Nitration of functional tyrosyl residues in rabbit muscle phosphorylase b

Upon reaction of rabbit muscle phosphorylase $\text{b}$ with tetranitromethane in a stoichiometric ratio with respect to the tyrosyl content, 2 out of 34 phenolic groups per mole of monomer (M.W. 95,000) were nitrated with an almost complete loss of activity. Only one residue per monomer was nitrated in the presence of AMP, the major part of the activity being preserved. The sedimentation pattern of modified phosphorylase $\text{b}$ showed that, following nitration in the absence of AMP, the enzyme was fully dissociated into monomers, whereas, when the enzyme was nitrated in its presence, the dimeric structure was retained.