Vitamin B12 Essay

INTRODUCTION

Vitamin B12 is an essential water soluble vitamin that must be carefully regulated to prevent deficiency related complications in the nervous, gastrointestinal, and cardiovascular systems (e). This review begins with a discussion of B12 absorption and role as a cofactor for L-methylmalonyl-CoA mutase and methionine synthase (e). Laboratory findings are interpreted along with associated disease conditions. Finally, assay methods are discussed including blood smear and complete blood count, B12 competitive-binding immunoenzymatic assay, homocysteine tandem mass spectrometry, methylmalonic acid liquid chromatography tandem mass spectrometry, and the Schilling test.

DISCUSSION

Role of Vitamin B12

Vitamin B12 is commonly obtained in the diet through consumption of meat, nuts, eggs, milk, shellfish, and green leafy vegetables (j). B12 has a large complex structure based around a corrin ring, which resembles porphyrin with a cobalt ion at the center (l). For this reason compounds with vitamin B12 activity are called cobalamin (k). Cyanocobalamin is the form typically found in fortified foods, which is converted in the body to the active forms methylcobalamin and 5-deoxyadenosylcobalamin (k).

Ingested vitamin B12 is typically protein bound, and is released by pepsin and gastric acid before binding with R-proteins in the stomach (a). Proteases in the small intestine then release B12, which binds the protein intrinsic factor (IF), protecting B12 from further degradation by intestinal bacteria (a). Binding IF is required for absorption by the intestinal epithelium, and subsequent entry into circulation (j).

Vitamin B12 is a cofactor for methionine synthase, which regenerates methionine from homocysteine and participa...

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...its role as a coenzyme for L-methylmalonyl-CoA mutase and methionine synthase (e). Serum vitamin B12 less than 145 pg/mL is considered deficient, and is commonly caused by pernicious anemia and atrophic gastritis (k) (h). B12 is measured with a competitive-binding immunoenzymatic assay that measures concentration based on an inverse relationship to absorbance (i). Methylmalonic acid and homocysteine are associated metabolites that increase in concentration with B12 deficiency and act as disease markers (b). The Schilling test is a less common method that involves administration of oral radiolabeled B12, saturation of B12 binding proteins, and subsequent detection of radiolabeled B12 in urine to assess absorption rate (g). Current laboratory methods are rapid and specific, providing clinicians with accurate information in the diagnosis of vitamin B12 deficiency.