Reutilization of preformed pyrimidines for the synthesis of DNA in rat liver and kidney

Hepatic purine and pyrimidine metabolism: implications for antiviral chemotherapy of viral hepatitis

The use of nucleoside analogues as antiviral agents is expanding. For most nucleoside analogues, intracellular phosphorylation is the major prerequisite for activity. Antiviral activity may be limited by poor uptake, absence of appropriate activating enzymes, catabolism, and competition from endogenous nucleotides. Appreciation of these factors, which are species-, tissue- and cell-specific is important in the understanding of the pharmacology and toxicology of nucleoside analogues. The use of nucleoside analogues against the agents of viral hepatitis is inherently problematic for many reasons including active hepatic nucleoside catabolism, probable absence of virus-specific activating enzymes, competition from endogenous nucleotides synthesised de novo or derived from RNA turnover, and factors related to mitochondrial toxicity. Despite these drawbacks, some nucleoside analogues have been found efficacious against hepatitis B virus and it is likely that as knowledge of their mechanism of action accumulates, their efficacy can be improved both by rational drug design and by use in combination with other drugs, including interferon.

Trifluoromethylanilines--their effect on DNA synthesis and proliferative activity in parenchymal organs of rats

Reactive isomeric 3- and 4-trifluoromethylanilines (3-,4-TFMA), and control aniline itself, induced the following effects on biosynthesis of DNA in the liver, kidney, thymus and spleen of rats: (a) The administration of 4-TFMA initially suppressed the utilization of labeled thymidine for splenic DNA synthesis during the early prereplicative stage. However, with progressing time the incorporation of the labeled marker began to increase and in 30 h its level exceeded the controls by more than 200%. As expected, aniline administration resulted in mild depression of incorporation during the whole period studied. (b) 4-TFMA caused a significant increase of incorporation of labeled thymidine into DNA thymine also in the thymus. After administration of aniline the utilization of labeled thymidine for the synthesis of DNA thymine in thymus was suppressed during the first 16 h. (c) The dose-response curve showed a linear increase of incorporation in the spleen within the dose range between 0.125 and 0.500 mmol/kg of 4-TFMA. (d) It appears that enhanced incorporation of labeled thymidine into splenic and thymic DNA is a phenomenon specific for compounds bearing the CF3 group on the 4-position of the phenyl ring, such as 4-TFMA and 4-TFMPD. On the contrary, the analogous 3-CF3 substituted derivatives had no effect. Increased incorporation of labeled thymidine into spleen and thymus DNA apparently represents an increased DNA synthesis and cellular proliferation in lymphatic organs. The proliferative response was possibly evoked by the preceding hemolysis or by other toxic effects caused by the drug.

Effect of 3,7-bis-(4-trifluoromethylphenyl)-1,5,3,7-dioxadiazocane on pyrimidine and DNA synthesis in rat organs

The administration of the lipophilic 3,7-bis-(4-trifluoromethylphenyl)- 1,5,3,7-dioxadiazocane (TFMPD) to rats induced the following effects on the biosynthesis of DNA in the liver, kidney, thymus and spleen: (a) The utilization of [3H]thymidine for the synthesis of liver DNA thymine was decreased after the administration of a single dose of the drug. The depression of the specific activities of DNA pyrimidines of liver DNA in experimental groups was observed also after an injection of [14C]orotic acid. (b) A decreased incorporation of labeled thymidine had occurred also in the spleen during the prereplicative period. Thereafter the specific activity of DNA thymine was higher than in the control group. (c) The observed mitogenic response in the spleen showed a protracted effect; after the administration of a single dose of the drug the specific activity of DNA thymine as well as the thymidine kinase activity of spleen cytosol have been rising up to the ninth day. The same holds true for DNA thymine of the thymus; the activity of thymidine kinase was not affected. (d) Both the single and repeated doses of TFMPD had no marked effect on the levels of microsomal cytochromes P-450 and b5 in the liver and kidney.

The stimulatory effect of muramyl dipeptide (MDP) on the proliferation processes in parenchymal organs of rats

The administration of muramyl dipeptide (MDP) in a single dose of 2 mg/kg increased markedly the utilization of 3H-thymidine for the synthesis of DNA thymine in liver, kidney and thymus, while no effect has been observed in spleen. Higher doses of MDP (5 mg and 10 mg) did not further increase the effect, but, on the contrary, in some tissues such as thymus, the effect was abolished. The repeated administration of 1 mg/kg of MDP for 7 days, contrary to single administration, had no effects. The measurement of proliferation activity in the experiment where DNA in the organs has been previously labeled with 3H-thymidine, has shown that the repeated administration of MDP (1 mg/kg) led to a more rapid decay in specific activity of DNA thymine in liver, kidney and thymus, but no effect was seen in spleen. The activities of thymidine kinase in the cytosole fraction of thymus and spleen after a single administration of MDP (1 mg/kg), are influenced in different ways. While in thymus the enzyme activity increased between 16 and 24 h, not being markedly changed in the early intervals, in spleen a significant decrease in phosphorylation of 14C-thymidine can be observed as early as 2 h after administration, and the decrease persists up to 48 h.

Rat spleen cytoplasmic nucleotidase: characterization and its physiological significance

1. A cytoplasmic nucleotidase, which preferably hydrolyzed 5'-dUMP, was investigated in rat spleen. 2. Total activity of the nucleotidase increased about 3-fold in the spleen of anemic rat was caused by phenylhydrazine administration. This increase was repressed by the injection of methotrexate, an inhibitor of DNA synthesis. 3. Activities of heme oxygenase or acid phosphatase did not correlate to the change of the nucleotidase activity. 4. The nucleotidase catalyzed dephosphorylation of 3'(5')-dUMP, 3'(5')-dTMP and 3'-UMP more readily, in the presence of Mg2+. Its optimum pH was around 6.0-6.5. It was stimulated by the addition of deoxyinosine. 5. These catalytic properties and tissue distribution of the enzyme, abundant in the thymus, spleen and intestine, were similar to that of 5'(3')-nucleotidase in rat liver [Fritzson P. (1978) Adv. Enzym. Regul. 16, 43-61]. 6. A possible physiological significance of the nucleotidase is in reutilization of preformed pyrimidine nucleotides.

Effect of clofibrate on DNA synthesis in rat liver and kidney

A single dose of clofibrate (400 mg/kg), given to rats, increased the incorporation of (3H)thymidine into liver DNA, in a period of 20-30 h after administration. However, (3H)thymidine incorporation into hepatic DNA of rats treated repeatedly was identical to that of control animals. After the administration of a single dose of clofibrate a small increase in (3H)thymidine incorporation also occurred in kidney DNA; repeated doses, however, resulted in a marked suppression of labeling.

Re-utilization of pyrimidine nucleotides during rat liver regeneration

The changes in the specific radioactivities of the pool of total acid-soluble uridine nucleotides and of uridine and cytidine components of total cellular and nuclear RNA were monitored in regenerating rat liver for 12 days after partial hepatectomy. Evidence is presented for the re-utilization of pyrimidine nucleotides derived from cytoplasmic RNA degradation for the synthesis of new RNA. The extent of recycling was assessed and the true rate of rRNA turnover determined more accurately. The reutilization of the uridine components of RNA was 7.0%/day during the proliferative and 3.2%/day during the post-proliferative phase, whereas that of the cytidine nucleotides was more pronounced (9.6%/day and 18.1%/day respectively). The results reveal the existence of partial compartmentalization of pyrimidine ribonucleoside triphosphate pools in the nucleus and cytoplasm of rat liver cells.

Depletion of blood plasma cytidine due to increased hepatocellular salvage in D-galactosamine-treated rats

Pyrimidine nucleosides in blood plasma of rats were identified by different procedures, including chemical peak shift methods, before their quantification by reversed-phase high-performance liquid chromatography. The concentrations of uridine, cytidine, and deoxycytidine were 1.0 +/- 0.2, 10.6 +/- 1.9, and 33.4 +/- 5.4 mumol/l, respectively. Six hours after the administration of D-galactosamine, the level of circulating cytidine was severely depressed to 25% of control values; uridine decreased to 54% while deoxycytidine remained unchanged. 24 h after the dose of the amino sugar, the levels of cytidine and uridine returned to control values in blood plasma. Total acid-soluble uridine, cytidine, guanosine, and adenosine was determined by reversed-phase HPLC after treatment of the neutralized acid-soluble supernatant of freeze-clamped rat livers with phosphodiesterase and alkaline phosphatase. Six hours after its administration, D-galactosamine induced a 2.2-fold and a 1.6-fold rise in total acid-soluble uridine and cytidine, respectively. Co-administration of N-(phosphonoacetyl)-L-aspartate, an inhibitor of de novo pyrimidine synthesis, suppressed the increase in total acid-soluble uridine observed after D-galactosamine alone, but was without effect on the enhancement of total cytidine. Three hours after D-galactosamine and 15 min after [2-14C] cytidine, there was a rapid fall of the labeled nucleoside in blood plasma to 49% of control animals accompanied by a 2.8-fold rise in the total radioactivity of rat liver homogenates. From these results it can be concluded that the hepatocellular rise in total acid-soluble cytidine after D-galactosamine, in contrast to the increase in total acid-soluble uridine, originates from the phosphorylation of blood plasma cytidine via the salvage pathway. The depletion of circulating cytidine in the presence of hepatocellular UTP deficiency points to the importance of the liver and the hepatic UTP level for the clearance of blood plasma cytidine.