Kinetics of the interaction of hemin liposomes with heme binding proteins

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Abstract
As a model for the transport of hemin across biological membranes, sonicated phosphatidylcholine liposomes with incorporated hemin were characterized. The interaction of the hemin liposomes with the heme binding proteins albumin, apomyoglobin and hemopexin was examined as a function of liposome charge and cholesterol content. In all cases, there was an almost complete transfer of hemin from liposome to protein; a rapid phase and a slow phase were observed for the transfer. For negatively charged liposomes (with 11% dicetyl phosphate), the rapid and slow phases showed observed rates of transfer of .apprx. 2 and 0.01/s, respectively, for all 3 proteins. The presence of cholesterol in the liposomes decreased the observed rates by a factor of 2, and positively charged liposomes (with 11% stearylamine) showed .apprx. 1/5 the observed rates of negatively charged liposomes. The observed rates were independent of protein concentration, indicating that the rate-determining step is hemin efflux from the lipid bilayer. The hemin interaction with the phospholipid bilayer is suggested to be primarily hydrophobic with some electrostatic character. The 2 phases are suggested to arise from 2 different populations of hemin within the liposomes and are interpreted as arising from 2 different orientations of hemin within the bilayer.