DAMAGE TO HUMAN ALPHA-1-PROTEINASE INHIBITOR BY AQUEOUS CIGARETTE TAR EXTRACTS AND THE FORMATION OF METHIONINE SULFOXIDE

The effects of aqueous extracts of cigarette tar (ACT) on human alpha-1-proteinase inhibitor (alpha,PI) are examined by determining alterations in the elastase inhibitory capacity (EIC), amino acid residue content, and electrophoretic behavior of the protein. Hydrogen peroxide generated in ACT by autoxidative processes accounts for the major portion of the loss of EIC. This is indicated by several lines of evidence, including the fact that anaerobic incubations of ACT with alpha,PI cause negligible loss of EIC. The hydrogen peroxide content of the ACT was estimated by measuring the ability of the extracts to oxidize methionine to methionine sulfoxide; hydrogen peroxide concentrations that model those in ACT cause a similar loss in EIC. Exposure of alpha-1PI to ACT leads to methionine sulfoxide as the only detectable amino acid residue modification and explains the loss of EIC. This is the first report to directly demonstrate methionine sulfoxide formation in alpha-1PI exposed to cigarette smoke components in vitro. Similar amounts of methionine sulfoxide are found in a,PI exposed to hydrogen peroxide at the concentrations predicted to be formed in ACT. Nondenaturing PAGE reveals that alpha-1PI exposed to ACT, but not hydrogen peroxide, shows changes in electrophoretic behavior; the changes are nonoxidative in nature and are not related to the loss of EIC. Studies on the effect of chelators on ACT-mediated damage to alpha-1PI indicate some role for metal ions; however, none of the chelators completely protect alpha-1PI. We suggest that water-soluble components of the cigarette smoke particulate phase ("tar"), in addition to gas-phase components, may damage a,PI and contribute to local antiproteinase deficiencies in smokers' lungs. This damage probably is caused by oxidative modification of at least one methionyl residue necessary for proteinase inhibitory function.