ABSTRACT
Glutamine synthetase is a crucial enzyme for the maintenance of nitrogen homeostasis. Its activity appears to be finely regulated in vivo in response to continuously fluctuating amino acid levels in the various body constituents. Escherichia coli glutamine synthetase is highly regulated by a complex control mechanism including repression/derepression of enzyme synthesis, cumulative feedback inhibition by metabolic effectors, and by chemical intercon version of the enzyme. Of the various preparations of glutamine synthetase isolated from mammalian tissues, most work has been carried out on the sheep brain enzyme. In early work, Waelsch and co-workers investigated the metabolic fate of l-glutamate and concluded, from the specific activities of isolated glutamate and glutamine, that glutamate metabolism is compartmented in the brain. There have been several conflicting reports on the activity of glutamine synthetase in mammalian organs. Much of this confusion appears to have resulted from inadequate assay procedures so that it is worth commenting on the procedures employed.
