The binding of human serum transferrin to its specific receptor reconstituted into liposomes

Human placental transferrin receptor (HPTR), purified following a procedure based on affinity chromatography step, was reconstituted by the detergent dialysis method into various kinds of phosphatidylcholine vesicles and the receptor ability to bind 125I-labelled human serum transferrin (HST) was then evaluated. In our experimental conditions, the binding of the labelled protein to its specific receptor showed several features, in particular: (1) in cholesterol/1-alpha-dipalmitoylphosphatidyl choline (CHO/DPPC) liposomes, a positive cooperatively of the transferrin binding resulted at the lowest cholesterol/phospholipids (C/P) ratio; 1-alpha-dioleylphosphatidyl choline (DOPC) and phosphatidic acid (PA) containing liposomes showed an opposite binding curve trend; (2) the apparent dissociation constant (K'd) did not change significantly as a function of the lipid composition, being always around 1.00 x 10(-6) M; (3) the encapsulation capacity of liposomes decreased from 27% to about 13% with increasing amounts of cholesterol and was around 20% in the presence of DOPC or PA; about 8-13% of this receptor was found to be functional; (4) receptor-loaded liposomes treated with polyclonal anti-HPTR rabbit antibodies showed a remarkable binding decrease for transferin. All these results seem to point out the crucial role played by the environment in the binding behaviour of the transferrin receptor.