The Protein‐Mediated Transfer of Phosphatidylcholine between Membranes

Abstract

The phosphatidylcholine exchange protein from bovine liver catalyzes the transfer of phosphatidylcholine between rat liver mitochondria and sonicated liposomes. The effect of changes in the liposomal lipid composition and ionic composition of the medium on the transfer have been determined. In addition, it has been determined how these changes affected the electrophoretic mobility i.e. the surface charge of the membrane particles involved. Transfer was inhibited by the incorporation of negatively charged phosphatidic acid, phosphatidylserine, phosphatidylglycerol and phosphatidylinositol into the phosphatidylcholine-containing vesicles ; zwitterionic phosphatidylethanolamine had much less of an inhibitory effect while positively charged stearylamine stimulated. The cation Mg2+ and, to a lesser extent, K + overcame the inhibitory effect exerted by phosphatidic acid, in that concentration range where these ions neutralized the negative surface charge most effectively. Under conditions where Mg2+ and K + affected the membrane surface charge relatively little inhibition was observed. In measuring the protein-mediated transfer between a monolayer and vesicles consisting of only phosphatidylcholine, cations inhibited the transfer in the order La3+ > M$+ 2 Ca2+ > K + = Na+. Inhibition was not related to the ionic strength, and very likely reflects an interference of these cations with an electrostatic interaction between the exchange protein and the polar head group of phosphatidylcholine.The phosphatidylcholine exchange protein from bovine liver functions as a carrier of phosphatidylcholine between membrane interfaces [l]. 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. inhibition of transfer.The former studies implied that physical chemical properties of the interface may have an effect on the activity of the exchange protein. In the present study this concept has been elaborated by correlating the protein-mediated transfer of phosphatidylcholine between rat liver mitochondria and liposomes with the surface charge of these membrane structures. Comparable studies on the relationship between surface charge and phospholipase action have indicated that the surface charge of a phospholipid interface may control the formation of the proper enzyme-phospholipid complex [4].The surface charge of a membrane is the resultant of two counteracting factors, namely, the surface charge density of the membrane and the counte...

Interaction of the Phosphatidylcholine Exchange Protein with Phospholipids

“…This indicates a local disturbance about the tryptophan residues without a major change in the polarity of the environment. In previous studies [7,8] it has been demonstrated that the exchange protein transfers phosphatidylcholine to and from the phospholipid vesicles used in this study. This implies that the titration curves illustrated in Fig.…”

The distribution of phosphatidylinositol in microsomal membranes from rat liver after biosynthesis de novo. Evidence for the existence of different pools of microsomal phosphatidylinositol by the use of phosphatidylinositol-exchange protein

“…Egg-yolk phosphatidylcholine, phosphatidic acid and phosphatidylinositol from rat liver were prepared as previously described (Wirtz et al, 1976a). Trypsin was obtained from Merck (Darmstadt, Germany), and chymotrypsin and CTP were from Boehringer (Mannheim, Germany).…”

Modification of the Phosphatidylcholine‐Transfer Protein from Bovine Liver with Butanedione and Phenyiglyoxal

“…[7-'4C]Phenylglyoxal (specific activity 1.7 x 10'l counts min-' mol-') synthesized from [14C]acetophenone by the method of Riley and Gray [24] was a generous gift of Dr E. A. M. Fleer and W. C. Puijk. Phosphatidic acid (sodium salt) was prepared from egg-yolk phosphatidylcholine (Makor, Jerusalem, Israel) with phospholipase D extracted from Savoy cabbage [15]. Sephadex G-25 was obtained from Pharmacia (Uppsala, Sweden).…”