Effect of acceptor membrane phosphatidylcholine on the catalytic activity of bovine liver phosphatidylcholine transfer protein.
Protein-mediated transfer of phosphatidylcholine (PC) by bovine liver phosphatidylcholine transfer protein (PC-TP) was examined using a vesicle-vesicle assay system. Donor and acceptor membranes were prepared from Escherichia coli phospholipids and limiting amounts of egg yolk PC. PC transfer between vesicles of E. coli lipid/egg PC was markedly higher than transfer of PC from vesicles of E. coli lipid/egg PC to vesicles of E. coli lipid. Kinetic parameters of the interaction between PC-TP and E. coli lipid vesicles with or without PC was investigated. The apparent dissociation constants of the complex formed between PC-TP and these vesicles were determined kinetically and from double-reciprocal plots of intrinsic PC-TP fluorescence intensity increase versus vesicle concentration. The magnitude of the dissociation constant decreased as the PC content of the vesicles increased from 0 to 5 mol%. In addition, kinetic analysis revealed that the presence of PC in acceptor vesicles increased both the association and dissociation of PC-TP from vesicles. The effect of membrane PC molecules on transfer rates was examined using bis-phosphatidylcholine, a dimeric PC molecule which is not transferred by PC-TP. Rates of PC transfer to acceptor vesicles comprised of E. coli lipid/bis-PC were virtually identical to rates observed with acceptors vesicles prepared from E. coli lipid. The results suggest that transfer of PC by PC-TP is enhanced only when insertion of protein-bound PC occurs concurrently with the extraction of a molecule of membrane PC, i.e., a concerted, one-step catalytic mechanism for phospholipid exchange.
Runquist
E A
EA
Department of Biochemistry, University of Kansas Medical Center, Kansas City 66103.
Helmkamp
G M
GM
D000727:Androgen-Binding Protein; D000818:Animals; D002352:Carrier Proteins / Q000378:metabolism*; D002417:Cattle; D004926:Escherichia coli / Q000378:metabolism; D007202:Indicators and Reagents; D007700:Kinetics; D008099:Liver / Q000378:metabolism*; D008563:Membrane Lipids / Q000378:metabolism*; D010713:Phosphatidylcholines / Q000138:chemical synthesis / Q000378:metabolism*; D049294:Phospholipid Transfer Proteins
D000727:Androgen-Binding Protein; D002352:Carrier Proteins; D007202:Indicators and Reagents; D008563:Membrane Lipids; D010713:Phosphatidylcholines; D049294:Phospholipid Transfer Proteins