Defining the minimal structural requirements for partial agonism at the type I myo-inositol 1,4,5-trisphosphate receptor

The novel synthetic analogues d-3-fluoro- myo-inositol 1,5-bisphosphate-4-phosphorothioate, [3F-Ins(1,5)P 2-4PS], d-3-fluoro- myo-inositol 1,4-bisphosphate-5-phosphorothioate [3F-Ins(1,4)P 2-5PS], and d-3-fluoro- myo-inositol 1-phosphate-4,5-bisphosphorothioate [3F-Ins(1)P-(4,5)PS 2] were utilised to define the structure-activity relationships which could produce partial agonism at the Ca 2+ mobilising myo-inositol 1,4,5-trisphosphate [Ins(1,4,5)P 3] receptor. Based on prior structure-activity data we hypothesised that the minimal structural requirements for Ins(1,4,5)P 3 receptor partial agonism, were phosphorothioate substitution of the crucial vicinal 4,5-bisphosphate pair accompanied by another structural perturbation, such fluorination of 3-position of the myo-inositol ring. All the analogues fully displaced [ 3H]Ins(1,4,5)P 3 from a single Ins(1,4,5)P 3 binding site in pig cerebellar membranes [3F-Ins(1,5)P 2-4PS (IC 50=26 nM), 3F-Ins(1,4)P 2-5PS (IC 50=80 nM) and 3F-Ins(1)P-(4,5)PS 2 (IC 50=109 nM) cf. Ins(1,4,5)P 3 (IC 50=11 nM)]. In contrast, 3F-Ins(1,5)P 2-4PS (IC 50=424 nM) and 3F-Ins(1,4)P 2-5PS (IC 50=3579 nM) were weak full agonists at the Ca 2+ mobilising Ins(1,4,5)P 3 receptor of permeabilised SH-SY5Y neuroblastoma cells, being respectively 4- and 36-fold less potent than Ins(1,4,5)P 3 (EC 50=99 nM). While 3F-Ins(1)P-(4,5)PS 2 (EC 50=11345 nM) was a partial agonist releasing only 64.3±1.9% of the Ins(1,4,5)P 3-sensitive intracellular Ca 2+ pools. 3F-Ins(1)P-(4,5)PS 2 was unique among the Ins(1,4,5)P 3 receptor partial agonists so far identified in having a relatively high affinity for the Ins(1,4,5)P 3 binding site, accompanied by a significant loss of intrinsic activity for Ca 2+ mobilisation. This improved affinity was probably due to the retention of the 1-position phosphate, which enhances interaction with the Ins(1,4,5)P 3 receptor. 3F-Ins(1)P-(4,5)PS 2 may be an important lead compound for the development of efficient Ins(1,4,5)P 3 receptor antagonists © 1997 Federation of European Biochemical Societies. All rights reserved.