mechanism). This functionality is lost in vitamin B₁₂ deficiency, and can be measured clinically as an increased methylmalonic acid (MMA) level. Unfortunately, an elevated MMA, though sensitive to B₁₂ deficiency, is probably overly sensitive, and not all who have it actually have B₁₂ deficiency. For example, MMA is elevated in 90–98% of patients with B₁₂ deficiency; however 20–25% of patients over the age of 70 have elevated levels of MMA, yet 25–33% of them do not have B₁₂ deficiency. For this reason, assessment of MMA levels is not routinely recommended in the elderly. There is no "gold standard" test for B₁₂ deficiency because as a B₁₂ deficiency occurs, serum values may be maintained while tissue B₁₂ stores become depleted. Therefore, serum B₁₂ values above the cut-off point of deficiency do not necessarily indicate adequate B₁₂ status The MUT function cannot be affected by folate supplementation, which is necessary for myelin synthesis (see mechanism below) and certain other functions of the central nervous system. Other functions of B₁₂ related to DNA synthesis related to MTR dysfunction (see below) can often be corrected with supplementation with the vitamin folic acid, but not the elevated levels of homocysteine, which is normally converted to methionine by MTR.
2. MTR, also known as methionine synthase, is a methyltransferase enzyme, which uses the MeB₁₂ and reaction type 2 to catalyze the conversion of the amino acid homocysteine (Hcy) back into methionine (Met) (for more see MTR's reaction mechanism). This functionality is lost in vitamin B₁₂ deficiency, and can be measured clinically as an increased homocysteine level in vitro. Increased homocysteine can also be caused by a folic acid deficiency, since B₁₂ helps to regenerate the tetrahydrofolate (THF) active form of folic acid. Without B₁₂, folate is trapped as 5-methyl-folate, from which THF cannot be recovered unless a MTR process reacts the 5-methyl-folate with homocysteine to produce methionine and THF, thus decreasing the need for fresh sources of THF from the diet. THF may be produced in the conversion of homocysteine to methionine, or may be obtained in the diet. It is converted by a non-B₁₂-dependent process to 5,10-methylene-THF, which is involved in the synthesis of thymine. Reduced availability of 5,10-methylene-THF results in problems with DNA synthesis, and ultimately in ineffective production cells with rapid turnover, in particular blood cells, and also intestinal wall cells which are responsible for absorption. The failure of blood cell production results in the once-dreaded and fatal disease, pernicious anemia. All of the DNA synthetic effects, including the megaloblastic anemia of pernicious anemia, resolve if sufficient folate is present (since levels of 5,10-methylene-THF still remain adequate with enough dietary folate). Thus the best-known "function" of B₁₂ (that which is involved with DNA synthesis, cell-division, and anemia) is actually a facultative function which is mediated by B₁₂-conservation of an active form of folate which is needed for efficient DNA production. Other cobalamin-requiring methyltransferase enzymes are also known in bacteria, such as Me-H4-MPT, coenzyme M methyl transferase.