Demonstration of ammonia utilization for purine biosynthesis by the intact cell and characterization of the enzymatic activity catalyzing this reaction

Incorporation of intraportally infused [15N]ammonia into urinary uric acid in cockerels pretreated with methionine sulfoximine

1. Measurements were made in situ to determine the incorporation of intraportally infused ammonia-15N into urinary uric acid in cockerels pre-treated with methionine sulfoximine (MSM), a glutamine synthetase inhibitor. 2. The incorporation of 15N into urinary uric acid was 34% of the infused amount in MSM-treated birds. This was not significantly different from the value of 46% for control birds. 3. Pre-treatment with MSM inhibited the activity of liver glutamine synthetase to 7% of the control value and decreased the incorporation of the infused ammonia-15N into plasma glutamine amide-N to 3% of the control. 4. Increases in glutamine concentrations in the blood, liver and kidney caused by the infusion of ammonia were also completely inhibited by the MSM treatment (P less than 0.05). 5. It is concluded that in the cockerel ammonia-N can be incorporated into uric acid other than by glutamine formation.

Effects of varying doses of methionine sulfoximine on liver glutamine synthetase activity and time courses of blood and urinary nitrogenous compounds in the chicken (Gallus domesticus)

1. The activity of liver glutamine synthetase was inhibited to 7-12% of the control activity by an intracardiac injection with methionine sulfoximine (MSM) at dosages of 20, 50, 75 and 100 mg/kg body wt. 2. Plasma glutamine concentrations in all the MSM treatments decreased sharply, then reached steady-state levels within 0.5-2.5 hr, which were almost proportional to a dose of MSM. 3. Blood ammonia concentration sharply increased to a steady-state level attained at 4.5 hr, which was proportional to a dose of MSM. The excretion rate of urinary ammonia augmented linearly up to the dose dependent maximum rates within 2-5 hr. 4. Plasma uric acid concentration dropped linearly by about 6.4 mg/100 ml at doses of 50, 75 and 100 mg MSM and by 3.7 mg/100 ml at a dose of 20 mg MSM within 2.5 hr, then recovered a little. 5. The decreases in excretion rates of urinary uric acid for the first 4 hr were almost the same at doses of 50 mg and larger, being twice as large as that of the control chicken. 6. Any doses of MSM affected neither the time course of excretion rate of total urinary nitrogen nor its total amounts for 7 hr after MSM treatment.

Some regulatory and integrative aspects of purine nucleotide biosynthesis and its control: an overview

The regulation and integration of purine nucleotide biosynthesis is considered from the viewpoint of the main groups of reaction sequences involved and with respect to some specific organs and tissues. Inhibiting either IMP dehydrogenase or adenylosuccinate synthetase in rat liver in vitro reduced the rate of purine do novo synthesis with respect to the purine remaining in the tissue and did not materially affect the rate with respect to the purines extruded into the incubation medium. These results are considered in contrast to the results of previous studies in cultured lymphoblasts. The relative activities of purine de novo synthesis and of purine salvage have been assessed in different tissues by the activities of amidophosphoribosyltransferase and hypoxanthine phosphoribosyltransferase (HPRT), respectively. Changes in purine de novo synthesis as measured by [14C]formate incorporation into cellular purines were reflected in the amidophosphoribosyltransferase activities. The capacity of different tissues to synthesize purines de novo is widespread and the role of the liver as the main site of purine de novo synthesis in vivo and exporting purines to other tissues appears questionable. Regulatory mechanisms may well be tissue specific. The age-related changes in the activity of the purine de novo synthesis and purine salvage pathways, respectively, in the brain suggest that it is physiological or neuropharmacological functions of the developed brain rather than cell division and organogenesis which require a high level of purine salvage relative to purine de novo synthesis. This is compatible with the observation that purine de novo synthesis alone can meet the needs for additional purine nucleotides which lectin induced lymphocyte transformation involves. The mechanism whereby purine de novo synthesis is initiated during lectin induced lymphoblast transformation remains obscure.