USMLE (Fach) / Biochemistry - Nutrition (Lektion)

Vorderseite Vitamin B12 (cobalamin)
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Cofactor for methionine synthase (transfers CH3 groups as methylcobalamine) and methylmalonyl-CoA mutase. Important for DNA synthesis.

- Found in animal products. Synthesized only by microorganisms. - Very large reserve pool (several years) stored primarily in the liver.

Deficiency:- Macrocytic, megaloblastic anemia; hypersegmented PMNs- Paresthesias and subacute combined degeneration (degeneration of dorsal columns, lateral corticospinal tracts, and spinocerebellar tracts) due to abnormal myelin.- ↑ serum homocysteine and methylmalonic acid levels, along with 2˚ folate deficiency.- Prolonged deficiency → irreversible nerve damage.

- Deficiency caused by malabsorption (eg, sprue, enteritis, Diphyllobothrium latum, achlorhydria, bacterial overgrowth, alcohol excess), lack of intrinsic factor (eg, pernicious anemia, gastric bypass surgery), absence of terminal ileum (surgical resection, eg, for Crohn disease), or insufficient intake (eg, veganism).- Anti-intrinsic factor antibodies diagnostic for pernicious anemia.- Folate supplementation can mask the hematologic symptoms of B12 deficiecny, but not the neruologic symptoms.

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