Kinetic comparisons between soluble and membrane-bound glutaminase preparations from pig brain

The kinetic properties of membrane-bound glutaminase from pig brain have been compared with those of the soluble enzyme that had been preincubated with either Tris/HCl buffer or Tris/phosphate/borate buffer. The two preparations of the soluble enzyme were similar in the dependence of their activities on pH and in their inhibition by borate and some glutamine analogues. They differed markedly in their responses to phosphate and their inhibition by glutamate. The membrane-bound enzyme differed from the other preparations in showing a sigmoid dependence of initial velocity on glutamine concentration at higher pH values and being activated by borate ions. The apparently cooperative behaviour that has been previously reported for soluble preparations of glutaminase may have resulted from failure to measure the true initial rate of the reaction. The Km values for glutamine and the inhibitor and activator constants for glutamate and phosphate respectively were such as to indicate that changes in the concentrations of these compounds might affect the glutaminase activity in vivo. None of the enzyme preparations was inhibited significantly by ammonium ions at concentrations up to 50 mM. Considerable differences were found between the three preparations in their activation by compounds containing sulphate or sulphonate groups.