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 Ki 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 microM, 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.