Pezacka E H
Department of Cell Biology, Cleveland Clinic Foundation, Ohio 44195.
Biochim Biophys Acta. 1993 Jun 11;1157(2):167-77. doi: 10.1016/0304-4165(93)90061-c.
Two enzymes involved in the intracellular metabolism of cobalamin have been identified and characterized: cyanocobalamin beta-ligand transferase and microsomal cob(III)alamin reductase. The beta-ligand transferase is a cytosolic enzyme utilizing FAD, NADPH and reduced glutathione. The product of the reaction has been identified as glutathionyl-cobalamin. NADH-linked cob(III)alamin reductase has been found in two subcellular fractions: microsomal and inner mitochondrial membrane. The product of the reduction catalyzed by the microsomal enzyme has been identified as cob(II)alamin. In cbl C mutant fibroblasts, the specific activities of cyanocobalamin beta-ligand transferase and cob(III)alamin reductase were markedly decreased and have varied from 3%-30% and 36%-42% of normal, respectively. The specific activity of mitochondrial cob(III)alamin reductase was only 30% of normal in two cbl C mutants and normal in remaining mutant cell lines. In the cbl D cells, the specific activities were 33% and 55%. Mitochondrial cob(III)alamin reductase was not affected by cbl D mutation. Methionine synthase, L-methylmalonyl-CoA mutase and microsomal cytochrome c and b5 reductases are not affected by both mutations. The cbl E mutation affects only the activity of methionine synthase. These results support the hypothesis that the early enzymatic steps of intracellular metabolism of cobalamin are similar in the synthesis of both methylcobalamin and adenosylcobalamin and these steps are altered by the cbl C and cbl D mutations.
氰钴胺素β-配体转移酶和微粒体钴胺素（III）还原酶。β-配体转移酶是一种利用黄素腺嘌呤二核苷酸（FAD）、烟酰胺腺嘌呤二核苷酸磷酸（NADPH）和还原型谷胱甘肽的胞质酶。该反应的产物已被鉴定为谷胱甘肽基钴胺素。与烟酰胺腺嘌呤二核苷酸（NADH）相关的钴胺素（III）还原酶已在两个亚细胞组分中发现：微粒体和线粒体内膜。微粒体酶催化还原的产物已被鉴定为钴胺素（II）。在cbl C突变型成纤维细胞中，氰钴胺素β-配体转移酶和钴胺素（III）还原酶的比活性显著降低，分别为正常水平的3%-30%和36%-42%。在两个cbl C突变体中，线粒体钴胺素（III）还原酶的比活性仅为正常水平的30%，而在其余突变细胞系中则正常。在cbl D细胞中，比活性分别为33%和55%。线粒体钴胺素（III）还原酶不受cbl D突变的影响。甲硫氨酸合酶、L-甲基丙二酰辅酶A变位酶以及微粒体细胞色素c和b5还原酶均不受这两种突变的影响。cbl E突变仅影响甲硫氨酸合酶的活性。这些结果支持以下假说：钴胺素细胞内代谢的早期酶促步骤在甲基钴胺素和腺苷钴胺素的合成中相似，并且这些步骤会因cbl C和cbl D突变而改变。
