Comparative study on histidine modification by diethylpyrocarbonate in human serotransferrin and lactotransferrin.

Serotransferrin (STF)** [l] and lactotransferrin (LTF) [2] are iron binding glycoproteins and the iron complex formed by LTF is known to be more stable especially at low pH [2]. It has generally been accepted that two or three tyrosine residues are involved in complex formation with each atom of Fe3+ in the molecule of STF [3-61 and LTF [7,8], Electron paramagnetic resonance studies have shown that in STF two nitrogen atoms are also co-ordinated with each iron [6,9] and that, in the Cu2+ LTF -bicarbonate complex, at least one nitrogen ligand is present at each metal binding site [8]. The results obtained by Bezkorovainy et al. [4,10] after alkylation with bromoacetic acid sug gest that in STF the nitrogen atoms belong to histidine residues. The present paper is dealing with identification of the nitrogen ligand present in the Fe3+ LTF complex. The reactivity of His residues of both STF and LTF as well as that of their apoderivatives was studied using DEP. This specific reagent for His modification [ 1 l-l 31 reacts more rapidly (1.5 hr) than bromo-