myo-inositol 1,4,6-trisphosphorothioate and myo-inositol 1,3, 6-trisphosphorothioate: partial agonists with very low intrinsic activity at the platelet myo-inositol 1,4,5-trisphosphate receptor.
Racemic mixtures and enantiomerically pure D-isomers of both myo-inositol 1,3,6-trisphosphorothioate [Ins(1,3,6)PS(3)] and myo-inositol 1,4,6-trisphosphorothioate [Ins(1,4,6)PS(3)], prepared by total synthesis, were examined in Ca(2+) flux and binding assays. Both D-Ins(1,3,6)PS(3) and D-Ins(1,4,6)PS(3) were shown to be low intrinsic activity partial agonists at the platelet myo-inositol 1,4, 5-trisphosphate [Ins(1,4,5)P(3)] receptor, releasing less than 20% of the Ins(1,4,5)P(3)-sensitive Ca(2+) store. D-Ins(1,4,6)PS(3) displaced specifically bound [(3)H]Ins(1,4,5)P(3) from rat cerebellar membranes, although displacement was some 34-fold weaker than by D-Ins(1,4,5)P(3). D-Ins(1,4,6)PS(3) displaced [(3)H]Ins(1,4, 5)P(3) from cerebellar membranes with roughly twice the affinity of DL-Ins(1,4,6)PS(3) (IC(50) value = 1.4 +/- 0.35 microM compared with 2.15 +/- 0.13 microM), whereas D-Ins(1,3,6)PS(3) displaced [(3)H]Ins(1,4,5)P(3) with roughly twice the affinity of DL-Ins(1,3, 6)PS(3) (IC(50) value = 17.5 +/- 5.8 microM compared with 34 +/- 10 microM), confirming that the activity of both these phosphorothioates resides in their D-enantiomers. Increasing concentrations of either D-Ins(1,3,6)PS(3) or D-Ins(1,4,6)PS(3) were able to partially antagonize Ca(2+) release induced by submaximal concentrations of Ins(1,4,5)P(3), an inhibition that could be overcome by increasing the concentration of Ins(1,4,5)P(3), suggesting competition for binding at the Ins(1,4,5)P(3)-R. The only low-efficacy partial agonists at the Ins(1,4,5)P(3)-R discovered to date have been phosphorothioates; the novel D-Ins(1,3,6)PS(3) and D-Ins(1,4,6)PS(3) can now be added to this small group of analogs. However, D-Ins(1,4,6)PS(3) has a relatively high affinity for the Ins(1,4,5)P(3)-R but maintains the lowest efficacy of all the partial agonists thus far identified. As such, it may be a useful tool for pharmacological intervention in the polyphosphoinositide pathway and an important lead compound for the development of further Ins(1,4,5)P(3)-R antagonists.
Murphy
C T
CT
Department of Pharmacy and Pharmacology, University of Bath, Bath, UK. c.t.murphy@bath.ac.uk
Riley
A M
AM
Mills
S J
SJ
Lindley
C J
CJ
Potter
B V
BV
Westwick
J
J
D000818:Animals; D001792:Blood Platelets / Q000187:drug effects* / Q000378:metabolism; D002118:Calcium / Q000378:metabolism*; D015220:Calcium Channels / Q000378:metabolism*; D002462:Cell Membrane / Q000187:drug effects / Q000378:metabolism; D066298:In Vitro Techniques; D015544:Inositol 1,4,5-Trisphosphate / Q000031:analogs & derivatives* / Q000494:pharmacology; D053496:Inositol 1,4,5-Trisphosphate Receptors; D009946:Organothiophosphorus Compounds / Q000494:pharmacology*; D010539:Permeability; D011817:Rabbits; D051381:Rats; D018160:Receptors, Cytoplasmic and Nuclear / Q000378:metabolism*; D014316:Tritium
D015220:Calcium Channels; D053496:Inositol 1,4,5-Trisphosphate Receptors; D009946:Organothiophosphorus Compounds; D018160:Receptors, Cytoplasmic and Nuclear; C406148:inositol 1,3,6-trisphosphorothioate; C086879:inositol 1,4,6-trisphosphorothioate; D014316:Tritium; D015544:Inositol 1,4,5-Trisphosphate; D002118:Calcium