The interaction of anions with native and phenylglyoxal-modified human serum transferrin

The interaction of various anions with human serum transferrin was investigated due to the concomitant binding of iron and a synergistic anion to form the transferrin-anion-iron complex. Two tetrahedral oxyanion oxidizing agents, periodate and permanganate, were found to partially inactivate transferrin when used at equimolar ratios of oxidizing agent to protein active sites. Hypochlorite, a strong oxidizing agent with little structural similarity to periodate and permanganate, had little effect on iron-binding activity when used at similar low molar ratios of reagent to transferrin active sites. Transferrin treated with a 3:1 molar ratio of periodate or permanganate to active sites lost 74 or 67% of its iron-binding capacity, respectively. The composition of the buffer affected the extent of transferrin inactivation by periodate and permanganate; for example, the extent of inactivation by periodate was threefold greater in a borate buffer than in a phosphate buffer. Comparative oxidations in buffer systems suggest the following order of affinity of three buffer anions for the apotransferrin metal-binding center: phosphate > bicarbonate > borate. The interaction of phosphate ions with the iron-transferrin complex was also examined due to the increased susceptibility to periodate inactivation of iron-saturated transferrin in phosphate buffer (M. H. Penner, R. B. Yamasaki, D. T. Osuga, D. R. Babin, C. F. Meares, and R. E. Feeney (1983) Arch. Biochem. Biophys. 225, 740-747). The apparent destabilization of the iron-transferrin complex in phosphate buffer was found to be due to the competitive removal of iron by phosphate from the iron-protein complex. We found that phenylglyoxal-modified Fe-transferrin, with no loss of bound iron, was much more resistant to iron removal by phosphate and other competitive chelators.