Binding of fluorescent-labeled phosphatidylcholine to rat liver nonspecific lipid transfer protein.

The fluorescent-labeled phospholipid, 1-palmitoyl-2-[12-[(7-nitro-2,1,3-benzoxadiazol-4-yl)amino]dodecan oyl] phosphatidylcholine (P-C12-NBD-PC), is highly self-quenched when contained as a high mole percent in phospholipid vesicles. The addition of nonspecific lipid transfer protein (nsLTP), which is identical to sterol carrier protein2, to P-C12-NBD-PC self-quenching vesicles results in a large increase in the fluorescence yield. Evidence is presented that demonstrates that this fluorescence increase results from the binding of P-C12-NBD-PC to one or more low dielectric sites on the nsLTP molecule and that the P-C12-NBD-PC.nsLTP complex can exist free in aqueous solution. The binding is inhibited in the presence of mersalyl, a thiol reagent, and can be reversed with excess cysteine. Transfer of the nsLTP-bound P-C12-NBD-PC to phospholipid vesicles is also demonstrated. The half-times for loading and unloading the P-C12-NBD-PC.nsLTP complex are both 3 orders of magnitude faster than the spontaneous P-C12-NBD-PC transfer between vesicles. These data demonstrate that nsLTP has the necessary properties to bind and carry lipids between membranes.