Effect of N-ethylmaleimide on rat liver phosphatidylcholine-specific and non-specific transfer protein activities: its dependence on donor liposome composition.

Phosphatidylcholine exchange between liposomes and mitochondria catalyzed by rat liver phosphatidylcholine transfer protein is strongly stimulated by N-ethylmaleimide (NEM) when PC/PI (molar ratio, 4:1) donor liposomes are used. In the presence of PC/PE or PC liposomes the exchange activity by this protein is unaffected. In the same experimental conditions, the activity of rat liver non-specific transfer protein is always stimulated by N-ethylmaleimide with all the types of liposomes tested in the order PC/PI greater than PC/PE greater than PC. Since the effect of NEM depends on the type of liposomes used and appears to be similar for both phospholipid transfer proteins, the possibility that their mode of action implies the formation of a ternary complex should be considered. As far as non-specific transfer protein is concerned, its interaction could vary depending on the nature of the exchanging membranes. Data are also presented indicating that when the two transfer proteins are together their activity is additive, therefore suggesting a specific role in phospholipid biomembrane assembly for each of them.