Studies on tyrosine phenol lyase. Modification of essential histidyl residues by diethylpyrocarbonate

Tyrosine phenol-lyase of Escherichia intermedia is inactivated by treatment with diethylpyrocarbonate at pH 6.0 AND 4 degrees. Spectrophotometric studies show that the inactivation is stoichiometric, with a modification of 2 histidyl residues per molecule of the enzyme. Finding that this inactivation is largely reversed by treatment with hydroxylamine indicates that the inactivation is mainly due to modification of the histidyl residues. No changes in the sulfhydryl content or in the aromatic amino acids are observed as a result of this modification. The modified tyrosine phenol-lyase retains most of its ability to form a nearly normal complex with its coenzyme, pyridoxal phosphate. This has been shown by studies of its absorption, by the determination of pyridoxal phosphate, and by reduction of the holoenzyme with tritiated sodium borohydride. The modified enzyme also appears to form a Schiff base intermediate with L-alanine. The modified holoenzyme fails to catalyze the exchange of the alpha-hydrogen of L-alanine with tritium from tritiated water. This is consistent with a catalytic role for modified histidyl residues at the active site of the enzyme; this role is the removal of the alpha-hydrogen of substrates.