The effects of lipid structure on the kinetics of spontaneous transfer of a series of phosphatidylcholines have been determined. Donors, which were model-reassembled high-density lipoproteins composed of apo A-I, 1-palmitoyl-2-oleoylphosphatidylcholine, and a trace of a radiolabeled lipid, were mixed with acceptors, which were human low-density lipoproteins. Within a series of phosphatidylcholines, the addition of double bonds and methylene units, respectively, increased and decreased the rate of transfer in a predictable way. An equation that predicts the rates of transfer of a large number of diacylglycerides and phosphoglycerides from any lipoprotein has been empirically derived from these data. The transfers of phosphatidylcholines that contain superpolyunsaturated fatty acids (four or more double bonds) do not obey the derived equation, probably due to limitations on the number of conformational degrees of freedom in these lipids. The range of measured transfer halftimes extends from less than 2 h to more than 12 days. Thus, the spontaneous transfer halftimes of some (but not all) lipids are short compared to the lifetime of lipoproteins in plasma. These results suggest that some lipids transfer among lipoproteins and cells via a spontaneous mechanism while others require specific transfer factors or hydrolysis to achieve this within a physiologically significant time frame.
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