IN-VITRO METHIONINE OXIDATION OF RECOMBINANT HUMAN LEPTIN

Purpose, To investigate the role and importance of the four methionine s in recombinant human leptin, and the effect of methionine oxidation in leptin structural stability and biological activity. Methods. Oxidi zed leptin derivatives were prepared in the presence of H2O2 and t-but ylhydroperoxide, separated by RP-HPLC, and characterized by peptide ma pping and LC/MS. Their biophysical and biological properties were stud ied. Results. Six major species of oxidized leptins were detected: two mono-oxidized, one di-oxidized, two tri-oxidized, and one tetra-oxidi zed. Further oxidation at cystine disulfide was also detected. Kinetic analysis indicated that oxidation at Met(1) and Met(69) proceeded fir st and independently. In 48 mM t-butylhydroperoxide, the pseudo first- order rate constants, k(1) and k(69), were 1.5 x 10(-3) and 2.3 x 10(- 4) min(-1). No change in the secondary or tertiary structure was detec ted for Met(1) mono-oxidized and Met(1), Met(69) di-oxidized leptins. The Met(1) mono-oxidized leptin retained full potency as compared to n ative leptin. A slight decrease of thermostability and a significant l oss of the in vitro bioactivity were observed for Met(1), Met(69) di-o xidized leptin. Both Met(55) and Met(137) were not oxidized in t-butyl hydroperoxide but only in H2O2. They appeared to be much less accessib le to oxidation and might interact with the hydrophobic core structure of the leptin molecule. Conclusions. The oxidation of leptin occurred in the order of Met(1) > Met(69) >> Met(55) approximate to Met(137), and the importance for maintaining leptin structural integrity was Met (55) approximate to Met(137) >> Met(69) approximate to Met(1). Met(69) , but not Met(1), plays a critical role in the protein stability and a ctivity.