The metabolism of inositol 1,4,5-trisphosphate [Ins(1,4,5)P] was examined in yeast cells and germ tubes of Methods have been developed for analysis of the two key metabolic enzymes, Ins(1,4,5)P kinase and phosphatase. ATP-dependent Ins(1,4,5)P kinase activity was detected predominantly in the soluble fraction of cell extracts and exhibited a of approximately 9 μM. The apparent of Ins(1,4,5)P phosphatase for Ins(1,4,5)P was approximately 480 μM. The slow rate of dephosphorylation of Ins(1,4,5)P to inositol bisphosphate suggests a lower importance of the phosphatase within cells compared to the kinase. Since both yeast cells and germ tubes of rapidly phosphorylated Ins(1,4,5)P to inositol tetrakisphosphate and inositol penta/hexakisphosphate, it is suggested that Ins(1,4,5)P has an important role as a precursor for production of these compounds. A sustained increase in cellular Ins(1,4,5)P levels was observed during germ tube formation and, prior to the onset of germination between 1 and 2 h incubation, the Ins(1,4,5)P content increased up to eightfold. Transien increases in the level of Ins(1,4,5)P were also observed during yeast-like growth of The possible role and relative importance of Ins(1,4,5)P as a precursor for inositol polyphosphates and in signal transduction involving Ca release from internal stores is discussed.


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