Control of Pyrimidine Biosynthesis in Human Lymphocytes

Interaction between the urea cycle and the orotate pathway: studies with isolated hepatocytes

Two enzymes catalyze the synthesis of carbamylphosphate (CP) in the liver. One is intramitochondrial and utilizes ammonia to make CP for ureagenesis; the second is cytoplasmic and utilizes glutamine to produce CP for pyrimidine biosynthesis. The extent to which the metabolic independence of the two pathways is abridged by the use of a common precursor was examined with measurements of the incorporation of [14C]NaHCO3 into orotic acid, uridine nucleotides, and urea in isolated hepatocytes. Pyrimidine synthesis was markedly stimulated by physiological concentrations of ammonia, and the stimulation was antagonized by ornithine. At intracellular concentrations of ornithine and levels of ammonia found in the portal circulation, some 90% of pyrimidine synthesis was ammonia-dependent. When the glutamine-dependent activity was released from feedback inhibition with galactosamine, the ammonia-dependent incorporation still accounted for 2/3 of pyrimidine synthesis. These results do not support the widely held view that the cytoplasmic enzyme is the sole source of CP for pyrimidine biosynthesis in the liver. They suggest instead that the bulk of the CP incorporated into hepatic pyrimidines is of mitochondrial origin. However, an experiment with intact animals failed to provide decisive evidence on this interpretation. Pyrimidine biosynthesis was sharply inhibited by the addition of uridine, but ureagenesis was unaffected. When physiological levels of ammonia were provided, the sensitivity of pyrimidine biosynthesis to uridine was lost. Although inhibition of the ammonia-dependent enzyme by pyrimidines has been observed with cell-free preparations, it was not evident in the intact cell. Thus, to the extent that the CP consumed in pyrimidine biosynthesis is of mitochondrial origin, feedback control of the orotate pathway appears to be thwarted.

Stimulation by 6-azauridine of carbamoyl phosphate synthesis for pyrimidine biosynthesis in mouse spleen slices

1. Slices of spleen from anaemic mice were incubated with [14C]bicarbonate in the presence and absence of 6-azauridine and the amounts of 14C that entered the de novo pyrimidine biosynthetic pathway were assessed and compared. Compounds analyzed included carbamoylaspartate, dihydroorotate, orotate plus its derivatives, acid-soluble uracil and cytosine 5'-nucleotides, nucleic acid pyrimidines, free pyrimidine bases and nucleosides. As the intracellular levels of carbamoyl phosphate and acid-soluble deoxyribonucleotides are known to be relatively low, the radioactivities of these compounds were not measured. Degradation of labelled uridine was limited in this tissues, therefore the radioactivity of degradative products of pyrimidines was not considered. 2. When the slices were incubated with 0.5 mM 6-azauridine for 10 min and then with [14C]bicarbonate for an additional 10 min and 30 min, the sum of radioactivity found in the above compounds, which represents the total amount of 14C that entered the pyrimidine pathway, was 2.1 and 2.3 times greater than when the tissue slices were incubated in the absence of the analogue. 3. When the 14C distribution among the carbon atoms of the molecules of labelled carbamoylaspartate and uracil was investigated, we found that more than 90% of the total 14C in these compounds derived directly from carbamoyl phosphate and the remaining portion was from aspartate, either in the presence or absence of 6-azauridine. 4. There was no indication that 6-azauridine altered [14C]bicarbonate permeation through the cell membrane or its intracellular metabolism. 5. These results, along with the pattern of early intermediate accumulation seen in the presence of 6-azauridine, indicate that 6-azauridine stimulates the production of carbamoyl phosphate for the pyrimidine biosynthetic pathway in the mouse spleen. 6. Of the radioactive early intermediates which accumulated, only orotate, its derivatives (orotidine and orotidine 5'-monophosphate) or both appeared in the medium, presumably the result of leakage through the cell membranes. 7. Stimulation of the pyrimidine pathway was not observed in the case of Ehrlich ascites tumour cells incubated under similar conditions with 6-azauridine.

Metabolic fate of pyrimidines and purines in dietary nucleic acids ingested by mice

1. In order to study the metabolism and tissue utilization of pyrimidines or purines ingested as dietary nucleic acid components, [14C]uracil, [14C]cytosine-labeled RNA, [14C]thymine-labeled DNA, or [14C]adenine-labeled RNA was fed to mice. 2. Absorption and catabolism of each ingested radioactive material were rapid; 80% or more of the ingested radioactivity was excreted as catabolic products over an 8-h period. 3. Utilization of the ingested radioactive materials for tissue synthesis of nucleic acids was limited under the usual conditions, the extent being 2--5%, 4 h after feeding. Such acid-insoluble radioactivity was localized principally in gastrointestinal tissue, and much lesser amounts, albeit significant, were found in the liver. 4. With increase in the dose of dietary nucleic acids, the amounts of utilized (nucleic acids and nucleotides) and utilizable (nucleosides and free bases) forms of uracil and cytosine and of adenine were increased in all tissues examined. Relationship between the dose and utilization together with additional findings support the view that gastrointestinal tissue and the liver utilize and degrade a greater part of the exogenous nucleic acid bases before their entry into the systemic circulation. 5. The metabolism of DNA thymine was unique in that it was significantly utilized for DNA synthesis in tissues other than the gastrointestinal tissue and liver to a comparative extent. The spleen was particularly active in this respect, and the hyperplastic, hematopoietic spleen was three times more active than the normal spleen. 6. Principal components of partially digested products in the intestinal lumen 1 h after the ingestion were uridine (33%) and cytidine (22%) in the case of [14C]uracil, [14C]cytosine-labeled RNA and inosine (53%) in the case of [14C]adenine-labeled RNA, in accordance with the view that purines and pyrimidines in nucleic acids are absorbed mainly in the form of nucleosides.