Roles for hydroxyl groups of D-myo-inositol 1,4,5-trisphosphate in the recognition by its receptor and metabolic enzymes

Positional isomers of D-myo-inositol 1,4,5-trisphosphate (Ins(1,4,5)P₃), Ins(1,4,6)P₃, and Ins(1,3,6)P₃ were examined to explore the roles for hydroxyl groups of Ins(1,4,5)P₃, recognized by metabolic enzymes and by the receptor. Ins(1,4,6)P₃ and Ins(1,3,6)P₃ inhibited the dephosphorylation of [³H] Ins(1,4,5)P₃ by the 5-phosphatase present in erythrocyte ghosts. The K(i) value for the former compound was slightly lower than that for Ins(1,4,5)P₃, while that for the latter compound was higher. In an assay of [³H] Ins(1,4,5)P₃ 3-kinase catalyzed by rat brain cytosol, Ins(1,4,6)P₃ was as potent as Ins(1,4,5)P₃ in inhibiting the phosphorylation, whereas Ins(1,3,6)P₃ at concentrations up to 30 μM, was without effect. Ins(1,4,6)P₃ and Ins(1,3,6)P₃ at higher concentrations were effective in inhibiting [³H]Ins(1,4,5)P₃ binding to purified Ins(1,4,5)P₃ receptor. It would thus appear that the 2- and 3-hydroxyl groups of Ins(1,4,5)P₃ are not primarily involved in recognition by metabolic enzymes, while 6-hydroxyl is slightly or severely involved in recognition by the phosphatase or kinase, respectively. On the other hand, although both 3-OH and 6-OH of Ins(1,4,5)P₃ are important in binding by the Ins(1,4,5)P₃ receptor to evoke the release of Ca²⁺, the 6-OH seems to be the more important.