TY - CHAP M1 - Book, Section TI - Chapter 147. Vitamin B12 and Folic Acid A1 - Watkins, David A1 - Rosenblatt, David S. A2 - Rudolph, Colin D. A2 - Rudolph, Abraham M. A2 - Lister, George E. A2 - First, Lewis R. A2 - Gershon, Anne A. Y1 - 2011 N1 - T2 - Rudolph's Pediatrics, 22e AB - Cobalamin (vitamin B12) is a complex organometallic molecule that is synthesized by many bacteria and is obtained in the human diet from meat, fish, and dairy products. It is not present in plant foods, so strict vegetarians are at risk for dietary deficiency. Derivatives of cobalamin are required for the activity of two enzymes: (1) methylcobalamin (MeCbl) is generated during the catalytic cycle of methionine synthase, a cytoplasmic enzyme that catalyzes methylation of homocysteine to methionine (see Chapter 138), and (2) 5′-deoxyadenosylcobalamin (AdoCbl) is required for the mitochondrial enzyme methylmalonyl-CoA mutase to catalyze the conversion of methylmalonyl-CoA, formed during catabolism of branched-chain amino acids and odd-chain fatty acids, to succinyl-CoA (see Chapter 137). Therefore, inborn errors of cobalamin metabolism result in isolated methylmalonic aciduria, isolated homocystinuria, or combined methylmalonic aciduria and homocystinuria, depending on which step in cobalamin metabolism is affected.1-4 In these disorders, hypomethioninemia usually occurs together with hyperhomocysteinemia. Elevated homocysteine levels are associated with an increased risk of thrombosis; decreased methionine is associated with abnormalities of the white matter of the nervous system. Elevated levels of methylmalonic acid are usually associated with metabolic acidosis. SN - PB - The McGraw-Hill Companies CY - New York, NY Y2 - 2024/03/28 UR - accesspediatrics.mhmedical.com/content.aspx?aid=6725723 ER -