Metabolism of Phosphatidylinositol in Reticulocytes — Isoproterenol Affects the Turnover of Polyphosphoinositides
Pulse-chase experiments with rabbit reticulocytes and erythrocytes prelabelled with inorganic 32P-phosphate revealed a high turnover of the phosphomonoester groups of polyphosphoinositides in reticulocytes which declines strongly during maturation. Reticulocytes incorporated H-inositol into phosphatidylinositol (PI), phosphatidylinositol-4-phosphate (PIP) and phosphatidylinositol-4, 5-bisphosphate (PIP2) in the proportion of 17: 1: 1.8, respectively. A slow release of 3H-labelled inositol phosphates was found in reticulocytes, indicating a very low phosphodiesterase activity (Phospholipase C) in the intact cells. Determination of the 32P/3H-ratio in double-labelled phosphatidylinositides in reticulocytes demonstrated an about 3000-fold turnover of the phosphodiester groups as compared with that of the phosphgdiester group. (-)-isoproterenol stimulates the incorporation of 32P-phosphate into the polyphosphoinositides in reticulocytes but did not influence the release of 3H-inositol phosphates. A rapid response to (-)-iso-proterenol (0.1 uM) with an increase of up to 60% was found in the 32P-labelling of PIP2. The effect was antagonized by alprenolol. The stimulation of the polyphosphoinositide phosphate turnover might be mediated by an elevated concentration of cyclic AMP in reticulocytes.
KeywordsInositol Phosphate Specific Radioactivity Inositol Trisphosphate Mature Erythrocyte Inositol Ring
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