SPECTROSCOPIC STUDY OF THE INTERACTION OF ALUMINUM IONS WITH HUMAN TRANSFERRIN

Transferrin is the plasma protein responsible for transporting Fe3+ fr om the absorption to the utilization site. Interactions of apo- and ho lo-transferrin with Al3+ were studied by circular dichroism (CD), UV-v isible, and fluorescence spectrometry. Binding of Al3+ to both metal-i on binding sites of apo-transferrin was confirmed by fluorescence stud ies. No interaction of Al3+ with holo-transferrin was observed, indica ting that Al3+ cannot displace Fe3+ under the experimental conditions employed. An increase in tryptophan fluorescence (lambda(max) at 330 n m) by excitation at either 280 or 295 nm was observed after Al3+ inter action with apo-transferrin. There was no shift in wavelength of the f luorescence band of apo-transferrin after interaction with Al3+, but t he intensity did increase. Since excitation at 295 nm is specific for tryptophan residues, tryptophan but not tyrosine must be responsible f or the change in fluorescence intensity. Decreased fluorescence is the result of Fe3+ binding to apo-transferrin. The CD spectrum of apo-tra nsferrin was slightly affected in the far UV by Al3+ binding, but a sa lient change was noted in the near UV at similar to 288 nm where tyros ine and tryptophan absorb. It is concluded that a small conformational change in the protein was induced by Al3+ binding to apo-transferrin.