Effect of divalent cations on phopholipid spontaneous and catalyzed exchange. A spin-lanel study of the rat liver non-specific exchange protein activity

By using the electron paramagnetic resonance assay of spin-labeled phosphatidylcholine exchange between three different types of donor vesicles and acceptor mitochondria, a progressive inhibition of the activity of the rat liver non-specific phospholipid exchange protein is observed upon increasing concentration of Mg and Ca ions. This observation indicates a possible membrane surfacial negative charge involvement in the process. A similar behaviur is observed also for the spontaneous spin- labeled lecithin exchange. The latter seem to be related to the catalyzed exchange, since acceptor mitochondria subjected to extensive spontaneous exchange reached an equilibrium phospholipid composition that was no longer affected by the subsequent protein action.