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L-asparaginase from Escherichia coli B. Chemical modifications of tyrosyl residues.

Abstract l-Asparaginase from Escherichia coli B was treated with tetranitromethane in amounts ranging from equimolar to 10 molar excess. At low ratios of the reagent the tetrameric enzyme aggregates to larger molecular weight species in the absence of any detectable nitrotyrosine formation. At the higher ratios of the reagent the enzyme first polymerizes and the resulting polymers are subsequently nitrated. Although the covalent bonds joining the subunits together in the aggregate are unaffected by 8 m urea or sodium dodecyl sulfate, fluorescence and difference spectroscopy indicate that the microenvironment surrounding the aromatic amino acid side chains in the aggregates is perturbed in the presence of these denaturants. In addition, the polymeric forms still retain a portion of their catalytic function. The similarity in titration behavior of the phenolic hydroxyl groups in the tetranitromethane-induced polymers and in the unmodified enzyme suggests that the subunits are not exclusively crosslinked through the phenolic side chains of tyrosine. The tyrosyl residues of asparaginase could be modified by iodination without the formation of large molecular weight aggregates and without causing dissociation into subunits.