Yoshinaga Masafumi, Ueki Tatsuya, Yamaguchi Nobuo, Kamino Kei, Michibata Hitoshi
Molecular Physiology Laboratory, Graduate School of Science, Hiroshima University, Kagamiyama 1-3-1, Higashi-Hiroshima 739-8526, Japan.
Biochim Biophys Acta. 2006 Mar;1760(3):495-503. doi: 10.1016/j.bbagen.2006.01.008. Epub 2006 Feb 9.
Some ascidians accumulate vanadium in vanadocytes, which are vanadium-containing blood cells, at high levels and with high selectivity. However, the mechanism and physiological significance of vanadium accumulation remain unknown. In this study, we isolated novel proteins with a striking homology to glutathione transferases (GSTs), designated AsGST-I and AsGST-II, from the digestive system of the vanadium-accumulating ascidian Ascidia sydneiensis samea, in which the digestive system is thought to be involved in vanadium uptake. Analysis of recombinant AsGST-I confirmed that AsGST-I has GST activity and forms a dimer, as do other GSTs. In addition, AsGST-I was revealed to have vanadium-binding activity, which has never been reported for GSTs isolated from other organisms. AsGST-I bound about 16 vanadium atoms as either V(IV) or V(V) per dimer, and the apparent dissociation constants for V(IV) and V(V) were 1.8 x 10(-4) M and 1.2 x 10(-4) M, respectively. Western blot analysis revealed that AsGSTs were expressed in the digestive system at exceptionally high levels, although they were localized in almost all organs and tissues examined. Considering these results, we postulate that AsGSTs play important roles in vanadium accumulation in the ascidian digestive system.
一些海鞘会在钒细胞（即含钒血细胞）中大量且高选择性地积累钒。然而，钒积累的机制和生理意义仍不清楚。在本研究中，我们从积累钒的海鞘萨氏海鞘（Ascidia sydneiensis samea）的消化系统中分离出了与谷胱甘肽转移酶（GSTs）具有显著同源性的新型蛋白质，命名为AsGST-I和AsGST-II，据认为该海鞘的消化系统参与钒的摄取。重组AsGST-I的分析证实，AsGST-I具有GST活性并形成二聚体，其他GSTs也是如此。此外，AsGST-I被发现具有钒结合活性，这在从其他生物体分离出的GSTs中从未有过报道。每个二聚体的AsGST-I结合约16个钒原子，以V(IV)或V(V)形式存在，V(IV)和V(V)的表观解离常数分别为1.8×10⁻⁴ M和1.2×10⁻⁴ M。蛋白质免疫印迹分析显示，尽管AsGSTs几乎存在于所有检测的器官和组织中，但在消化系统中的表达水平异常高。考虑到这些结果，我们推测AsGSTs在海鞘消化系统中的钒积累中发挥重要作用。
