Cobalamin

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Contents 1 More Specific Terms 2 Introduction 3 More General Terms 4 Additional Terms 5 Internet Database 6 References

More Specific Terms

• adenosylcobalamin; cobamamide (vitamin B12)
• cyanocobalamin (vitamin B12, Betalin-12, Redisol, Rubramin-PC)
• hydroxycobalamin (vitamin B12)
• methylcobalamin (vitamin B12)

Introduction

• Vitamin B12 assumes 3 biological forms, & a pharmaceutical form.
  
• hydroxycobalamin
• methylcobalamin
• adenosylcobalamin
• cyanocobalamin (pharmaceutical form)
  

• Co+2-containing cofactor in methyltransferase reactions including:

• methylation of homocysteine*, in association with folic acid ( enzyme: methionine synthase; cofactor: methylcobalamin)
• metabolism of propionate ( propionyl CoA) formed from catabolism of odd chain fatty acids, cholesterol & the amino acids methionine, valine & isoleucine ( enzyme: methylmalonyl CoA mutase; cofactor: adenosylcobalamin)
  

• 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:

• MMACHC

• Dietary sources include:

• meat
• dairy products
• vegetable products

• fermented soy
• yeast
• sea vegetables

• See cyanocobalamin for pharmaceutical form of vitamin B12.

More General Terms

• corrin
• B vitamin
• benzimidazole

Additional Terms

• Co+2
• cubilin (intrinsic factor-cobalamin receptor, intrinsic factor-vitamin B12 receptor, 460 kD receptor, intestinal intrinsic factor receptor, CUBN, IFCR)
• intrinsic factor (IF)
• methionine synthase; 5-methyltetrahydrofolate:homocysteine methyltransferase; vitamin-B12 dependent methionine synthase; tetrahydropteroyl-glutamate methyltransferase; MS (MTR)
• methylcobalamin deficiency
• methylmalonate; methylmalonic acid (MMA)
• methylmalonyl Coenzyme A mutase; succinyl CoA mutase; methylmalonyl CoA isomerase (MUT, MCM)
• Schilling test
• transcobalamin
• vitamin B12 deficiency
• vitamin B12 in serum/plasma

Internet Database

PubChem: 3035220
PubChem: 5287729
PubChem: 2823
PubChem: 5311498
PubChem: 452304

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. Drug Information & Medication Formulary, Veterans Affairs, Central California Health Care System, 1st ed., Ravnan et al eds, 1998
4. 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. Stryer Biochemistry WH Freeman & Co, New York, 1988 pg 508
7. Prescriber's Letter 8(10):55 2001
8. Harrison's Online, Chapter 107, McGraw Hill, 2002
9. Geriatrics Review Syllabus, American Geriatrics Society, 5th edition, 2002-2004
10. Carmel R, Clinical Chemistry. 48:407-409. 2002 <PubMed> PMID: [1] <Internet> [2]

vitamin B12; cobalamin