Abstract
The effects of membrane composition on heme binding to large unilamellar vesicles were examined using 30 separate phospholipid mixtures. Although there was some variation, most lecithins with Tm values less than or equal to 20 degrees C showed overall equilibrium partition constants equal to approximately 5 x 10(5) and association and dissociation partition rate constants equal to approximately 3 x 10(6) s-1 and 7 s-1, respectively, for CO-heme binding at 30 degrees C. A sharp decrease in the association rate for CO-heme uptake was observed as the lipid vesicles changed from liquid-crystalline to the gel phase. The addition of dicetyl phosphate or dimyristoylphosphatidylglycerol, which are negatively charged at neutral pH, decreased the affinity of the vesicles for CO-heme. The association rate and equilibrium partition constants for CO-heme uptake in unsaturated lecithins were unaffected by cholesterol content at levels up to 40%/mol. The affinity of saturated dimyristoylphosphatidylcholine (DMPC) vesicles for CO-heme decreased with increasing cholesterol content at 30 degrees C. This effect appears to be related to the influence of cholesterol on the DMPC phase transition temperature (Tm) since at low temperatures (less than or equal to 20 degrees C) little CO-heme binds to vesicles composed of DMPC even in the absence of cholesterol.
Highlights
5 20 “C showed overall equilibrium partition constants equal to -5 x lo6 and association and dissociation partition rate constants equal to -3 x lo6 s-’ and 7 s-*, respectively, for CO-heme binding at 30 ‘C. A sharp decrease in the association rate for CO-heme uptake was observed as the lipid vesicles changed from liquidcrystalline to the gel phase
The affinity of saturated dimyristoylphosphatidylcholine (DMPC) vesicles for CO-heme decreased with increasing cholesterol content at 30 “C. This effect appears to be related to the influence of cholesterol on the DMPC
DCP (5:4:1) vesicles was tested by examining hemin binding to liposomes containing egg lecithin, DMPC alone, and mixtures of DMPC, cholesterol, and DCP using gel filtration chromatography
Summary
5 20 “C showed overall equilibrium partition constants equal to -5 x lo and association and dissociation partition rate constants equal to -3 x lo s-’ and 7 s-*, respectively, for CO-heme binding at 30 ‘C. A sharp decrease in the association rate for CO-heme uptake was observed as the lipid vesicles changed from liquidcrystalline to the gel phase. Charged phospholipids decreased the relative change in surface pressure They concluded that this decrease was the result of charge repulsion between the propionic acid groups of hemin and the phospholipid head groups and that these unfavorable interactions limit the amount of heme which can intercalate into the monolayer. They drew a parallel between their surface pressure results and the relative toxicity of hemin in different membrane systems
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