Phosphatidylinositol distribution and translocation in sonicated vesicles. A study with exchange protein and phospholipase C.

The distribution of phosphatidylinositol and phosphatidylcholine in sonicated phospholipid vesicles (phosphatidylcholine : diphosphatidylglycerol : phosphatidylinositol, 90 : 5 : 5 mol%) has been determined by the use of exchange protein from beef heart and phosphatidylinositol-specific phospholipase C from Staphylococcus aureus. Approximately 70% of the phosphatidylinositol in the sonicated vesicles was accessible to the exchange protein and 70--75% was accessible to the phospholipase C. A similar proportion (65%) of the phosphatidylcholine was accessible to the exchange protein suggesting that phosphatidylinositol was not preferentially located in either surface of the phospholipid bilayer. The rate of translocation of both phospholipids was very slow but the rate for phosphatidylcholine (t 1/2 = 4--7 days) appeared to be greater than that for phosphatidylinositol (t 1/2 = 8--60 days). Production of asymmetric vesicles by removing phosphatidylinositol from the outer surface with either exchange protein or phospholipase C did not induce rapid phospholipid translocation.