Effect of diets and hormones on two urea cycle enzymes

INFLUENCE OF PANCREATIC HORMONES ON ENZYMES CONCERNED WITH UREA SYNTHESIS IN RAT LIVER

1. The activities of enzymes of the urea cycle [carbamoyl phosphate synthetase, ornithine transcarbamoylase, argininosuccinate synthetase, argininosuccinase (these last two comprising the arginine-synthetase system) and arginase] have been measured in control, alloxan-diabetic and glucagon-treated rats. In addition, measurements were made on alloxan-diabetic rats treated with protamine-zinc-insulin. 2. Treatment of rats with glucagon for 3 days results in a marked increase in the activities of three enzymes of the urea cycle (carbamoyl phosphate synthetase, argininosuccinate synthetase and argininosuccinase). The pattern of change in the alloxan-diabetic group is very similar to that of the glucagon-treated group, although the magnitude of the change was much greater. 3. Comparison was made of the actual and potential rate of urea synthesis in normal and diabetic rats. In both groups the potential rate of urea production, as measured by the activity of the rate-limiting enzyme, argininosuccinate synthetase, slightly exceeds the actual rate of synthesis by liver slices in the presence of substrates. The relative activities of the actual and potential rates were similar in the two groups of animals, this ratio being 1:0.70. 4. In the alloxan-diabetic rats treated with protamine-zinc-insulin for 2.5 or 4 days there was a marked increase in liver weight. This was associated with a rise in the total hepatic activity of the urea-cycle enzymes located in the soluble fraction of the cell (the arginine-synthetase system and arginase) after 2.5 days of treatment. After 4 days of treatment the concentration of these enzymes/g. of liver decreased, and the total hepatic content then reverted to the untreated alloxan-diabetic value. 5. No effects of glucagon or of insulin in vitro could be found on the rate of urea production by liver slices. 6. The present results are discussed in relation to how far this pattern of change is typical of conditions resulting in a high urea output, and comparison has been made with other values in the literature.

Urea as a selective inhibitor of argininosuccinate lyase

The effect of urea on various ornithine cycle enzymes has been investigated. It was demonstrated that argininosuccinate lyase was the only ornithine cycle enzyme inhibited by urea in a competitive manner. Based on the data presented the possible role of urea in maintaining a physiological range of intracellular and extracellular urea concentration by controlling hepatic ureogenesis was discussed.

Effect of ornithine and lactate on urea synthesis in isolated hepatocytes

1. In hepatocytes isolated from 24 h-starved rats, urea production from ammonia was stimulated by addition of lactate, in both the presence and the absence of ornithine. The relationship of lactate concentration to the rate of urea synthesis was hyperbolic. 2. Other glucose precursors also stimulated urea production to varying degrees, but none more than lactate. Added oleate and butyrate did not stimulate urea synthesis. 3. Citrulline accumulation was largely dependent on ornithine concentration. As ornithine was increased from 0 to 40 mM, the rate of citrulline accumulation increased hyperbolically, and was half-maximal when ornithine was 8-12 mM. 4. The rate of citrulline accumulation was independent of the presence of lactate, but with pyruvate the rate increased. 5. The rate of urea production continued to increase as ornithine was varied from 0 to 40 mM. 6. It was concluded that intermediates provided by both ornithine and lactate are limiting for urea production from ammonia in isolated liver cells. It was suggested that the stimulatory effect of lactate lies in increased availability of cytosolic aspartate for condensation with citrulline.

[Control activities and regulation by the glucocorticoids of three urea-cycle enzymes in rat foetal liver: argininosuccinate synthetase, argininosuccinase and arginase (author's transl)]

The activity of three urea-cycle enzymes, argininosuccinate synthetase, argininosuccinase and arginase have been studied in the foetal and new-born liver of rats. The activity increases with regularity between 17.5 days of pregnancy and birth in control foetuses. The lack of corticosteroid from 18.5 days of pregnancy decreases the activity of argininosuccinate synthetase. After administration of cortisol (hydrocortisone), to these 18.5 day-old foetuses lacking of corticosteroids, both activities of argininosuccinate synthetase and arginase are enhanced.

Studies on urea cycle enzymes in rat liver during acute uraemia

Activities of urea cycle enzymes were measured in the liver of starved rats 12 and 48 h after bilateral nephrectomy. Control experiments (sham-operated, starved rats) revealed that the activities of only two enzymes of the cycle are altered in the uraemic state: argininosuccinic acid synthetase (EC 6.3.4.5.), which is considered to be rate limiting for urea production and carbamyl phosphate synthetase (EC 2.7.2.5.). Alterations in ornithine concentration of the liver, a possible cause of an increased urea production rate, could not be detected previously (21). Our present results do not support the concept that a decrease of the activity of ornithine-delta-amino transferase (EC 2.6.1.13), leading to an increase in the ornithine content of the liver is responsible for the accelerated urea production rate in the liver of acute uraemic rats.

Influence of glucagon, 6-N,2'-O-dibutyryladenosine 3':5'-cyclic monophosphate and triamcinolone on the arginine synthetase system in perinatal rat liver

1. The administration of triamcinolone (19-190mug/animal) to postnatal rats increased the arginine synthetase system activity 1.2-2.5-fold above control values 24h after exposure to the hormone. Cortisol (hydrocortisone), however, increased the arginine synthetase system activity only when larger (190mug/animal) or repeated daily doses were given. Glucagon (100mug/animal) stimulated arginine synthetase system activity only after the second postnatal day. None of these agents increased the activity in 19.5-21.5-day foetuses after intrauterine administration. 2. The viability of foetal rat liver explants maintained in organ culture for up to 54h was validated both by ultramicroscopic examination and by incorporation of radioactive leucine and orotic acid. 3. In organ cultures of foetal rat liver explants (18.5 days to term), triamcinolone (20mug/ml of medium) evoked a 2.8-4.3-fold increase after 24h of incubation. This increase was completely inhibited by actinomycin D (25mug/ml) or cycloheximide (10mug/ml). Cortisol (5-50mug/ml) or glucagon (0.067-67mug/ml) also increased the arginine synthetase system activity above the respective control values, but there was no increase in activity with insulin (0.05-0.25i.u./ml). 4. Maximum concentrations of glucagon (67mug/ml), dibutyryl cyclic AMP (6-N,2'-O-dibutyryladenosine 3':5'-cyclic monophosphate) (0.1mm) and triamcinolone (20mug/ml) incubated for 24h with foetal rat liver explants each produced between a two-and three-fold increase in the activity of the arginine synthetase system. Combinations of maximum amounts of glucagon and the cyclic nucleotide did not produce a greater effect than either agent alone. However, the combination of dibutyryl cyclic AMP with triamcinolone appeared to produce somewhat less than additive effects. 5. The effects of the cyclic nucleotide and triamcinolone were evident after 12h of incubation and increased steadily throughout the 24h of observation. This time-course of increased enzyme activity is very much slower than that reported for the induction of other enzymes in explant cultures of foetal rat liver.

Adaptive enzyme changes in liver and muscle of rats during protein depletion and refeeding

1. Young rats were kept on a 6% casein diet for 4–7 weeks and then either killed or returned to a stock diet. Control rats were given a stock diet.

2. Argininosuccinate lyase (EC 4.3.2.1, L-argininosuccinate arginine-lyase) was assayed in liver, and amino acid activating enzymes were assayed in liver and muscle of rats after depletion and while they were refed on the stock diet. Levels of enzymes were compared with those in the control rats.

3. Amino acid activating enzymes were markedly increased in liver at the end of the depletion period, but returned almost to normal levels after 4–6 days of refeeding. In muscle the enzyme levels were not decreased after depletion, but began to rise after 4 days of refeeding.

4. Argininosuccinate lyase activity in depleted female rats was about half that in control rats and rose again slowly on refeeding. Male rats were hardly affected by depletion.

5. It is suggested that these enzyme changes play a part in the adaptation of the rat to a reduced protein intake.

The metabolism of mucopolysaccharides. 3. The effect of adrenocortical hormones on enzymes involved in glucuronide synthesis and metabolism in connective tissue

The levels of activity of hexokinase, phosphoglucomutase, uridine diphosphoglucose pyrophosphorylase, and uridine diphosphoglucose dehydrogenase, which are involved in the synthesis of glucuronic acid, and the levels of glucuronosyltransferase and β-glucuronidase, which are involved in its metabolism, were studied in connective tissue from polyvinyl sponge implants from sham-operated and adrenalectomized rats which had received "replacement therapy" of individual steroids.The levels of activity of these enzymes were significantly increased in sham-operated rats which had been treated with corticosterone, cortisone, or hydrocortisone. In adrenalectomized animals, the increased or decreased enzyme activities were restored to normal levels by "therapy" with corticosterone, cortisone, or hydrocortisone. In some cases there was an over-correction at the particular level of hormone treatment. In both sham-operated and adrenalectomized animals, treatment with deoxycorticosterone, progesterone, or Reichstein's substance S frequently, but not always, resulted in little or no change in activity of the enzymes.

The relationship between nutritive value of dietary protein and activity of liver arginae and kidney transamidinase enzymes

1. The protein efficiency ratio of three protein sources was determined with rats by a depletion-repletion method. The sources were: a groundnut product, a methionine-supplemented groundnut product and lactalbumin.

2. Livers obtained from the test animals were assayed for arginase activity, and kidneys for transamidinase activity (glycine amidinotransferase).

3. The measurements indicated that there was an inverse relationship between arginase activity and the nutritive value of the dietary protein.

4. Transamidinase activity was also influenced by nutritive value. Only the unsupplemented groundnut product, which had the lowest nutritive value, failed to produce a significant increase of transamidinase activity over basal levels.

5. The findings are discussed from the standpoint of physiological function and needs. It is suggested that observed levels of arginase activity are not necessarily related to amounts of urea excreted; similarly, transamidinase activity may be well in excess of physiological requirements.