Chemical structure of Vitamin B12
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Vitamin B12 Deficiency

The term 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 artefact 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...

VACTERL association
Van der Woude syndrome
Van Goethem syndrome
Varicella Zoster
Variegate porphyria
Vasovagal syncope
VATER association
Velocardiofacial syndrome
Ventricular septal defect
Viral hemorrhagic fever
Vitamin B12 Deficiency
VLCAD deficiency
Von Gierke disease
Von Hippel-Lindau disease
Von Recklinghausen disease
Von Willebrand disease

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.


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 haem, 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. (p.32)


B12 cannot be made by plants or by animals, as the only type of organism that have the enzymes required for the synthesis of B12 are bacteria (eubacteria, archaebacteria).


Coenzyme B12's reactive C-Co bond participates in two types of enzyme-catalyzed reactions: (p.675)

  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).


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Vitamin B12 deficiency, infertility and miscarriage
From Townsend Letter for Doctors and Patients, 7/1/01 by Tori Hudson

Infertility as a result of vitamin B12 deficiency may be related to anovulation, abnormal cell division in the fertilized ovum or a lack of implantation due to megaloblastoid changes in the endometrium. Recurrent early fetal loss may also be associated with vitamin B12 deficiency, though rare.

The obstetrics histories of 14 patients presenting With 15 episodes of vitamin B12 deficiency were analyzed. Infertility of two to eight years had been present in four episodes, and recurrent fetal loss was a feature in 11. Two of these 11 cases had never had a full-term delivery. Case 1 had had seven spontaneous abortions before the finding of vitamin B12 deficiency. Treatment with B12 resulted in their first full-term delivery within nine months followed by two more full term deliveries. In six cases, miscarriages were followed by a period of infertility. In one case, after 3 miscarriages followed by six years of infertility, severe megaloblastic anemia was found with ahemoglobin of 5.8 g/dL and a serum B12 level of [less than] 50 pg/mL. Following parenteral B12 administration, the patient became pregnant immediately and delivered a full-term infant.

In 10 patients, a full-term delivery occurred after vitamin B12 therapy. Six of them almost immediately conceived.

Recurrent fetal loss in the presence of B12 deficiency may be due to elevated homocysteine levels and a resulting thrombophilic state. Pregnancy is already a prethrombotic state, and the homocysteinemia and increased risk of thrombovascular disease may cause thrombosis and inadequate perfusion of the placenta, preeclampsia, abruptio placentae, fetal growth retardation and stillbirth.

This study demonstrates the importance of measuring B12 levels in the evaluation of infertility or recurrent spontaneous abortions. Since folate therapy, high-folate diets, iron deficiency or thalassemia minor may mask a B12 deficiency, serum measurements are crucial.

Bennett M. Vitamin B12 deficiency, infertility and recurrent fetal loss. J Reprod Med 2001; 46:209-212

COPYRIGHT 2001 The Townsend Letter Group
COPYRIGHT 2001 Gale Group

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