Specificity of the Glutamine‐Binding Site Involved in the Regulation of Glutamine‐Synthetase Activity in Hepatoma Tissue‐Culture Cells

Glutamine accelerates the degradation of glutamine synthetase in hepatoma tissue culture cells. Compounds structurally related to glutamine were tested for their ability to mimic or antagonize this effect of glutamine. 6‐Diazo‐5‐oxo‐ l ‐norleucine, like glutamine depressed the activity of glutamine synthetase in hepatoma tissue culture cells. l ‐Methionine sulfone, albizzine, l ‐methionine sulfoxide, l ‐γ‐glutamyl hydrazide and γ‐ N ‐methyl‐ l ‐glutamine (listed in order of decreasing potency) were antagonists which prevented the effect of glutamine on glutamine synthetase activity. These antagonists had little effect on glutamine transport or protein synthesis of hepatoma tissue culture cells and their effects were reversible. The effects of compounds on glutamine synthetase activity in cell‐free extracts of the cells were examined. Diazo‐oxonorleucine and albizzine inhibited neither the transferase nor the synthetase activity of glutamine synthetase. This observation is interpreted to mean that the glutamine‐binding site involved in the regulation of glutamine synthetase activity of hepatoma tissue culture cells is not the active site of the enzyme.