Role of argininosuccinate synthetase in the regulation of urea synthesis in the rat and argininosuccinate synthetase-associated metabolic disorder in man

Abstract

Regulation of urea synthesis and the role of ASS were studied in rats subjected to acute dietary transitions from high to low protein or vice versa and in rats injected with an ammonium salt. Significant increase/decrease in urea excretion and urea in the liver preceded increase/decrease of ASS during the dietary transitions. Shortly after the switch of diet from high to low protein, the ratios of the rate of urea synthesis from ammonium chloride to the activity of ASS in the perfused liver decreased to much lower values than those of rats fed on the low protein diet. In contrast, during the acute transition from low to high protein diet, the ratios increased above the values of rats fed on the high protein diet. The results indicated that regulatory factors other than the activity of ASS may influence urea synthesis. The concentrations of ornithine and acetylglutamate capable of stimulating urea synthesis markedly changed shortly after the dietary switch. The fact that fluctuations of ornithine and acetylglutamate concentrations were greater and occurred prior to the activity changes of ASS strongly suggests a possible regulatory function of these amino acids in urea synthesis. ASS may be rate-limiting only during the dietary transition from low to high protein. Intraperitoneal injection of an ammonium salt caused an increase in ammonia and urea in the liver of fed or starved rats. The increase in urea was accompanied by increases in acetylglutamate, ornithine and citrulline. In perfused liver, the addition of an ammonium salt to the perfusate caused increases in acetyl-glutamate and citrulline as observed in vivo, while the concentration of ornithine, one-fifth of that in vivo, did not respond to the addition. From these results, we conclude that ammonia, a direct substrate for urea synthesis, may control the concentrations of both ornithine and acetylglutamate in the liver, directly or indirectly, although perfused liver lacks some factors that keep the concentration of ornithine at the in vivo level and cause its increase in response to ammonia. In another part of the work, enzymological analysis in the liver of 4 cases of so-called adult-type citrullinemia with neuro-psychiatric symptoms of late-onset (24–48 years old) caused by ASS abnormalities was performed. One patient in whose family there was a hereditary tendency, judging from their serum citrulline levels, had an abnormal ASS with very high K m values for citrulline and aspartate. This was similar to the cases of neonatal-type citrullinemia summarized by Shih (13). We also observed another type of citrullinemia: decreased ASS activity in the liver of three patients was associated with the decrease in the enzyme protein. ASS activity in the kidney, however, was not decreased at all. The kinetic properties of the enzymes of the patients were quite similar to those of healthy control. No hereditary tendency was observed in these families. Thus, we assume that adult-type citrullinemia results from an epigenetic abnormality as well as a genetic one and heterogeneity in the cause may explain the higher incidence of citrullinemia in Japan.