Oxidation of either methionine 351 or methionine 358 in alpha(1)-antitrypsin causes loss of anti-neutrophil elastase activity

Hydrogen peroxide is a component of cigarette smoke known to be essential f or inactivation of alpha(1)-antitrypsin, the primary inhibitor of neutrophi l elastase, To establish the molecular basis of the inactivation of alpha(1 )-antitrypsin, we determined the sites oxidized by hydrogen peroxide, Two o f the nine methionines were particularly susceptible to oxidation, One was methionine 358, whose oxidation was known to cause loss of antielastase act ivity. The other, methionine 351, was as susceptible to oxidation as methio nine 358, Its oxidation also resulted in loss of anti-elastase activity, an effect not previously recognized, The equal susceptibility of methionine 3 58 and methionine 351 to oxidation was confirmed by mass spectrometry. To v erify this finding, we produced recombinant alpha(1)-antitrypsins in which one or both of the susceptible methionines were mutated to valine, M351V an d M358V were not as rapidly inactivated as wild-type alpha 1-antitrypsin, b ut only the double mutant M351V/M358V was markedly resistant to oxidative i nactivation. We suggest that inactivation of alpha(1)-antitrypsin by oxidat ion of either methionine 351 or 358 provides a mechanism for regulation of its activity at sites of inflammation.