We have studied the interaction of liposomes of dipalmitoyl- and dimyristoylphosphatidylcholine (DPPC and DMPC, respectively) with apolipoprotein A-I (apoA-I) from human plasma high density lipoproteins (HDL) by chromatography on Sepharose 4B and kinetic turbidimetric methods. The incubation and chromatography of apoA-I and highly turbid DMPC mixtures at the phospholipid gel→liquid crystalline transition temperature, Tc, revealed the complete incorporation of DMPC into a lipid-protein complex which scattered very little light. Similar experiments conducted above (30°C) and below (22°C) Tc showed much less complex formation even when incubated for much longer times. Over similar time periods DPPC failed to associate completely with apoA-I even at its Tc (41.5°C). We have determined the rates of association of DPPC and DMPC with apoA-I as a function of temperature by measuring the rate of disappearance of liposomal turbidity. Below and above the Tc of DMPC, the rate of its association with apoA-I was slow, but increased by a factor of 500 to 1000 at Tc. A similar set of experiments substituting DPPC for DMPC showed very slow reaction rates even at the Tc of the former lipid. The relatively high rate of interaction of apoA-I and DMPC at Tc was assigned to the increased permeability of the DMPC matrix produced by a high percentage of boundary lipid which we viewed as a lattice defect between coexisting gel and liquid crystalline phases. The slow decrease in rate between Tc and Tc + 15°C was assigned to the retention lattice defects composed of ordered and disordered populations of DMPC. The absence of a fast reaction between DPPC and apoA-I was probably due to its lower permeability even at its Tc. These results were suggested to be important in predicting the rate of transfer of apoA-I from HDL to phospholipids and may be important in regulating the activity of the enzyme, lecithin:cholesterol acyltransferase.
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