Irreversible inhibition of Ca²⁺ release in saponin-Treated macrophages by the photoaffinity derivative of inositol-1,4,5-Trisphosphate

D-myo-inositol-1,4,5-trisphosphate (InsP₃) is a putative intracellular second messenger for the mobilization of Ca^2- from intracel-lular stores, in particular, the endoplasmic reticulum^1-6. Specific binding sites on the endoplasmic reticulum may participate in the InsP₃-induced release of Ca^2- from the Ca^2- pool^6-8. To examine the specific binding sites on the endoplasmic reticulum, we synthesized an arylazide derivative of InsP₃ for photoaffinity labelling; InsP₃ coupled to p-azidobenzoic acid (InsP₃-pAB) using N,N′-carbonyldiimidazole (GDI) was obtained at a 9-11% yield. Here, we report that InsP₃-pAB, but not an arylazide derivative of inositol-1,4-bisphophate (Ins(1,4)P₂), causes the irreversible inhibition of InsP₃-induced release of Ca ^2- in saponin-permeabilized photo-irradiated macrophages. The irreversible inhibition by InsP₃-pAB after photo-irradiation was prevented by a 10-fold excess of unmodified InsP₃.