THE BINDING OF HUMAN SERUM TRANSFERRIN TO ITS SPECIFIC RECEPTOR RECONSTITUTED INTO LIPOSOMES

Human placental transferrin receptor (HPTR), purified following a proc edure based on affinity chromatography step, was reconstituted by the detergent dialysis method into various kinds of phosphatidylcholine ve sicles and the receptor ability to bind I-125-labelled human serum tra nsferrin (HST) was then evaluated. In our experimental conditions, the binding of the labelled protein to its specific receptor showed sever al features, in particular. (1) in cholesterol/1-alpha-dipalmitoylphos phatidyl choline (CHO/DPPC) liposomes, a positive cooperativity of the transferrin binding resulted at the lowest cholesterol/phospholipids (C/P) ratio; 1-alpha-dioleylphosphatidyl choline (DOPC) and phosphatid ic acid (PA) containing liposomes showed an opposite binding curve tre nd; (2) the apparent dissociation constant (K'(d)) did not change sign ificantly 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 wit h polyclonal anti-HPTR rabbit antibodies showed a remarkable binding d ecrease for transferrin. All these results seem to point out the cruci al role played by the environment in the binding behaviour of the tran sferrin receptor.