1. Glutamine synthetase activity has been determined in extracts of rat cardiac and skeletal muscle and kidney, after treatment to ensure that the rate of synthesis was proportional to time of incubation and to amount of extract added. The activity was measured by two methods, with hydroxylamine as substrate. 2. No activity was detected in rat heart extract by either method. The activity in skeletal muscle was of the order of 20mumol of glutamylhydroxamate synthesized/h per g of tissue under optimum conditions. The activity in kidney extracts was 180mumol/h per g of tissue when measured as ferric hydroxamate. 3. The activity in both skeletal-muscle and kidney extracts was inhibited by P(i). The inhibition is competitive for the muscle enzyme, with a K(i) of 12mm. For the kidney enzyme the inhibition is non-competitive, and less marked. Possible enzyme mechanisms that would lead to these types of inhibition are discussed. 4. Several observations are reported that suggest that the enzymes from muscle and kidney are not identical. 5. Growth hormone, either in vivo or in vitro, did not affect the measured glutamine synthetase activity of tissue extracts.
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