The spatial structure of lipids in human leukocytes: studies by nonradiative energy transfer

The spatial structure of lipids in living human lymphocytes and granulocytes has been studied using the energy transfer between lipophilic fluorescent probes. One of the probes, an energy donor (DMC), was localized in the lipid interior, whereas another donor (K-68) and an energy acceptor (DSP-12) were near the lipid/water interface. The energy transfer in lymphocytes was the same as in artificial lipid membranes (liposomes). Obviously, in lymphocytes as in liposomes, both donors are localized near the lipid surface (the distance from the donors to the lipid surface is less than Forster's radius R0, i.e., 3.4-5 nm). On the contrary, in granulocytes, the energy transfer from K-68 was 2.2 times more efficient than from the lipid-immersed DMC. It was suggested that a fraction of DMC molecules was immersed into some lipid particles, and the distance from the molecules to the surface of the particles was greater than R0. The positions of the DMC fluorescence spectrum maxima in lymphocytes and in liposomes were the same, but in granulocytes the spectrum was blue-shifted (as in the case of lipoproteins). After subcellular fractionation the DMC fluorescence intensity correlated only with phospholipid concentration in different fractions but not with protein or nucleic acid concentrations. It was suggested that lipid organelles are the main source of the DMC fluorescence. The studies of cell-lipoprotein model mixtures support the suggestion that lipids in lymphocytes are mainly present as lamellar structures (membranes); the presence of lipoprotein-like particles of rather small radius can not be excluded either. On the other hand, in addition to membrane lipids, granulocytes have lipid-containing particles similar to large serum very low density lipoproteins.