Chemical modification and 1H-NMR studies on the receptor-binding region of human interleukin 6

Oxidation of the Met residues of human interleukin 6 (IL-6) molecule has been performed. Reactivity of Met for the oxidation reaction was found to decrease in the order of Met50, Met118, Met185, Met162, and Met68. Chemical modifications involving oxidation and carboxypeptidase A digestion of IL-6 have led to the assignments of the methyl proton resonances of Met162 and Met185, respectively. The hydroxynitrobenzyl chromophore attached to Trp158 in the IL-6 molecule showed a different absorption spectrum when the labeled IL-6 was bound to the soluble IL-6 receptor. This result indicates that Trp158 is near the receptor-binding region in IL-6. On the basis of the 1H-NMR and chemical modification data, it has been concluded that Trp158 is in spatial proximity to Met162, His165 and Met185. The receptor-binding activity decreased with an increase in the number of oxidized Met residues. Of these five Met residues, Met162 was the residue in which the receptor-binding activity decreased in the most parallel degree with that of the oxidation reaction.