N-lobe versus C-lobe complexation of bismuth by human transferrin

Interactions of recombinant N-lobe of human serum transferrin (hTF/2N) with Bi3+, a metal ion widely used in medicine, have been investigated by both UV and NMR spectroscopy. The bicarbonate-independent stability constant for Bi3+ binding (K*) to hTF/2N was determined to be log KS 18.9+/-0.2 in 5 mM bicarbonate/10 mM Hepes buffer at 310 K, pH 7.4. The presence of Fe3+ in t he C-lobe of intact hTF perturbed Bi3+ binding to the N-lobe, whereas bindi ng of Bi3+ to the C-lobe was unaffected by the presence of Fe3+ in the N-lo be. Reactions of Bi3+ (as bismuth nitrilotriacetate or ranitidine bismuth c itrate) with hTF/2N in solutions containing 10 mM bicarbonate induced speci fic changes to high-field H-1-NMR peaks. The H-1 co-ordination shifts induc ed by Bi3+ were similar to those induced by Fe3+ and Ga3+, suggesting that Bi3+ binding causes similar structural changes to those induced by hTF/2N. C-13-NMR data showed that carbonate binds to hTF/2N concomitantly with Bi3.