[Modification of phosphorylase b histidine residues by diethylpyrocarbonate]

Interaction of phosphorylase B with diethylpyrocarbonate was studied. It was found that modification of the histidine residues is accompanied by a change in the adsorption spectrum of phosphorylase B at around 240--245 nm, as well as by inhibition of the enzyme activity. The addition of hydroxylamine to the modified protein results in an elimination of the absorption at round 240 nm and a complete restoration of the enzyme activity. The apparent constants of the phosphorylase B inactivation rates and the rates of the histidine residues modification are shown to be the values of the same order. In the presence of substrate (glucose-1-phosphate) one of the histidine residues (calculated per 1 subunit) does not enter the reaction with diethylpyrocarbonate, a protection of the enzyme against inactivation being observed. In the presence of activator (AMP) four histidine residues are inaccessible for diethylpyrocarbonate; however, the degree of inhibition of the enzyme activity does not change. The phosphorylase B molecule, modified in the presence of AMP, is incapable of dissociating into monomers.