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 investigat ion, the subcellular localization of scyllo-PI and the relative rates of bi osynthesis and accumulation of [P-32]phosphoric acid ([(32)Pi])-labeled scy llo- and myo-phosphoinositides in the plasma membrane and intracellular mem brane pools were investigated. About 25% of the [(32)Pi]-labeled phospholip ids were present in plasma membrane and 75% in intracellular membranes. Inc orporation of [(32)Pi] into scyllo-PI was greater than into myo-PI in both the plasma membranes and intracellular membranes at all time points investi gated, thus suggesting a higher rate of biosynthesis; however, the data do not preclude reduced breakdown of labeled scyllo-PI as a contributing facto r. In vitro studies were conducted to investigate the presence of cytidined iphosphate diacylglycerol (CDP-DC):scyllo-inosilol 3-phosphatidyltransferas e (scyllo-PI synthase) and to optimize enzymatic activity. The inclusion of nonionic detergents (Brij 58 and Triton X-100) effected significant enhanc ement in the biosynthesis of scyllo-PI, whereas anionic, cationic, and zwit terionic detergents had little or no effect. This is the first evidence for CDP-DG:scyllo-inositol 3-phosphatidyltransferase activity.