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首页> 美国卫生研究院文献>Proceedings of the National Academy of Sciences of the United States of America >Absence of an intracellular cobalamin-binding protein in cultured fibroblasts from patients with defective synthesis of 5′-deoxyadenosylcobalamin and methylcobalamin
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Absence of an intracellular cobalamin-binding protein in cultured fibroblasts from patients with defective synthesis of 5′-deoxyadenosylcobalamin and methylcobalamin

机译：5-脱氧腺苷基钴胺素和甲基钴胺素合成有缺陷的患者培养的成纤维细胞中缺乏细胞内钴胺素结合蛋白

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摘要

Three distinct classes of human mutations (cbl A, cbl B, and cbl C) cause defective synthesis of cobalamin (Cbl; vitamin B12) coenzymes. Cultured fibroblasts from that unique class (cbl C) deficient in the synthesis of both Cbl coenzymes, 5′-deoxyadenosylcobalamin (AdoCbl) and methylcobalamin (MeCbl), were used to explore the underlying defect. We compared the uptake of transcobalamin II(TC II)-bound cyano[57Co]cobalamin (CN-Cbl) by cbl cells with that of other control and mutant cell lines. Although the cbl C cells initially took up CN-[57Co]Cbl normally, they were unable to retain it. To characterize this “leak” further, cell extracts were prepared following incubation and chromatographed on Sephadex G-150. After incubations of 1-2 hr, most of the CN-[57Co]Cbl accumulated by control cells was still bound to TC II; the remainder was free. Thereafter, an ever-increasing fraction of the labeled Cbl eluted with an intracellular cobalamin-binding protein (ICB); more than 80% of the total was so bound after 76 hr incubations. ICB had an apparent molecular weight similar to that of several Cbl “R” binders (about 120,000), but was distinguished from them by its failure to react with specific anti-“R”binder antiserum. Significantly, no ICB was detected in extracts of three different cbl C lines even aftr prolonged incubations, whereas its appearance in cbl A, cbl B, and mutase apoenzyme mutants was normal. We propose: that ICB is required for retention of cobalamins by cells; and that cbl C cells “leak” cobalamins and show defective synthesis of Cbl coenzymes because they lack this intracellular binder.
机译：三种不同类别的人类突变（cbl A，cbl B和cbl C）引起钴胺素（Cbl；维生素B12）辅酶的合成缺陷。来自缺乏独特的Cbl辅酶5'-脱氧腺苷基钴胺素（AdoCbl）和甲基钴胺素（MeCbl）的合成的独特类（cbl C）的培养的成纤维细胞被用于探索潜在的缺陷。我们比较了cbl细胞与其他对照和突变细胞系对跨钴胺素II（TC II）结合的氰基[ 57 Co]钴胺素（CN-Cbl）的摄取。尽管cbl C细胞最初正常吸收CN-[ 57 Co] Cbl，但它们无法保留。为了进一步表征这种“泄漏”，孵育后制备细胞提取物，并在Sephadex G-150上进行色谱分离。温育1-2小时后，对照细胞积累的大部分CN-[ 57 Co] Cbl仍与TC II结合；其余的是免费的。此后，用细胞内钴胺素结合蛋白（ICB）洗脱的标记的Cbl的比例不断增加；经过76小时的孵育后，有80％以上的分子被如此结合。 ICB的表观分子量类似于几种Cbl“ R”粘合剂的分子量（约120,000），但与之不同之处在于它无法与特定的“ R”粘合剂抗血清反应。重要的是，即使在延长培养时间后，在三个不同的cbl C系的提取物中也未检测到ICB，而在cbl A，cbl B和突变酶脱辅酶突变体中它的出现是正常的。我们建议：ICB是细胞保留钴胺素所必需的；并且cbl C细胞“泄漏”钴胺素并显示出Cbl辅酶的合成缺陷，因为它们缺乏这种细胞内结合剂。

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