Glutaminase from pig renal cortex. II. Activation by inorganic and organic anions.

The Tris-HCl and phosphate-borate forms of glutaminase (EC 3.5.1.2, L-glutamine amidohydrolase) are both activated and protected from inactivation by phosphate and carboxylic acids. The Tris-HCI enzyme is less sensitive to this activation, but more sensitive to glutamate inhibition than the phosphate-borate enzyme. Plots of activity against concentration of activating anions are generally sigmoidal, and plots against phosphate become more sigmoidal by addition of glutamate. However, such plots for the phosphate-borate enzyme become hyperbolic in the presence of cu-ketoglutarate. In addition to spontaneous activation by phosphate and carboxylic acids, assumed to be of allosteric nature, a timedependent activation has been shown at higher protein concentrations. The time-dependent activation occurs following incubation of the Tris-HCl enzyme with phosphate or phosphate plus borate. The time-dependem activation in the presence of phosphate plus borate changes the properties of the Tris-HCl enzyme so that they become similar to those of the phosphate-borate enzyme. It is suggested that time-dependent activation reflects a transition of the TrisHCl enzyme to higher molecular forms.