Selective covalent binding of acrylonitrile to Cys 186 in rat liver carbonic anhydrase III in vivo

Covalent binding of reactive chemical species to tissue proteins is a common, but poorly understood, mechanism of toxicity. Identification of the proteins and the specific amino acid residues within the proteins that are chemically modified will aid our understanding of the toxification/detoxification mechanisms involved in covalent binding. Acrylonitrile (AN) is a commercial vinyl monomer that is acutely toxic and readily binds to tissue proteins. Total covalent binding of AN to tissue proteins is highly correlated with acute toxicity. Two-dimensional PAGE and autoradiography were used to locate proteins in male rat liver cytosol that are radiolabeled following administration of [2,3-(14)C]AN in vivo. Four intensely labeled spots were prominent in the autoradiogram and formed an apparent "charge-train" at approximately 30 kDa. Tryptic peptide mapping by matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) MS was used to identify all of the spots as carbonic anhydrase III (CAIII). HPLC of the tryptic digests combined with MALDI-TOF MS was used to localize the radiolabel to tryptic fragment T22 containing amino acids 171-187. This tryptic fragment contains two Cys residues (Cys181 and Cys186) in the rat CAIII sequence. Electrospray ionization ion-trap MS was used to sequence the peptide and establish that only Cys186 was labeled. Thus, although AN is considered to be highly reactive, our data indicate that it does not react indiscriminately with rat CAIII but rather is selective for one out of five Cys residues. Rat liver CAIII has previously been shown to protect cells against oxidative stress. Our data suggest that CAIII is also capable of scavenging reactive xenobiotics and may help prevent covalent binding to more critical macromolecules.