Parsons_1975_J.Biol.Chem_250_5660

Adenosine triphosphate phosphoribosyltransferase is inactivated rapidly by bulky alkylating reagents in a biphasic reaction. The initial inactivation rate is dependent upon an optimal concentration of histidine and is more rapid at low enzyme concentrations and low ionic strength. A histidine-free dimer form of the enzyme is the proposed reactive species. The dimer is shown by ultraviolet difference spectroscopy to bind histidine about 1 order of magnitude more weakly than the hexameric form of the enzyme. Alkylated enzyme is similar to native enzyme in dissociation and histidine-binding properties. Native enzyme must exist at significant levels in at least five different conformational and dissociative states to account for the inactivation behavior.