SUMMARY: In a sudden increase in ammonia supply or a decrease in glucose supply can result in a rapid and extensive reduction in glutamine synthetase activity, a fall that is much faster than can be accounted for by the immediate repression of enzyme synthesis followed by the dilution of existing enzyme by the other cell proteins produced during continued growth. This decrease in enzyme activity is not caused by the build-up within the cell of an inhibitor of the enzyme, by the rapid turn-over of the enzyme, or by the conversion of the enzyme from a more active to a less active form, but by the specific inactivation of the enzyme. Inactivation appears to be largely irreversible, and studies using inhibitors of protein synthesis indicate that the reappearance of the enzyme after removal of ammonia is dependent on protein synthesis.

Glutamine synthetase activity can be estimated by using either the transferase assay or the synthetase assay: the ratio of these activities is virtually constant in preparations from yeast growing under a wide range of conditions and during inactivation and reappearance of the enzyme.


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