PHOSPHOLIPID-BINDING AND TRANSFER BY THE NONSPECIFIC LIPID-TRANSFER PROTEIN (STEROL CARRIER PROTEIN-2) - A KINETIC-MODEL

The nonspecific lipid-transfer protein (nsL-TP) from bovine liver was studied by measuring the binding and transfer of the fluorescent phosp holipid 1-palmitoyl-2-[6-(1-pyrenyl)-hexanoyl]-sn- glycero-3-phosphoch oline (PamPyrGroPCho). A kinetic model is presented involving three st eps: (a) interaction of nsL-TP with a membrane surface; (b) equilibrat ion of PamPyrGroPCho monomers between the membrane and nsL-TP; and (c) dissociation of the nsL-TP/PamPyrGroPCho complex from the membrane su rface. Steady-state analysis of the model yielded theoretical equation s describing both binding and transfer kinetics. Computer analysis, us ing these equations, showed good fits with the experimental results an d several kinetic constants could be calculated. From these constants it was inferred that incorporation of acidic phospholipids into vesicl es enhanced the interaction of nsL-TP with the membrane interface (ste p a), without affecting the equilibrium binding of phospholipid monome rs to nsL-TP (step b). As a result, the rate of nsL-TP-mediated PamPyr GroPCho transfer from donor to acceptor vesicles was greatly affected. Under the conditions of incubation, incorporation of the acidic lipid s in the donor membrane vesicles stimulated transfer, whereas incorpor ation of these lipids in the acceptor membranes could lead to a virtua lly complete inhibition of transfer. From the results it is concluded that the formation of a soluble lipid-nsL-TP complex is the key step i n nsL-TP-mediated phospholipid transfer.