Kinetic study of methionine oxidation in human parathyroid hormone

Human parathyroid hormone (hPTH) has two methionine (Met) residues at positions 8 and 18. These are oxidized to Met sulfoxide in the presence of an oxidant such as H2O2. In this study, the rate constants of Met oxidation were estimated in the 2–10 pH range. Between pH 4–7, the rate constants were constant. The rate constant of Met18 was greater than that of Met8 at all pH values examined. In the presence of urea, both rate constants increased with rise of urea concentration until the difference between them was negligible. In comparison with 1–34 hPTH, both rate constants of hPTH were smaller. From these results, we suggest that differences of folding, especially in the proximity of the Met residues, are reflected by the rate constants for Met oxidation.