Kinetic model of the protein-mediated phosphatidylcholine exchange between single bilayer liposomes

Abstract

The phosphatidylcholine exchange protein from beef liver catalyzes the exchange of phosphatidylcholine between single bilayer liposomes (Hellings et al. (1974), Eur. J. Biochem. 47, 601). A model has been proposed which describes the kinetics of this exchange. Steady-state equations have been derived from the model and have been used for the derivation of the theoretical rate equation. Computer analysis shows a good fit with the experimental results. It follows from the analysis that the apparent dissociation constant of the exchange protein-liposome complex decreases with an increasing phosphatidic acid content of the liposomes. This suggests that in this model system it is the phospholipid composition of the membranes involved that regulates the amount of exchange protein available to function as a carrier of phosphatidylcholine.

The Protein‐Mediated Transfer of Phosphatidylcholine between Membranes

“…Previously in studying the transfer of phosphatidylcholine between single bilayer liposomes, it was demonstrated that the transfer decreased with an increase of negatively charged phosphatidic acid in the liposomes [2,3]. An analysis of the kinetic data indicated that the apparent dissociation constant of the exchange protein-liposome complex decreased in a parallel fashion resulting in an inhibition of transfer [3]. In the present study it has been confirmed that the incorporation of phosphatidic acid into phosphatidylcholine liposomes inhibits transfer (Fig.…”

“…It is presumably by this mechanism that in the present study the protein transfers ['4C]phosphatidylcholine from the 14C-labelled mitochondria to the liposomes. Previously in studying the transfer of phosphatidylcholine between single bilayer liposomes, it was demonstrated that the transfer decreased with an increase of negatively charged phosphatidic acid in the liposomes [2,3]. An analysis of the kinetic data indicated that the apparent dissociation constant of the exchange protein-liposome complex decreased in a parallel fashion resulting in an inhibition of transfer [3].…”

“…The protein-mediated transfer of phosphatidylcholine between mitochondria and liposomes will be a function of the phosphatidylcholine pools available in these two membrane particles [3]. Recently evidence was provided that only phosphatidylcholine present in the outer liposomal monolayer participated in the transfer process [26].…”

“…Recently it was demonstrated that the protein-mediated transfer of phosphatidylcholine between phosphatidylcholine liposomes was inhibited by incorporation of phosphatidic acid or phosphatidylinositol into these liposomes [2]. Steady state analysis of the kinetic data indicated that the apparent dissociation constant of the exchange protein-liposome complex decreased with an increasing phosphatidic acid content of the liposomes [3]. This was interpreted to mean that an increase of the negative surface charge facilitated the interaction of the exchange protein with the membrane interface resulting in less protein free in the medium to function as a carrier i.e.…”

The Primary Structure of the Phosphatidylcholine‐Exchange Protein from Bovine Liver

“…-Val-Val-Tyr-Trp-Gln-Val-(positions 98 -104) and -Tyr-Val-Tyr-Val-(positions 114-117). At present, studies are in progress to establish how the endogenous binding site relates to the membrane interface when the protein forms a 'collision' complex with that interface [14].…”