Second messenger specificity of the inositol trisphophate receptor: Reappraisal based on novel inositol phosphates

To further understand how the second messenger D-myo-inositol 1,4,5- trisphosphate [Ins(1,4,5)P₃] interacts with its intracellular receptor, we injected 47 highly purified inositol phosphate (InsP) positional isomers in Xenopus oocytes and compared their potency in releasing intracellular Ca²⁺. The potency of the Ca²⁺-releasing InsPs spanned four orders of magnitude. Seven compounds, including the novel inositol 1,2,4,5-tetrakisphosphate [D/L- Ins(1,2,4,5)P₄] and D/L-Ins(1,4,6)P₃, had a very high potency. All of these highly active InsPs shared the following structure: two D-trans-equatorial phosphates (eq-P) and one equatorial hydroxyl (eq-OH) attached to ring carbons D-4, D-5, and D-6 (or to the structurally equivalent D-1, D-6, and D- 5 carbons). This permissive structure was not sufficient for Ca²⁺ release, because it was also found in two inactive compounds, Ins(1,6)P₂ and Ins(1,3,6)P₃. To be active, InsPs also required the structural equivalent of a D-3 eq-OH and/or a D-1 eq-P. Together, our data reveal how the structure of the InsP molecule affects its ability to release Ca²⁺.