Metabolism of Inositol(1,4,5)trisphosphate by a Soluble Enzyme Fraction from Pea (Pisum sativum) Roots

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Abstract
Metabolism of the putative messenger molecule d-myo-inositol(1,4,5)trisphosphate [Ins(1,4,5)P3] in plant cells has been studied using a soluble fraction from pea (Pisum sativum) roots as enzyme source and [5-32P]Ins(1,4,5)P3 and [2-3H]Ins(1,4,5)P3 as tracers. Ins(1,4,5)P3 was rapidly converted into both lower and higher inositol phosphates. The major dephosphorylation product was inositol(4,5)bisphosphate [Ins(4,5)P2] whereas inositol(1,4)bisphosphate [Ins(1,4)P2] was only present in very small quantities throughout a 15 minute incubation period. In addition to these compounds, small amounts of nine other metabolites were produced including inositol and inositol(1,4,5,X)P4. Dephosphorylation of Ins(1,4,5)P3 to Ins(4,5)P2 was dependent on Ins(1,4,5)P3 concentration and was partially inhibited by the phosphohydrolase inhibitors 2,3-diphosphoglycerate, glucose 6-phosphate, and p-nitrophenylphosphate. Conversion of Ins(1,4,5)P3 to Ins(4,5)P2 and Ins(1,4,5,X)P4 was inhibited by 55 micromolar Ca2+. This study demonstrates that enzymes are present in plant tissues which are capable of rapidly converting Ins(1,4,5)P3 and that pathways of inositol phosphate metabolism exist which may prove to be unique to the plant kingdom.