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

All 9 racemic regiosomers (15 enantiomerically) of myo-inositol tetrakisphosphates (IP₄s): DL-Ins(1,2,4,5)P₄ [A], DL-Ins(1,2,4,6)P₄ [B], Ins(1,2,3,5)P₄ [C], Ins(1,3,4,6)P₄ [D], Ins(2,4,5,6)P₄ [E], DL-Ins(1,3,4,5)P₄ [F], DL-Ins(1,2,5,6)P₄ [G], DL-Ins(1,2,3,4)P₄ [H] and DL-Ins(1,4,5,6)P₄ [I] were investigated for their ability to bind to the D-myo-inositol 1,4,5-trisphosphate [Ins(1,4,5)P₃] receptor in bovine adrenal cortical membranes, and for their ability to mobilize ⁴⁵Ca²⁺ from Ins(l,4,5)P₃-sensitive Ca²⁺ stores in permeabilized Chinese hamster ovary (CHO) cells. DL-Ins(1,2,4,5)P₄ (K₁ = 11 nM) bound to Ins(1,4,5)P₃ receptors with an affinity only 2-fold lower than Ins(1,4,5)P₃ (K₁ = 6 nM), Ins(1,2,3,5)P₄, Ins(1,3,4,6)P₄, Ins(2,4,5,6)P₄, DL-Ins(1,3,4,5)P₄, DL-Ins(1,2,3,4)P₄ and DL-Ins(1,4,5,6)P₄ bound with affinities of between 0.4 - 0.7 μM. DL-Ins(1,2,4,6)P₄ and DL-Ins(1,2,5,6)P₄ bound to the Ins(1,4,5)P₃ receptor with low affinity (approximately 2-3 μM). All but one of the IP₄s mediated release of ⁴⁵Ca²⁺ from stores of permeabilized CHO cells with a similar rank order of potency as that for Ins(1,4,5)P₃ receptor binding, being between 16-fold and 50-fold less potent at releasing ⁴⁵Ca²⁺ compared with their apparent binding affinities to the Ins(1,4,5)P₃ receptor. The notable exception was Ins(1,2,3,5)P₄, which showed an approximately 200-fold lower potency compared with its affinity for the Ins(1,4,5)P₃ receptor. Ins(1,2,3,5)P₄ may be a useful lead compound for the rational design of novel synthetic Ins(1,4,5)P₃ 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)P₃ receptor.