MYOINOSITOL 1,4,5-TRIPHOSPHATE AND RELATED-COMPOUNDS PROTONATION SEQUENCE - POTENTIOMETRIC AND P-31 NMR-STUDIES

The protonation sequence of myo-inositol 1,4,5-triphosphate [Ins(1,4,5 )P3], of its dehydroxylated analogue, Cyhx(1,2,4)P3, of two diphosphor ylated inositol phosphates, Ins(1,4)P2 and Ins(4,5)P2 and of one inosi tol monophosphate, Ins(1)P1, have been determined. Potentiometric and P-31 NMR studies have been performed in a 0.1 mol dm-3 solution of tet raethylammonium perchlorate 25-degrees-C (medium 1) and, in addition, for Ins(1,4,5)P3, in a 0.2 mol dm-3 KCl solution at 37-degrees-C (medi um 2). In the case of the inositol diphosphate, microconstants related to each individual phosphate could be calculated and interpreted acco rding to the position of the phosphates around the inositol ring. Cyhx (1,2,4)P3 bears an independent phosphate (P4) and two additional phosp hates (P1 and P2) equally sharing the bound protons. For Ins(1,4,5)P3, strong interactions between the phosphate groups are possible, due to the presence of the hydroxy groups. The chemical shifts of the monoan ionic and dianionic phosphate forms of the studied compounds are discu ssed. The superimposition of binding data with the NMR titration curve s of Ins(1,4,5)P3 emphasizes the particular importance of P5 in bindin g of Ins(1,4,5)P3 to its receptor.