Abstract

Adenophostins A and B, which are metabolic products of the fungusPenicillium brevicompactum, are potent agonists at thed-myo-inositol-1,4,5-trisphosphate [Ins(1,4,5)P₃] receptor. In the current study, adenophostin A was ∼50-fold more potent than Ins(1,4,5)P₃at both releasing Ca²⁺ from the intracellular stores of permeabilized platelets and displacing [³H]Ins(1,4,5)P₃ from its receptor on rat cerebellar membranes. Various analogues bearing structural features found in the adenophostins and/or Ins(1,4,5)P₃ were examined to elucidate the molecular basis for the observed enhanced potency. 2-AMP did not induce Ca²⁺ release from permeabilized platelets or have any effect on Ins(1,4,5)P₃-induced Ca²⁺ release. Two carbohydrate-based analogues, (2-hydroxyethyl)-α-d-glucopyranoside-2′,3,4-trisphosphate and α,α′-trehalose-3,4,3′,4′-tetrakisphosphate, could induce release of Ca²⁺ and displace [³H]Ins(1,4,5)P₃ from its binding site on rat cerebellar membranes, although both were less potent than Ins(1,4,5)P₃. In common with adenophostin A, release of Ca²⁺ from the intracellular stores could be inhibited by heparin, and both analogues were metabolically resistant. This study is the first to demonstrate the activity of a synthetic disaccharide at the Ins(1,4,5)P₃ receptor and that the Ins(1,4,5)P₃ receptor is capable of accommodating an increased steric bulk. The minimal importance of the 2-hydroxyl group of Ins(1,4,5)P₃ (occupied by the pyranoside oxygen in adenophostin) was confirmed by comparing the activity ofdl-scyllo-Ins(1,2,4)P₃ [which differs from Ins(1,4,5)P₃ solely by the orientation of this hydroxyl group] with that of Ins(1,4,5)P₃. An analogue of this compound, namely,dl-6-CH₂OH-scyllo-Ins(1,2,4)P₃, which possesses an equatorial hydroxymethyl group analogous to the 5′-hydroxymethyl group of adenophostin, was found to be equipotent to Ins(1,4,5)P₃, demonstrating the tolerance of the Ins(1,4,5)P₃ receptor to additional steric bulk at this position.