Metabolism of phosphatidylinositol in the frog retina

The synthesis and the turnover of phosphatidylinositol in frog retinal rod outer segments and microsomes were studied by following the time course of incorporation into lipids of the following radioactive precursors: [3H]glycerol, 33PO4, and [3H]inositol. 1. Although all precursors were incorporated into lipid, glycerol was the only true pulse of radioactive substrate because the precursor pools of phosphate and inositol in the retina have a slow rate of turnover. 2. A precursor-product relationship exists between retinal microsomes and rod outer segments for phosphatidylinositol synthesized from glycerol. 3. The specific activity in the rod outer segment phosphatidylinositol derived from labeled glycerol was ten times that of the other glycerolipids. Since the labeled precursor for each phospholipid class is derived from a common pool of glycerol 3-phosphate, the synthesis rate of phosphatidylinositol in the retina is much greater than that of the other phospholipids. 4. Two pools of phosphatidylinositol were identified in the rod outer segments; one turned over with a t1/2 of about 3.5 days, while the other turned over at the same rate as the other phospholipids labeled with glycerol. 5. Turnover of phosphatidylinositol in the rod outer segments after glycerol injection was followed by an increase in specific radioactivity in 1,2-diacylglycerols, consistent with the latter being a lipolytic product of phosphatidylinositol in these membranes. 6. The present studies demonstrate a unique metabolism of phosphatidylinositol in the rod outer segments compared to the other phospholipids, and it is suggested that the rapid turnover of this phospholipid may be related to membrane fusion events associated with the assembly and/or turnover of rod outer segment membranes.