Hirata M, Takeuchi H, Riley A M, Mills S J, Watanabe Y, Potter B V
Department of Biochemistry, Faculty of Dentistry, Kyushu University, Fukuoka, Japan.
Biochem J. 1997 Nov 15;328 ( Pt 1)(Pt 1):93-8. doi: 10.1042/bj3280093.
The Ins(1,4,5)P3 regioisomers, Ins(1,4,6)P3 and Ins(1,3,6)P3, which can mimic the 1,4,5-arrangement on the inositol ring of Ins(1,4,5)P3, were examined for Ca2+ release by using four types of saponin-permeabilized cell possessing various abundances of receptor subtypes, with special reference to the relation of potency to receptor subtype. Ins(1,4,6)P3 and Ins(1,3,6)P3 were weak agonists in rat basophilic leukaemic cells (RBL cells), which possess predominantly subtype II receptors, with respective potencies of 1/200 and less than 1/500 that of Ins(1,4,5)P3 [the EC50 values were 0.2, 45 and more than 100 microM for Ins(1,4,5)P3, Ins(1,4,6)P3 and Ins(1,3,6)P3 respectively]. Similar rank order potencies were also evaluated for the displacement of [3H]Ins(1,4,5)P3 bound to RBL cell membranes by these regioisomers. However, they caused Ca2+ release from GH3 rat pituitary cells possessing predominantly subtype I receptors more potently; Ins(1,4,6)P3 and Ins(1,3,6)P3 evoked release at respective concentrations of only one-third and one-twentieth that of Ins(1,4,5)P3 (the EC50 values were 0.4, 1.2 and 8 microM for Ins(1,4,5)P3, Ins(1,4,6)P3 and Ins(1,3,6)P3 respectively). In COS-1 African green-monkey kidney cells, with the relative abundances of 37% of the subtype II and of 62% of the subtype III receptor, potencies of 1/40 and approx. 1/200 for Ins(1, 4,6)P3 and Ins(1,3,6)P3 respectively were exhibited relative to Ins(1,4,5)P3 (the EC50 values were 0.4, 15 and approx. 80 microM for Ins(1,4,5)P3, Ins(1,4,6)P3 and Ins(1,3,6)P3 respectively). In HL-60 human leukaemic cells, in spite of the dominant presence of subtype I receptors (71%), similar respective potencies to those seen with COS-1 cells were exhibited (the EC50 values were 0.3, 15 and approx. 100 microM for Ins(1,4,5)P3, Ins(1,4,6)P3 and Ins(1,3,6)P3 respectively). These results indicate that these regioisomers are the first ligands that distinguish between receptor subtypes; the present observations are of significance for the future design of molecules with enhanced selectivity.
肌醇（1,4,5）三磷酸（Ins(1,4,5)P3）的区域异构体肌醇（1,4,6）三磷酸（Ins(1,4,6)P3）和肌醇（1,3,6）三磷酸（Ins(1,3,6)P3）能够模拟Ins(1,4,5)P3肌醇环上的1,4,5排列方式。本研究使用了四种具有不同受体亚型丰度的皂素通透细胞，特别参考了效力与受体亚型的关系，检测了这两种区域异构体的钙离子释放情况。Ins(1,4,6)P3和Ins(1,3,6)P3在主要表达II型受体的大鼠嗜碱性白血病细胞（RBL细胞）中是弱激动剂，其效力分别为Ins(1,4,5)P3的1/200和小于1/500 [Ins(1,4,5)P3、Ins(1,4,6)P3和Ins(1,3,6)P3的半数有效浓度（EC50）值分别为0.2、45和大于100 μM]。这些区域异构体对结合于RBL细胞膜的[3H]Ins(1,4,5)P3的置换效力也呈现出类似的等级顺序。然而，它们能更有效地引起主要表达I型受体的GH3大鼠垂体细胞释放钙离子；Ins(1,4,6)P3和Ins(1,3,6)P3分别在仅为Ins(1,4,5)P3浓度的三分之一和二十分之一时就能诱发钙离子释放（Ins(1,4,5)P3、Ins(1,4,6)P3和Ins(1,3,6)P3的EC50值分别为0.4、1.2和8 μM）。在COS-1非洲绿猴肾细胞中，II型受体相对丰度为37%，III型受体相对丰度为62%，Ins(1,4,6)P3和Ins(1,3,6)P3相对于Ins(1,4,5)P3的效力分别为1/40和约1/200（Ins(1,4,5)P3、Ins(1,4,6)P3和Ins(1,3,6)P3的EC50值分别为0.4、15和约80 μM）。在HL-60人白血病细胞中，尽管I型受体占主导地位（71%），但其效力与COS-1细胞中的情况相似（Ins(1,4,5)P3、Ins(1,4,6)P3和Ins(1,3,6)P3的EC50值分别为0.3、15和约100 μM）。这些结果表明，这些区域异构体是首批能够区分受体亚型的配体；目前的观察结果对于未来设计具有更高选择性的分子具有重要意义。
