AN ACTIVITY IN RAT-TISSUES THAT MODIFIES NITROTYROSINE-CONTAINING PROTEINS

Homogenates from rat spleen and lung could modify nitrotyrosine-contai ning BSA, With incubation, nitrotyrosine-containing BSA lost its epito pe to a monoclonal antibody that selectively recognized nitrotyrosine- containing proteins. In the presence of protease inhibitors, the loss of the nitrotyrosine epitope occurred without protein degradation and hydrolysis, This activity was found in supernatant but not particulate fractions of spleen homogenates. The factor was heat labile, was sens itive to trypsin treatment, and was retained after passage through a m embrane with a 10-kDa retention. The activity was time- and protein-co ncentration dependent. The activity increased about 2-fold in spleen e xtracts with endotoxin (bacterial lipopolysaccharide) treatment of ani mals, suggesting that the activity is inducible or regulatable. Other nitrotyrosine-containing proteins also served as substrates, while fre e nitrotyrosine and some endogenous nitrotyrosine-containing proteins in tissue extracts were poor substrates. Although the product and poss ible cofactors for this reaction have not yet been identified, this ac tivity may be a ''nitrotyrosine denitrase'' that reverses protein nitr ation and, thus, decreases peroxynitrite toxicity. This activity was n ot observed in homogenates from rat liver or kidney, suggesting that t here may also be some tissue specificity for the apparent denitrase ac tivity.