Inactivation of rat renal phosphate-dependent glutaminase by 6-diazo-5-oxo-L-norleucine occurs only under conditions where the enzyme is catalytically active. The glutaminase activity and the rate of inactivation by the diazoketone exhibit very similar phosphate concentration-dependent activation profiles. Because of this phosphate dependency, it was not possible to differentiate an apparent protection by glutamine from the strong inhibition of inactivation caused by glutamate. The ability of glutamate to protect the glutaminase against inactivation is reversed by increasing concentrations of phosphate.The observed characteristics of inactivation by 6-diazo-5-oxo-L-norleucine differ considerably from those reported for the inactivation by L-2-amino-4-oxo-5-chloropentanoic acid. In addition, the presence of o-carbamoyl-L-serine was found to stimulate inactivation by 6-diazo-5-oxo-L-norleucine, but to protect the glutaminase against inactivation by the chloroketone. Preinactivation of the glutaminase by the diazoketone only slightly reduced the stoichiometry of binding of [5-14C]chloroketone. These observations suggest that 6-diazo-5-oxo-L-norleucine and L-2-amino-4-oxo-5-chloropentanoic acid interact with different sites on the glutaminase which are specific for binding glutamine and glutamate, respectively.