Comparison of DPPC and DPPG environments in pulmonary surfactant models
Michael Robert Morrow 1*, Sara Temple 1, June Stewart 1 and Kevin M. W. Keough 2
1 Memorial University of Newfoundland
2 Alberta Heritage Foundation for Medical Research
* To whom correspondence should be addressed. E-mail: myke{at}physics.mun.ca.
Submitted on December 6, 2006
Revised on December 28, 2006
Accepted on 14 March 2007
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Abstract |
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Deuterium nuclear magnetic resonance was used to monitor lipid acyl chain orientational order in suspensions of dipalmitoylphosphatidylcholine (DPPC) and dipalmitoylphosphatidylglycerol (DPPG) containing Ca2+ and the lung surfactant proteins SP-A and SP-B separately and together. In order to distinguish between protein-lipid interactions involving the PC and PG lipid head groups and to examine whether such interactions might influence spatial distribution of lipids within the bilayer, acyl chains on either the DPPC or the DPPG component of the mixture were deuterated. The lipid components of the resulting mixtures were thus either DPPC-d62/DPPG (7:3) or DPPC/DPPG-d62 (7:3) respectively. SP-A had little effect on DPPC-d62 chain order but did narrow the temperature range over which DPPG-d62 ordered at the liquid-crystal to gel transition. No segregation of lipid components was seen for temperatures above or below the transition. Near the transition, though, there was evidence that SP-A promoted preferential depletion of DPPG from liquid crystalline domains in the temperature range over which gel and liquid crystal domains coexist. SP-B lowered average chain order of both lipids both above and below the main transition. The perturbations of chain order by SP-A and SP-B together were smaller than by SP-B alone. This reduction in perturbation of the lipids by the additional presence of SP-A likely indicated a strong interaction between SP-A and SP-B. The competitive lipid-lipid, lipid-protein, and protein-protein interactions suggested by these observations presumably facilitate the reorganization of surfactant material inherent in the transformation from lamellar bodies to a functional surfactant layer.
Key Words:
deuterium NMR, lipid mixture, lipid-protein interaction, lung surfactant, model membrane, phase transition
