THE KINETICS OF RELAXIN OXIDATION BY HYDROGEN-PEROXIDE

In this study, hydrogen peroxide was used to study the oxidation of rh Rlx under various conditions. Oxidation of rhRlx occurred al both of t he two methionines on the B chain, Met B(4) and Met B(25), as expected from the three-dimensional structure of the molecule, which shows tha t these two residues are located on the surface of the molecule and ex posed to solvent. The reaction produced three different oxidized forms of rhRlx containing either Met B(4) sulfoxide, Met B(25) sulfoxide, o r both residues oxidized. The corresponding sulfone was not formed und er these conditions. The oxidation at the two methionines proceeded in dependently from each other but Met B(25) was oxidized at a significan tly faster rate than Met B(4). The fact that the rate of oxidation at Met B(25) was identical to the rate of oxidation of free methionine an d that of two model peptides mimicking the residues around Met B(4) an d Met B(25) suggests that the lower reactivity at Met B(4) was due to steric hindrance, and at least in this case, neighboring groups do not influence the oxidation kinetics of methionine residues. The reaction was independent of pH, ionic strength, and buffer concentration in th e range studied. The enthalpy of activation for the reaction was appro ximately 10-14 kcal mol-1, with an entropy of activation of the order of -30 cal K-1 mol-1. These data are consistent with previously publis hed mechanisms for organic sulfide oxidation by alkyl hydroperoxides.