Binding and activity of the nine possible regioisomers of myo-inositol tetrakisphosphate at the inositol 1,4,5-trisphosphate receptor

Abstract

All 9 racemic regiosomers (15 enantiomerically) of myo-inositol tetrakisphosphates (IP 4s): DL-Ins(1,2,4,5)P 4 [ A], DL-Ins(1,2,4,6)P 4 [ B], Ins(1,2,3,5)P 4 [ C], Ins(1,3,4,6)P 4 [ D], Ins(2,4,5,6)P 4 [ E], DL-Ins(1,3,4,5)P 4 [ F], DL-Ins(1,2,5,6)P 4 [ G], DL-Ins(1,2,3,4)P 4 [ H] and DL-Ins(1,4,5,6)P 4 [ I] [Chang S-K., Chang YT. Synthesis of all possible regioisomers of myo-inositol tetrakisphosphate. J Chem Soc Chem Commun 1995; 11–13] were investigated for their ability to bind to the D- myo-inositol 1,4,5-trisphosphate [Ins(1,4,5)P 3] receptor in bovine adrenal cortical membranes, and for their ability to mobilize 45Ca 2+ from Ins(1,4,5)P 3-sensitive Ca 2+ stores in permeabilized Chinese hamster ovary (CHO) cells. DL-Ins(1,2,4,5)P 4 (K i = 11 nM) bound to Ins(1,4,5)P 3 receptors with an affinity only 2-fold lower than Ins(1,4,5)P 3 (K i = 6 nM), Ins(1,2,3,5)P 4 Ins(1,3,4,6)P 4, Ins(2,4,5,6)P 4, DL-Ins(1,3,4,5)P 4, DL-Ins(1,2,3,4)P 4 and DL-Ins(1,4,5,6)P 4 bound with affinities of between 0.4–0.7 μM. DL-Ins(1,2,4,6)P 4 and DL-Ins(1,2,5,6)P 4 bound to the Ins(1,4,5)P 3 receptor with low affinity (approximately 2–3 μM). All but one of the IP 4s mediated release of 45Ca 2+ from stores of permeabilized CHO cells with a similar rank order of potency as that for Ins(1,4,5)P 3 receptor binding, being between 16-fold and 50-fold less potent at releasing 45Ca 2+ compared with their apparent binding affinities to the Ins(1,4,5)P 3 receptor. The notable exception was Ins(1,2,3,5)P 4, which showed an approximately 200-fold lower potency compared with its affinity for the Ins(1,4,5)P 3 receptor. Ins(1,2,3,5)P 4 may be a useful lead compound for the rational design of novel synthetic Ins(1,4,5)P 3 analogues possessing structure-activity profiles with relatively high binding affinity, but low intrinsic efficacy, and hence partial agonists and antagonists at the Ins(1,4,5)P 3 receptor.