Comparative oxidations of tyrosines and methionines in transferrins: Human serum transferrin, human lactotransferrin, and chicken ovotransferrin

Periodate treatments of apo human serum transferrin (HST), and apo chicken ovotransferrin (COT) were previously reported to cause a rapid loss of Fe⁺³ binding capacity, with a loss of 3 to 5 tyrosine residues [P. AZARI AND J. L. PHILLIPS (1970) Arch. Biochem. Biophys., 138, 32-38; K. F. GEOGHEGAN, J. L. DALLAS, AND R. E. FEENEY (1980) J. Biol. Chem., 255, 11429-11434]. The effects of periodate and hydrogen peroxide on human lactotransferrin (HLT), HST, and COT have been compared. All three apotransferrins were rapidly inactivated and lost ~4 to 5 tyrosine residues by 5 mm periodate treatment; their iron complexes had little or no inactivation and losses of ~1 to 2 tyrosine residues. All three iron transferrins were highly resistant to inactivation by 5 mm periodate in bicarbonate, with or without the addition of phosphate, while in phosphate (with ambient carbonate) Fe₂HLT was highly resistant, Fe₂COT slightly less resistant, and Fe₂HST much less resistant. Similar oxidations of methionines to the sulfoxides were found in both the apo and iron forms. After 150 min of 5 mm periodate treatment HST lost ~3 (apo 3.1, iron 2.8) of 9, HLT ~3 (apo 2.6, iron 2.9) of 6, and COT ~7 (apo 7.2, iron 7.2) of 11 methionines per mole of protein. In the presence of 8 M urea HST had essentially all of its methionine residues oxidized by periodate, but only lost part of its activity on renaturation. Treatment of all apo transferrins with 300 mm hydrogen peroxide resulted in little or no losses (<10%) in activity. HST lost approximately one-third of its methionines and no tyrosines during the 300 mm hydrogen peroxide treatment. Therefore the essentiality of tyrosines for all three transferrins was confirmed and the nonessentiality of methionines was demonstrated.