STRUCTURAL ANALOGS OF D-MYO-INOSITOL-1,4,5-TRISPHOSPHATE AND ADENOPHOSTIN-A - RECOGNITION BY CEREBELLAR AND PLATELET INOSITOL-1,4,5-TRISPHOSPHATE RECEPTORS

Adenophostins A and B, which are metabolic products of the fungus Peni cillium brevicompactum, are potent agonists at the D-myo-inositol-1,4, 5-trisphosphate [Ins(1,4,5)P-3] receptor. In the current study, adenop hostin A was similar to 50-fold more potent than Ins(1,4,5)P-3 at both releasing Ca2+ from the intracellular stores of permeabilized platele ts and displacing [H-3]Ins(1,4,5)P-3 from its receptor on rat cerebell ar membranes. Various analogues bearing structural features found in t he adenophostins and/or Ins(1,4,5)P-3 were examined to elucidate the m olecular 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)P-3-induced Ca2+ release. Two carbohydrate-based analogues, thyl) -alpha-D-glucopyranoside-2',3,4-trisphosphate and lpha,alpha'-trehalos e-3,4,3',4'-tetrakisphosphate, could induce release of Ca2+ and displa ce [H-3]Ins(1,4,5)P-3 from its binding site on rat cerebellar membrane s, although both were less potent than Ins(1,4,5)P-3. In common with a denophostin 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 di saccharide at the Ins(1,4,5)P-3 receptor and that the Ins(1,4,5)P-3 re ceptor is capable of accommodating an increased steric bulk. The minim al importance of the 2-hydroxyl group of Ins(1,4,5)P-3 (occupied by th e pyranoside oxygen in adenophostin) was confirmed by comparing the ac tivity of DL-scyllo-Ins(1,2,4)P-3 [which differs from Ins(1,4,5)P-3 so lely by the orientation of this hydroxyl group] with that of Ins(1,4,5 )P-3. An analogue of this compound, namely, DL-6-CH2OH-scyllo-Ins(1,2, 4)P-3, which possesses an equatorial hydroxymethyl group analogous to the 5'-hydroxymethyl group of adenophostin, was found to be equipotent to Ins(1,4,5)P-3, demonstrating the tolerance of the Ins(1,4,5)P-3 re ceptor to additional steric bulk at this position.