Chemical structure of Vitamin B12
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Cyanocobalamin

The name vitamin B12 (or B12 for short) is used in two different ways. In a broader sense it refers to a group of Co-containing compounds known as cobalamins - cyanocobalamin (an artifact formed as a result of the use of cyanide in the purification procedures), hydroxocobalamin and the two coenzyme forms of B12, methylcobalamin (MeB12) and 5-deoxyadenosylcobalamin (adenosylcobalamin - AdoB12). more...

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In a more specific way, the term B12 is used to refer to only one of these forms, cyanocobalamin, which is the principal B12 form used for foods and in nutritional supplements.

Pseudo-B12 refers to B12-like substances which are found in certain organisms, such as Spirulina spp. (blue-green algae, cyanobacteria). However, these substances do not have B12 biological activity for humans.

Structure

B12 is the most chemically complex of all the vitamins. B12's structure is based on a corrin ring, which, although similar to the porphyrin ring found in heme, chlorophyll, and cytochrome, has two of the pyrrole rings directly bonded. The central metal ion is Co (cobalt). Four of the six coordinations are provided by the corrin ring nitrogens, and a fifth by a dimethylbenzimidazole group. The sixth coordination partner varies, being a cyano group (-CN), a hydroxyl group (-OH), a methyl group (-CH₃) or a 5'-deoxyadenosyl group (here the C5' atom of the deoxyribose forms the covalent bond with Co), respectively, to yield the four B12 forms mentioned above. The covalent C-Co bond is the only carbon-metal bond known in biology.

Synthesis

B12 cannot be made by plants or by animals, as the only type of organisms that have the enzymes required for the synthesis of B12 are bacteria and archaea.

Functions

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

  1. 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 alcochol, or an amine.
  2. Methyl (-CH₃) group transfers between two molecules.

In humans there are only two coenzyme B12-dependent enzymes:

  1. MUT 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)
  2. MTR, a methyl transfer enzyme, which uses the MeB12 and reaction type 2 to catalyzes the conversion of the amino acid Hcy into Met (for more see MTR's reaction mechanism).

Read more at Wikipedia.org


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New England Journal of Drugs stays true to form
From Townsend Letter for Doctors and Patients, 1/1/05 by Alan R. Gaby

In a recent issue of the New England Journal of Medicine (2004;351:1323-1331), there was a review article on the diagnosis and management of Bell's palsy. In keeping with the tradition of certain major medical journals of either ignoring or misrepresenting the evidence supporting natural therapies, this article did not mention that intramuscular vitamin B12 appears to be an effective treatment.

[ILLUSTRATION OMITTED]

In a 1959 report, two patients with Bell's palsy of 1.5 and 4 years' duration, respectively, experienced complete or almost complete recovery within 20 days after beginning a series of daily or every-other-day cyanocobalamin injections. More recently, methylcobalamin (500 mcg intramuscularly, 3 times a week) was compared with prednisolone and with the combination of methylcobalamin and prednisolone in a randomized trial. The mean time until complete recovery of facial nerve function was significantly less (p < 0.001) in patients receiving methylcobalamin (1.95 weeks) or methylcobalamin plus prednisolone (2.05 weeks) than in those receiving prednisolone alone (9.60 weeks).

Comment: I sent a letter to NEJM, pointing out the potential importance of this safe and inexpensive alternative to acyclovir and glucocorticoids. While it has been my experience that sending letters to NEJM or JAMA typically results in a polite rejection letter, on rare occasions these journals behave unpredictably and actually publish my letter. So, I keep writing letters, because variable-interval reinforcement is difficult to extinguish, and because if you don't try to promote an attitude shift, you don't have a right to complain. Actually, complaining has gotten to be less fun over the years, but the need to set a dysfunctional healthcare system straight remains.

Mitra M, Nandi AK. Cyanocobalamin in chronic Bell's paisy. J Indian Med Assoc 1959;33:129-31.

Jalaludin MA. Methylcobalamin treatment of Bell's palsy. Methods Find Exp Clin Pharmacol 1995;17:539-44.

by Alan R. Gaby, MD

301 Dorwood Drive * Carlisle, Pennsylvania 17013

COPYRIGHT 2005 The Townsend Letter Group
COPYRIGHT 2005 Gale Group

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