INOSITOL TRISPHOSPHATE METABOLISM IN SUBCELLULAR-FRACTIONS OF BARLEY (HORDEUM-VULGARE L) MESOPHYLL-CELLS

Phosphatases in cytosolic fractions, vacuoles, and vacuolar membranes from barley (Hordeum vulgare L.) leaves were found to dephosphorylate inositol 1,4,5-trisphosphate (IP3). 1,4-inositol bis-phosphate (1,4-IP 2) is the main product of IP3 dephosphorylation by the cytosolic fract ion. The activity was strictly Mg2+ dependent. In contrast, IP3 dephos phorylation activity of both the soluble vacuolar and the tonoplast fr actions was inhibited up to 50% by Mg2+. When vacuolar membranes were incubated with IP3, 1,4-IP2 was produced only under neutral and slight ly alkaline conditions. Under acidic conditions, however, dephosphoryl ation yielded putative 4,5-inositol bisphosphate. Li+ (20 mm) and Ca2 (100 muM) strongly inhibited activity in the soluble vacuolar fractio n but had only a slight effect on the activities of the cytosolic and tonoplast fractions.