Exchange of phospholipids between microsomes and inner and outer mitochondrial membranes of rat liver

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1. 1. Sixty minutes after injection of 32P-labelled phosphate into rats, the specific activities of phosphatidylcholine and phosphatidylethanolamine were determined in mitochondria, outer and inner mitochondrial membranes, and microsomes. The phospholipid specific activities of the outer membrane were lower than those of the microsomes and 3 to 4 times higher than those of the inner membrane. This is compatible with the idea that 32P-labelled phospholipids, synthesized in the endoplasmic reticulum, exchange first with the outer mitochondrial membrane followed by an exchange between outer and inner mitochondrial membrane. 2. 2. Mitochondria were incubated with microsomes in the presence of a 105 000 × g supernatant fraction (the pH 5.1 supernatant). It was shown that both inner and outer mitochondrial membranes were involved in the exchange of phospholipids with microsomes. The phospholipids of the outer membrane exchanged to a larger extent than did those of the inner membrane. 3. 3. Adenylate kinase activity was determined in mitochondria as an index for the integrity of the outer membrane. It was found that the integrity was not seriously affected by incubating mitochondria with or without microsomes and supernatant. Electron micrographs of the same mitochondria supported this conclusion. This endorses the view that in vitro phospholipids exchange between inner membrane and microsomes via the outer membrane. 4. 4. Mitochondria isolated 60 min after injection of 32P-labelled phosphate were incubated at 37° in 0.25 M sucrose—1 mM EDTA (pH 7.8). It was demonstrated that the 3- to 6-fold difference between the specific activities of outer and inner membrane phospholipids was partially eliminated. It is concluded that phospholipids exchange between inner and outer membrane inside the mitochondrion. 5. 5. In the 105 000 × g supernatant fraction, a protein is present which stimulates the exchange of phospholipids between microsomes and mitochondria. Such a stimulatory protein appears to be lacking in the mitochondrion. Differences in phospholipid exchange between microsomes and outer mitochondrial membrane as compared to phospholipid exchange between outer and inner mitochondrial membrane are discussed with respect to this observation.