RESUMO
A soluble extract from pea (Pisum sativum L.) roots, when incubated with ATP and inositol 1,4,5-trisphosphate, produced an inositol tetrakisphosphate. The chromatographic properties of this inositol tetrakisphosphate, and of the products formed by its chemical degradation, identify it as inositol 1,4,5,6-tetrakisphosphate. No evidence was obtained for a 3-phosphorylation of inositol 1,4,5-trisphosphate. The importance of these observations with respect to inositol phosphates and calcium signalling in higher plants, is discussed.
RESUMO
Metabolism of the putative messenger molecule d-myo-inositol(1,4,5)trisphosphate [Ins(1,4,5)P(3)] in plant cells has been studied using a soluble fraction from pea (Pisum sativum) roots as enzyme source and [5-(32)P]Ins(1,4,5)P(3) and [2-(3)H]Ins(1,4,5)P(3) as tracers. Ins(1,4,5)P(3) was rapidly converted into both lower and higher inositol phosphates. The major dephosphorylation product was inositol(4,5)bisphosphate [Ins(4,5)P(2)] whereas inositol(1,4)bisphosphate [Ins(1,4)P(2)] 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)P(4). Dephosphorylation of Ins(1,4,5)P(3) to Ins(4,5)P(2) was dependent on Ins(1,4,5)P(3) concentration and was partially inhibited by the phosphohydrolase inhibitors 2,3-diphosphoglycerate, glucose 6-phosphate, and p-nitrophenylphosphate. Conversion of Ins(1,4,5)P(3) to Ins(4,5)P(2) and Ins(1,4,5,X)P(4) was inhibited by 55 micromolar Ca(2+). This study demonstrates that enzymes are present in plant tissues which are capable of rapidly converting Ins(1,4,5)P(3) and that pathways of inositol phosphate metabolism exist which may prove to be unique to the plant kingdom.
