3-POSITION MODIFICATION OF MYOINOSITOL 1,4,5-TRISPHOSPHATE - CONSEQUENCES FOR INTRACELLULAR CA2+ MOBILIZATION AND ENZYME RECOGNITION

The ability of the novel D-myo-inositol 1,4,5-trisphosphate (Ins(1,4,5)P3) analogues, L-chiro-inositol 2,3,5-trisphosphate (L-ch-Ins(2,3,5)P3) and D-3-deoxy-3-fluoro-myo-inositol 1,4,5-trisphosphate (3F-Ins(1,4,5)P3), to bind to the Ins(1,4,5)P3 receptor, mobilise intracellular Ca2+ stores and interact with metabolic enzymes has been investigated. L-ch-Ins(2,3,5)P3 and 3F-Ins(1,4,5)P3 were bound by the Ins(1,4,5)P3 receptor from bovine adrenal cortex with relatively high affinity (K(i) values 60.4 and 8.0 nM respectively) but with lower affinity than Ins(1,4,5)P3 (K(D) = 5.9 nM). Both analogues were apparent full agonists at the Ca2+ mobilising receptor in SH-SY5Y cells, but were less potent than Ins(1,4,5)P3 (EC5, L-ch-Ins(2,3,5)P3 = 1.4-mu-M, 3F-Ins(1,4,5)P3 = 0.37-mu-M and Ins(1,4,5)P3 = 0.12-mu-M). L-ch-Ins(2,3,5)P3 and 3F-Ins(1,4,5)P3 were resistant to Ins(1,4,5)P3 3-kinase, and were potent inhibitors of the enzyme (K(i) values 7.1 and 8.6-mu-M respectively). 3F-Ins(1,4,5)P3 was hydrolysed by Ins(1,4,5)P3 5-phosphatase at a similar rate to Ins(1,4,5)P3, but inhibited dephosphorylation of [H-3]Ins(1,4,5)P3 with high potency (apparent K(i) = 3.9-mu-M) L-ch-Ins(2,3,5)P3 was also recognised by the enzyme with high affinity (K(i) = 7.7-mu-M) but was resistant to hydrolysis. These results suggest that the environment around C-3 is of major importance for recognition not only by Ins(1,4,5)P3 3-kinase but also by Ins(1,4,5)P3 5-phosphatase.