The kinetic mechanism of cation-catalyzed phosphatidylglycerol transbilayer migration implies close contact between vesicles as an intermediate state.
We have investigated variations in the rate of Mn2+-catalyzed phosphatidylglycerol transbilayer migration [Lentz, Madden, & Alford (1982) Biochemistry 21, 6799] with changes in phospholipid and cation concentration over more than a 100-fold range of both parameters. The slope of a double logarithmic plot of the rate of transbilayer lipid migration versus lipid concentration was 1.7, suggesting that lipid redistribution was dependent on vesicle aggregation or collision. A model involving transitory dimerization of vesicles was able to account for the concentration dependence of the transbilayer redistribution rate. The observed variation in rate with the logarithm of Mn2+ concentration was complex: linear above 0.4 microM (corresponding to roughly 2.5 Mn2+ per vesicle) but involving a steeper dependence on Mn2+ below 0.04 microM (roughly four vesicles per Mn2+). The rate of transbilayer redistribution increased substantially between 37 and 56 degrees C, yielding a nonlinear Arrhenius plot. There was no evidence of either fusion or lipid exchange between vesicles at the low concentrations of Mn2+ needed for transbilayer redistribution. The data are consistent with a model suggesting transitory "micro-domains" of a dehydrated, interbilayer complex as involved in the transition state and are inconsistent with a model involving an inverted micelle-type structure for the transition state.
Lentz
B R
BR
Department of Biochemistry, University of North Carolina, Chapel Hill 27599-7260.
Whitt
N A
NA
Alford
D R
DR
Burgess
S W
SW
Yates
J C
JC
Nir
S
S
D001692:Biological Transport / Q000187:drug effects; D002384:Catalysis; D002449:Cell Aggregation / Q000187:drug effects; D007700:Kinetics; D008051:Lipid Bilayers / Q000378:metabolism*; D008345:Manganese / Q000494:pharmacology; D008433:Mathematics; D008561:Membrane Fusion / Q000187:drug effects*; D008954:Models, Biological; D010713:Phosphatidylcholines / Q000378:metabolism; D010715:Phosphatidylglycerols / Q000378:metabolism*; D013696:Temperature
D008051:Lipid Bilayers; D010713:Phosphatidylcholines; D010715:Phosphatidylglycerols; D008345:Manganese