TY - CHAP M1 - Book, Section TI - Vitamin B12 and Folic Acid A1 - Shchelochkov, Oleg A. A1 - Venditti, Charles P. A2 - Kline, Mark W. Y1 - 2018 N1 - T2 - Rudolph's Pediatrics, 23e 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 2 enzymes: methylcobalamin and 5′-deoxyadenosylcobalamin. Methylcobalamin is generated during the catalytic cycle of methionine synthase, a cytoplasmic enzyme that catalyzes methylation of homocysteine to methionine (see Chapter 133). 5′-Deoxyadenosylcobalamin (AdoCbl) is required for the mitochondrial enzyme methylmalonyl-coenzyme A (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 132). 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. Without treatment, hyperhomocysteinemia is usually accompanied by hypomethioninemia. Significantly elevated homocysteine levels may be 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 can lead to metabolic acidosis. SN - PB - McGraw-Hill Education CY - New York, NY Y2 - 2024/03/28 UR - accesspediatrics.mhmedical.com/content.aspx?aid=1182928867 ER -