EFFECTS OF LANTHANIDE IONS ON THE BINDING ABILITY OF ANNEXIN-V TO PHOSPHOLIPID VESICLE

The effects of lanthanide ions (Ln(3+)), La3+, Ce3+, Gd3+, and Tb3+, o n the binding ability of annexin V to phospholipid vesicle were studie d, and compared with that of Ca2+. The energy transfer following excit ation at 278 nm was accompanied by a small decrease (about 3%) in the fluorescence signal (310-315 nm) of the aromatic amino acid residues, followed by strong re-emission of annexin V-bound Tb3+ at 547 nm. The results of the titration of Tb3+-protein interactions using the rate d ialysis method suggest that there are two high affinity metal binding sites on annexin V, to which Tb3+ binds with an average dissociation c onstant, Kd, of about 2.11 x 10(-7) M. The Ln(3+) seemed to assist the anticoagulant action of annexin V because it binds more strongly to p hospholipid than Ca2+ does. Annexin V had the same ability to bind pho spholipids in 10 mu M Ln(3+) as it did in 1 mM Ca2+, showing that it b inds to anionic phospholipids with Ln(3+) more strongly than it does w ith Ca2+.