Identification of Amino Acids Involve in Indium Binding To Serum Human Apo-Transferrin

Indium is a heavy metal belonging to group IIIa. It is used as a radioimaging and chemotherapeutic agent in diagnosis and also in the treatment of cancers. It is believed that indium may interfere with iron metabolism and reduce cell growth in cancer tissue. The present report was established to study the binding of iron and indium to apo-transferrin (apo-tf) and to identify amino acids involved in this process. The pure human transferrin was used and iron and indium, as citrate complexes (1:20), were added to apo-tf. The binding constant was calculated using spectrofluorometric titration technique. Maximum wavelengths for excitation and emission of apo-tf were 300 and 335 nm, respectively. When apo-tf was complexed with iron, the emission was decreased 69%; whereas, the binding of indium to apo-tf increased the emission 29%. The approximate binding constant for iron-transferrin complexes were 1  108 M-1 and 0.11  108 M-1 , respectively. The pKa's of aspartate, histidine, tyrosine, lysine and arginin were identified. The data indicated that the indium competes with iron in binding to apo-tf. Although, the binding sites for these two ions seem to be similar, the binding of iron to apo-tf is approximately 9 times more tightly than indium.