Transfer of phosphatidylserine from liposomes or microsomes to mitochondria. Stimulation by a cell supernatant factor

Incubation of microsomes or sonicated dispersions of microsomal lipid containing (14-C)phosphatidylserine with rat liver mitochondria results in the transfer of radioactivity to the mitochondria. Transfer is time and temperature dependent and is stimulated by a factor in the 105 000 x g supernatant from liver homogenates. The supernatant factor is soluble at pH 5.1, stable to dialysis, and is inactivated by heating and by digestion with trypsin. Radioactivity accumulates in the mitochondria largley as phosphatidylethanolamine owing the activity of phosphatidylserine decarboxylase. Transfer to mitochondria, heat treated to inactivate the decarbosylase, is also promoted by the suppernatant factor, in which case the lipid appears to be incorporated into the mitochondrial membrane as unchanged phosphatidylserine.