Glutamine synthetase : III. Factors controlling its activity in the developing rat

Abstract

The developmental changes in glutamine synthetase activity in rat liver and brain have been studied and shown to be different in the two tissues. The liver enzyme exhibited a fall in activity after birth, followed by a slow recovery and a rapid increase in activity for a short interval of time, before a gradual rise to the adult level in about 40 days. The brain enzyme raised its activity gradually and steadily right after birth until it reached the adult level in about 18 days.The enzyme activity in the liver of developing rats appears to be affected by those compounds that also affect protein synthesis, ribonucleic acid synthesis, or deoxyribonucleic acid synthesis. When injected into young rats, puromycin, -p-fluorophenylalanine, ammonium lactate, actinomycin D, and 8-azaguanine lowered the enzyme activity in liver, whereas -ethionine, cortisol, -thyroxine, and 5-fluorouracil elevated it. The absence of effect of these compounds on the enzyme activity in vitro suggests that the changes observed may result from their effect on the amount of the enzyme formed in the liver. In most instances, the enzyme in the brain escaped the effect of these compounds.With repeated injections of puromycin to block the synthesis of glutamine synthetase, the enzyme in the liver and testis, respectively, lost one half of its original activity in about 14 hours and 7.5 hours. A very slight decrease in the activity, however, occurred in the brain and kidney.