The Effect of Tetranitromethane on Apo‐Aspartate‐Aminotransferase from Pig Heart

Both holo‐ and apo‐aspartate‐aminotransferase are sensitive to tetranitromethane; however, the inactivation which follows nitration is partial for the holoenzyme, and complete for the apoenzyme. Cysteinyl and tyrosyl residues are modified in both forms of the enzyme; in the apoenzyme a greater number of tyrosines are available to the reagent. The complete inactivation of the apoenzyme is correlated with the loss of only one tyrosyl residue, which is selectively modified under particular conditions. The loss of activity is caused by the failure of the nitrated apoenzyme to recombine with the coenzyme.