vitamin B12; cobalamin

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[1][2][3][4][5][6][7][8][9][10][11][12][13]

Introduction

Vitamin B12 assumes 3 biological forms, & a pharmaceutical form.

Co+2-containing cofactor in methyltransferase reactions including:

These are the only known functions of vitamin B12 in mammals[6].

* Also through this mechanism, plays a role in DNA synthesis

Absorption of vitamin B12:

Cobalamin is absorbed in association with intrinsic factor, a protein secreted by parietal cells in the gastric fundus. However, it is transcobalamin-1 that binds to vit B12 in the acidic environment of the stomach. Gastric acid facilitates release of cobalamin from binding in food sources. In the duodenum, transcobalamin-1 releases B12 for binding to intrinsic factor. The intrinsic factor/B12 complex is absorbed by attachment to specific receptors in the mucosal brush border of the ileum, i.e. cubilin. Amnionless is also necessary for efficient absorption of vitamin B12.

About 1% of an oral dose of vitamin B12 is absorbed by passive diffusion.[7] Within the ileal mucosa, the intrinsic factor is metabolized & cobalamin is transferred to transcobalamin-2. The transcobalamin-2/B12 complex is secreted into the portal circulation from which about 1/2 is taken up by the liver.

Most circulating vitamin B12 is bound to transcobalamin-1. However, it is probably metabolically inert since no cellular receptors for it have been identified.[10]

In plasma, transcobalamin-2/vit B12 (holo-TC2) is the metabolically active form of vit B12. 6-20% of plasma TC2 is saturated with vit B12. Elimination 1/2life of the holo-TC2 in plasma is 1 hour.[3] Most of the peripheral holo-TC2 clearance occurs via glomerular filtration followed by tubular uptake in the kidney. Tubular epithelium is abundant in TC2 receptors[4]

Liver stores of vitamin B12 are generally 2-5 mg.[9]

Other proteins that partipate in vitamin B12 metabolism include:

Dietary sources include:

Recommended daily allowance (adults)

See cyanocobalamin for pharmaceutical form of vitamin B12.

More general terms

More specific terms

Additional terms

Component of

References

  1. ↑ Saunders Manual of Medical Practice, Rakel (ed), WB Saunders, Philadelphia, 1996, pg 1017
  2. ↑ Mayo Internal Medicine Board Review, 1998-99, Prakash UBS (ed) Lippincott-Raven, Philadelphia, 1998, pg 411
  3. ↑ 3.0 3.1 Drug Information & Medication Formulary, Veterans Affairs, Central California Health Care System, 1st ed., Ravnan et al eds, 1998
  4. ↑ 4.0 4.1 Kaiser Permanente Northern California Regional Drug Formulary, 1998
  5. ↑ UCLA Intensive Course in Geriatric Medicine & Board Review, Marina Del Ray, CA, Sept 12-15, 2001
  6. ↑ 6.0 6.1 Stryer Biochemistry WH Freeman & Co, New York, 1988 pg 508
  7. ↑ 7.0 7.1 Prescriber's Letter 8(10):55 2001
  8. ↑ Harrison's Online, Chapter 107, McGraw Hill, 2002
  9. ↑ 9.0 9.1 Geriatrics Review Syllabus, American Geriatrics Society, 5th edition, 2002-2004
    Geriatric Review Syllabus, 9th edition (GRS9) Medinal-Walpole A, Pacala JT, Porter JF (eds) American Geriatrics Society, 2016
  10. ↑ 10.0 10.1 Carmel R, Clinical Chemistry. 48:407-409. 2002 <PubMed> PMID: https://www.ncbi.nlm.nih.gov/pubmed/11861432 <Internet> http://www.clinchem.org/cgi/content/full/48/3/407
  11. ↑ O'Leary F, Samman S. Vitamin B12 in health and disease. Nutrients. 2010 Mar;2(3):299-316. Review. PMID: https://www.ncbi.nlm.nih.gov/pubmed/22254022 Free PMC Article
  12. ↑ Malouf R, Areosa Sastre A. Vitamin B12 for cognition. Cochrane Database Syst Rev. 2003;(3):CD004326. Review. PMID: https://www.ncbi.nlm.nih.gov/pubmed/12918012
  13. ↑ 13.0 13.1 Khardori R Fast Five Quiz: Vitamin B12 Deficiency Medscape. December 22, 2020 https://reference.medscape.com/viewarticle/942750

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