On the mechanism of spontaneous transfer of lipids from isolated microsomal to mitochondrial membranes.

Spontaneous (protein independent) transfer of endogenously biosynthesized radioactive lipids (phosphatidic acid, phosphatidylcholine, and diglycerides) from isolated guinea pig liver microsomal to unlabelled mitochondrial membranes was studied as a function of addition of ATP, duration of incubation, and protein concentration of microsomal and mitochondrial membranes. It was found that transfer of phosphatidic acid, phosphatidylcholine, and diglycerides was not absolutely dependent on addition of ATP, although its presence enhanced the transfer of phosphatidylcholine. The highest amount of transferred biosynthesized phosphatidic acid, phosphatidylcholine, and diglycerides was observed after a relatively short incubation and amounted to around one-third of all biosynthesized microsomal lipids, but decreased with increasing incubation time. With increasing concentrations of proteins of mitochondria, a significant decrease of transfer of radioactive lipids from microsomal membranes was established. The same tendency, although to a lesser degree, was established with increasing concentrations of mitochondrial proteins. These results, together with previously published experiments from this laboratory, were taken as the evidence for the transfer of membrane-bound biosynthesized phosphatidic acid, phosphatidylcholine, and diglycerides from isolated guinea pig liver microsomal to mitochondrial membranes, by a mechanism which involves a close contact between the membrane-donor and the membrane-acceptor. Mechanisms for lipid transfer in biological membranes and in model membranes (liposomes) were compared and discussed.