School of Bionics, Tokyo University of Technology, 1404-1 Katakura-cho, Hachioji, Tokyo 192-0982, Japan.
J Clin Biochem Nutr. 2008 Mar;42(2):167-74. doi: 10.3164/jcbn.2008024.
Coenzyme Q10 (CoQ10) is essential for ATP production in the mitochondria, and is an important antioxidant in every biomembrane and lipoprotein. Due to its hydrophobicity, a binding and transfer protein for CoQ10 is plausible, but none have yet been isolated and characterized. Here we purified a CoQ10-binding protein from human urine and identified it to be saposin B, a housekeeping protein necessary for sphingolipid hydrolysis in lysosomes. We confirmed that cellular saposin B binds CoQ10 in human sperm and the hepatoma cell line HepG2 by using saposin B monoclonal antibody. The molar ratios of CoQ10 to saposin B were estimated to be 0.22 in urine, 0.003 in HepG2, and 0.12 in sperm. We then confirmed that aqueous saposin B extracts CoQ10 from hexane to form a saposin B-CoQ10 complex. Lipid binding affinity to saposin B decreased in the following order: CoQ10>CoQ9>CoQ7>>alpha-tocopherol>>cholesterol (no binding). The CoQ10-binding affinity to saposin B increased with pH, with maximal binding seen at pH 7.4. On the other hand, the CoQ10-donating activity of the saposin B-CoQ10 complex to erythrocyte ghost membranes increased with decreasing pH. These results suggest that saposin B binds and transports CoQ10 in human cells.
辅酶 Q10（CoQ10）是线粒体中 ATP 产生所必需的，并且是每个生物膜和脂蛋白中的重要抗氧化剂。由于其疏水性，可能存在与 CoQ10 结合和转运的蛋白质，但尚未分离和表征。在这里，我们从人尿中纯化了一种与 CoQ10 结合的蛋白质，并将其鉴定为脑苷脂 B，这是溶酶体中鞘脂水解所必需的管家蛋白。我们通过使用脑苷脂 B 单克隆抗体证实了细胞脑苷脂 B 在人精子和肝癌细胞系 HepG2 中与 CoQ10 结合。CoQ10 与脑苷脂 B 的摩尔比分别估计为尿中的 0.22，HepG2 中的 0.003 和精子中的 0.12。然后，我们证实了水性脑苷脂 B 从己烷中提取 CoQ10 形成脑苷脂 B-CoQ10 复合物。脑苷脂 B 对脂质的结合亲和力按以下顺序降低：CoQ10>CoQ9>CoQ7>>α-生育酚>胆固醇（无结合）。脑苷脂 B 与 CoQ10 的结合亲和力随 pH 值增加而增加，在 pH 值为 7.4 时达到最大结合。另一方面，脑苷脂 B-CoQ10 复合物向红细胞血影膜的 CoQ10 供体活性随 pH 值降低而增加。这些结果表明脑苷脂 B 在人细胞中结合和转运 CoQ10。
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