Competitive binding of bismuth to transferrin and albumin in aqueous solution and in blood plasma

Several bismuth compounds are currently used as antiulcer drugs, but their mechanism of action is not well established. Proteins are thought to be tar get sites. In this work we establish that the competitive binding of Bi3+ t o the blood serum proteins albumin and transferrin, as isolated proteins an d in blood plasma, can be monitored via observation of H-1 and C-13 NMR res onances of isotopically labeled [epsilon-C-13]Met transferrin. We show that Met(132) in the I132M recombinant N-lobe transferrin mutant is a sensitive indicator of N-lobe metal binding. Bi3+ binds to the specific Fe3+ sites o f transferrin and the observed shifts of Met resonances suggest that Bi3+ i nduces similar conformational changes in the N-lobe of transferrin in aqueo us solution and plasma. Bi3+ binding to albumin is nonspecific and Cys(34) is not a major binding site, which is surprising because Bi3+ has a high af finity for thiolate sulfur. This illustrates that the potential target site s for metals (in this case Bi3+) in proteins depend not only on their prese nce but also on their accessibility. Bi3+ binds to transferrin in preferenc e to albumin both in aqueous solution and in blood plasma.