Enhanced physical stability of human calcitonin after methionine oxidation.

ABSTRACT

Calcitonin is a blood-calcium-lowering peptide, present in different species, which inhibits the resorption of bone by osteoclasts. Human calcitonin (hCT) is one of the few calcitonin peptides, which contains a methionine residue; this residue is in position 8. Methionines are known to be readily oxidized to sulfoxides both in vivo and in vitro. The current work describes the effect of methionine oxidation on the physical stability of hCT. Aggregation kinetics of human calcitonin were studied at different pH values by intrinsic fluorescence spectroscopy, turbidity at 350 nm, microscopy analyses, Nile Red, and 1,8-ANS fluorescence emission. In all the experiments, methionine oxidation reduced the aggregation rate of human calcitonin. The effect of methionine oxidation was independent of pH. Fluorescence lifetime data also showed that the conformation of hCT in the aggregated state can be influenced by methionine oxidation. A hypothesis for the enhanced physical stability of oxidized hCT is presented and discussed.