Regulation of N‐Acetylglutamate Synthetase in Mouse Liver

Abstract

N-Acetyl-L-glutamate synthetase catalyzes the synthesis of N-acetyl-L-glutamate, an allosteric and essential activator of carbamoyl-phosphate synthetase I in the liver of ureotelic animals. The enzyme is activated specifically by arginine.1. Mice were fed laboratory chow (protein content, 21 x) for 3 h and, after various times, N-acetyl-L-glutamate synthetase activity in the sonicated mitochondria was measured both in the presence and absence of 1 niM L-arginine. The activity in the absence of arginine did not significantly change after the feeding, while the activity in the presence of arginine increased with time and reached a maximum (fivefold increase) 9 h after the start of feeding, then decreased gradually. The ratio of the activity with arginine to that without arginine was 1.6 at the start of feeding and 5.5 at 9 h.2. Similar postprandial changes were observed with the mice on a protein-free diet, when these mice had previously been on a protein diet. No such change was seen with a protein-free diet when the mice were previously given a protein-free diet for a few days.3. Cycloheximide injection, which inhibited the total protein synthesis in the mouse liver by 90%, did not inhibit the postprandial increase in arginine activation of the synthetase. Thus, protein synthesis is presumably not required for the change.4. The enzymes with a low and a high arginine sensitivity were purified over tenfold from mitochondrial extracts of fasted and fed animals respectively. The extent of arginine activation of the enzymes remained unchanged, low or high, throughout the purification. In Sephacryl S-300 chromatography both types of the enzyme were eluted as a single peak and corresponded to a M, of 220000-250000. The change in arginine activation may be due to a modification of the enzyme molecule without a gross change in the molecular weight. 5. The diet-dependent change in sensitivity of the synthetase to arginine presents a new type of regulation of this enzyme and this regulation may contribute to the prevention of a rapid decrease in hepatic acetylglutamate levels after food ingestion N-Acetyl-L-glutamate is a specific and obligatory allosteric activator of mitochondrial carbamoyl-phosphate synthetase