Proteins and the intracellular exchange of lipids. I. Stimulation of phospholipid exchange between mitochondria and microsomal fractions by proteins isolated from potato tuber.

1. An active fraction which stimulates the exchange of phospholipids between microsomal fractions and mitochondria was isolated by gel filtration from potato tuber homogenates. 2. This fraction is probably a protein since the stimulatory factor is nondialyzable and temperature-sensitive. The molecular weight of the active fraction is approximatively 22 000. 3. The addition of the stimulatory fraction increases essentially the exchange of the main phospholipids of the potato tuber organelles; phosphatidyl-inositol, phosphatidylcholine and phosphatidylethanolamine. 4. Inhibiting factors, probably lipid-degrading enzymes, are present in the total supernatant fraction and are responsible for the inhibiting effect on the exchange of lipids when the crude supernatant fraction is used. The "inhibition effect" is rather a degradation of the exchangeable phospholipid pools.