SPECIFICITY OF PHOSPHATIDYLCHOLINE EXCHANGE PROTEIN FROM BOVINE LIVER

The phosphatidylcholine exchange protein from bovine liver stimulates the specific transfer of phosphatidylcholine (PC) from rat liver microsomes to mitochondria or phospholipid vesicles. This study establishes which components of the PC molecule are essential to the specific interaction with the protein. Radiochemically labeled analogues of PC were synthesized with modifications in the polar and apolar moiety, and their transfer was measured between donor and acceptor vesicles. Relative to 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphorylcholine (egg yolk PC), transfer is inhibited or abolished when the distance between P and N is decreased or increased and a methyl group on the quaternary N is removed or substituted by an ethyl or propyl group. Transfer is much less affected when the ester bonds are replaced by ether or C-C bonds, the PC molecule contains 2 saturated fatty acids and the D stereoisomer is used. The protein probably has a binding site which interacts specifically with the phosphorylcholine head group and which cannot accommodate substantial configurational changes. Interaction with the apolar moiety of PC is less specific. However, lyso-PC is not transferred, suggesting that 2 hydrocarbon chains are required to stabilize the exchange protein-phospholipid complex. Interaction of [14C]PC-labeled exchange protein with vesicles of different phospholipid composition was analyzed by measuring the release of [14C]PC into these vesicles. Vesicles of egg PC or dimethylphosphatidylethanolamine function as acceptors, in contrast to vesicles of sphingomyelin or phosphatidylethanolamine.