Biosynthesis and localization of phosphatidyl‐scyllo‐inositol in barley aleurone cells

A novel isomer of phosphatidylinositol (PI), phosphatidyl‐scyllo‐inositol, was characterized in the aleurone cells of barley seeds. In this investigation, the subcellular localization of scyllo‐PI and the relative rates of biosynthesis and accumulation of [32P]phosphoric acid ([32Pi])‐labeled scyllo‐ and myo‐phosphoinositides in the plasma membrane and intracellular membrane pools were investigated. About 25% of the [32Pi]‐labeled phospholipids were present in plasma membrane and 75% in intracellular membranes. Incorporation of [32Pi] into scyllo‐PI was greater than into myo‐PI in both the plasma membranes and intracellular membranes at all time points investigated, thus suggesting a higher rate of biosynthesis; however, the data do not preclude reduced breakdown of labeled scyllo‐PI as a contributing factor. In vitro studies were conducted to investigate the presence of cytidinediphosphate diacylglycerol (CDP‐DG):scyllo‐inositol 3‐phosphatidyltransferase (scyllo‐PI synthase) and to optimize enzymatic activity. The inclusion of nonionic detergents (Brij 58 and Triton X‐100) effected significant enhancement in the biosynthesis of scyllo‐PI, whereas anionic, cationic, and zwitterionic detergents had little or no effect. This is the first evidence for CDP‐DG:scyllo‐inositol 3‐phosphatidyltransferase activity.