Evidence for the participation of histidine residues located in the 56 kDa C‐terminal polypeptide domain of ADP‐ribosyl transferase in its catalytic activity

Purified ADPRT protein was inactivated by the histidine specific reagent diethylpyrocarbonate, binding to two histidine residues, or by a relatively histidine selective photoinactivation method. Inactivation with up to 1.3 mM diethylpyrocarbonate was reversible by hydroxylamine. Enzymatic inactivation coincided with the loss of binding capacity of the enzyme protein to benzamide affinity matrix but not to DNA cellulose. Labelled diethylpyrocarbonate was identified exclusively in the 56 kDa carboxyl‐terminal polypeptide where 2 out of 13 histidine residues were modified by this reagent. It is proposed that histidine residues in the 56 kDa polypeptide may participate as initiator sites for polyADP‐ribosylation.