Selective interaction of chemical dyes with inositol 1,4,5-trisphosphate recognition sites

Inositol 1,4,5-trisphosphate (InsP3) is an important second messenger that interacts with a specific intracellular receptor and triggers a release of Ca2+ from intracellular stores. InsP3 is preferentially metabolized by two enzymes. A specific 5-phosphatase (InsP3 phosphatase) produces an inactive metabolite, inositol 1,4-bisphosphate, while a specific 3-kinase (InsP3 kinase) produces an active metabolite, inositol 1,3,4,5-tetrakisphosphate. With the goal of developing selective ligands of the diverse InsP3 recognition sites, we have studied the effects of some chemical dyes on the binding of InsP3 to its receptor and on the activity of its metabolic enzymes. Although these dyes possess similar chemical structures, they showed varied selectivities towards the three recognition sites. Thymol Blue was the most potent inhibitor of InsP3 binding activity, with an IC50 of 105 microM. Phenol Red demonstrated a higher selectivity for InsP3 phosphatase inhibition, with an IC50 of 100 microM. 3',3",5',5"-Tetraiodophenolsulfonephthalein showed its most potent inhibitory effect on InsP3 kinase activity, with an IC50 of 35 microM. Tetrabromophenol Blue potently inhibited InsP3 phosphatase and InsP3 kinase activities, with respective IC50 values of 25 and 12 microM. Phenolphthalein Diphosphate and Phenolphthalein Carbinol Disulfate demonstrated weak inhibitory effects towards the three recognition sites for InsP3. These results reveal certain structural clues that should help in the development of more selective inhibitors.