Mechanism of allopurinol-mediated inhibition and stabilization of human orotate phosphoribosyltransferase and orotidine phosphate decarboxylase

Depletion of plasma thymidine results in growth retardation and mitochondrial myopathy in mice overexpressing human thymidine phosphorylase

Plasma thymidine levels in rodents are higher than in other mammals including humans, possibly due to a different pattern and lower level of thymidine phosphorylase expression. Here, we generated a novel knock-in (KI) mouse line with high systemic expression of human thymidine phosphorylase to investigate this difference in nucleotide metabolism in rodents. The KI mice showed growth retardation around weaning and died by 4 weeks of age with a decrease in plasma thymidine level compared with the litter-control WT mice. These phenotypes were completely or partially rescued by administration of the thymidine phosphorylase inhibitor 5-chloro-6-(2-iminopyrrolidin-1-yl) methyl-2,4(1H,3H)-pyrimidinedione hydrochloride or thymidine, respectively. Interestingly, when thymidine phosphorylase inhibitor administration was discontinued in adult animals, KI mice showed deteriorated grip strength and locomotor activity, decreased bodyweight, and subsequent hind-limb paralysis. Upon histological analyses, we observed axonal degeneration in the spinal cord, muscular atrophy with morphologically abnormal mitochondria in quadriceps, retinal degeneration, and abnormality in the exocrine pancreas. Moreover, we detected mitochondrial DNA depletion in multiple tissues of KI mice. These results indicate that the KI mouse represents a new animal model for mitochondrial diseases and should be applicable for the study of differences in nucleotide metabolism between humans and mice.

UMP synthase activity expressed in deficient hamster cells by separate transferase and decarboxylase proteins or by linker-deleted bifunctional protein

Segments of the human UMP synthase cDNA coding for the orotate phosphoribosyl transferase (OPRT) and orotidylate decarboxylase (ODC) domains of the bifunctional protein UMP synthase were produced by polymerase chain reaction techniques and cloned into a eukaryotic expression vector. The separate OPRT and ODC vectors, along with a selectable marker, were cotransfected into UMP synthase-deficient hamster cells (Urd-C) that require exogeneous uridine for growth. Transfected Urd-C cells surviving selection in media without added uridine were isolated and designated transferase decarboxylase Urd-C (TDU). All of the selected colonies contained DNA corresponding to the OPRT and ODC expression vectors. Two cell lines (TDU3 and TDU5) integrated many more copies of the OPRT and ODC vectors into their genomes compared to the other TDU lines. A 28.6-kDa ODC protein band and a 24.4-kDa OPRT band were detected on western blots with UMP synthase-specific polyclonal antiserum. The OPRT activity of the TDU lines was up to 8.7 times the OPRT activity of control CHL cells, and the ODC activity was up to 12.5 times control levels. Both OPRT and ODC activities in the monofunctional proteins were less heat stable than in the bifunctional UMP synthase protein. The monofunctional OPRT protein was less stable than the ODC protein at 45 degrees C. Growth of transfected cells in 6-azauridine resulted in striking increases in activity and temperature stability for the monofunctional ODC protein. A UMP synthase bifunctional protein was constructed with a deletion of the suspected linker region joining the two catalytic domains. The linker-deleted UMP synthase showed no significant change in either OPRT or ODC activity or temperature stability. The increased stability of the bifunctional protein may be a factor in its evolutionary selection in mammalian cells.

Uridine-5'-phosphate synthase: evidence for substrate cycling involving this bifunctional protein

Uridine 5'-phosphate (UMP) synthase contains two sequential catalytic activities for the synthesis of orotidine 5'-phosphate (OMP) from orotate (EC 2.4.2.10, orotate phosphoribosyltransferase) and the decarboxylation of OMP to form UMP (EC 4.1.1.23, OMP decarboxylase). Previous kinetic studies had indicated that partial channeling of OMP might occur [T.W. Traut and M.E. Jones (1977) J. Biol. Chem. 252, 8374-8381]; in the presence of a nucleotidase, there was no measurable formation of orotidine from OMP under conditions where OMP was maintained at a steady-state concentration [T.W. Traut (1980) Arch. Biochem. Biophys. 200, 590-594]. Recently claims were made that (i) the steady-state activities of UMP synthase could be modeled by Michaelis-Menten kinetics, and (ii) the nucleotidase activity in Ehrlich ascites cells was insufficient to degrade any significant amount of OMP [R.W. McClard and K.M. Shokat (1987) Biochemistry 26, 3378-3384]. The present studies show that UMP synthase has cooperative kinetics toward OMP, and that a substrate cycle involving orotate phosphoribosyltransferase, cytoplasmic nucleotidase, and uridine phosphorylase maintains the cyclic interconversion: orotate----OMP----orotidine----orotate, etc. It is therefore the complex steady-state kinetics of UMP synthase in the presence of OMP, and the existence of a substrate cycle that account for the results which were interpreted as channeling in the earlier studies.

Structure-activity relationship of pyrimidine base analogs as ligands of orotate phosphoribosyltransferase

Eighty pyrimidine base analogs were evaluated as inhibitors of mouse liver orotate phosphoribosyltransferase (OPRTase, EC 2.4.2.10). Based on these findings and an extensive literature review, a structure-activity relationship has been formulated for the binding of pyrimidine base analogs to OPRTase. This study provides a basis for the rational design of new inhibitors of this enzyme, and several such compounds are proposed. Additionally, 4,6-dihydroxypyrimidine has been found to be a potent OPRTase inhibitor. Eleven OPRTase inhibitors were also evaluated as inhibitors of orotidine 5'-monophosphate decarboxylase (ODCase, EC 4.1.2.23). 5-Azauracil, 5-azaorotate, and barbituric acid inhibited ODCase significantly only after preincubation with PRPP and MgCl2 in the presence of cytosol.

Pyrimidine metabolism in peripheral and phytohemagglutinin-stimulated mammalian lymphocytes

1. Activity of uridine kinase was very low in ovine lymphocytes and in those of some pigs. Lymphocytes of other pigs showed a significantly higher activity of this enzyme. Activity of uridine kinase in lymphocytes of man, horse and cattle was intermediate. 2. Activity of uridine phosphorylase was higher than that of uridine kinase with lymphocytes of all species. 3. Activity of uridine kinase in equine lymphocytes increases at PHA-stimulation and also in porcine lymphocytes with a low activity at the start of the culture. Activity of uridine kinase decreased in porcine lymphocytes with a high activity at the start of the culture. 4. Activity of uridine phosphorylase increases at PHA-stimulation with equine and porcine lymphocytes and during culturing of non-stimulated porcine lymphocytes. 5. Activities of OPRT and ODC decrease in cultures of porcine lymphocytes with and without PHA. 6. Activity of OPRT in lysates of porcine lymphocytes is inhibited by purine nucleosides and by guanine and pyrimidine nucleotides.

Aminoimidazole carboxamide ribonucleoside toxicity: a model for study of pyrimidine starvation

Aminoimidazole carboxamide ribonucleoside (AIC-R), a purine precursor, has biphasic effects on the growth of Chinese hamster fibroblasts. At 200 microM AIC-R cell growth is almost completely arrested, while at 50 and 700 microM AIC-R cell growth is comparable to that observed in the absence of nucleoside. The growth inhibition produced by AIC-R is the consequence of inhibition of the orotate phosphoribosyltransferase-orotidylic decarboxylase (OPRT-ODC) reactions, as evidenced by a 87% reduction in the intracellular concentrations of UTP and CTP, accumulation of orotate in the medium, and restoration of normal growth by inclusion of 100 microM uridine in the medium. Inhibition of pyrimidine nucleotide synthesis at 200 microM AIC-R is associated with an 82% reduction in the intracellular concentration of PP-ribose-P and a 150% increase in the concentration of purine nucleotides. Restoration of cell growth to a normal rate at 700 microM AIC-R--a condition under which PP-ribose-P remains depressed and purine nucleotide concentrations are also depressed (40% of control)--and absence of toxicity at 50 microM AIC-R--a condition under which purine nucleotide concentrations are increased by 150% and PP-ribose-P concentration is normal--suggest that the inhibition of OPRT-ODC observed at 200 microM AIC-R is caused by the combination of the reduction in PP-ribose-P and increase in purine nucleotides. These studies provide a better understanding of the control of the OPRT-ODC reactions in the cell and provide additional insight into the basis of pyrimidine starvation induced by purine nucleosides.

Dependence of the aggregation and conformation states of uridine 5'-phosphate synthase on pyrimidine nucleotides. Evidence for a regulatory site

Uridine 5'-phosphate (UMP) synthase is multifunctional protein that contains the last two enzyme activities of the de novo pathway for UMP biosynthesis, orotate phosphoribosyltransferase (EC 2.4.2.10) and orotidine-5'-phosphate (OMP) decarboxylase (EC 4.1.1.23). We have previously reported that UMP synthase from mouse Ehrlich ascites cells can exist in at least three distinct aggregation and/or conformation states, as measured by changes in sedimentation through sucrose gradients [Traut, T. W., & Jones, M. E. (1979) J. Biol. Chem, 254, 1143-1150]. The major sedimenting species were a 3.6S monomer, a 5.1S dimer, and a 5.6S species. The formation of the 5.1S dimer from the 3.6S monomer occurs in the presence of ligands that are competitive inhibitors at the OMP decarboxylase catalytic site. This paper presents evidence for a regulatory site, distinct from either of the two catalytic sites, which appears to mediate the conversion of the 5.1S dimer to the 5.6S form upon binding certain pyrimidine nucleotides (OMP, UMP, and 6-azaUMP). Since UMP synthase sediments predominantly as a dimer in the presence of substrates, regulation of the aggregation/conformation state of this multifunctional protein may be physiologically significant.

The urinary excretion of orotic acid and orotidine, measured by an isotope dilution assay

Unknown concentrations of orotic acid can be measured by competition with a known amount of [carboxyl-14C]orotic acid for reaction with a limiting amount of phosphoribosylpyrophosphate in the presence of orotate phosphoribosyltransferase and orotidine monophosphate decarboxylase. The dilution of the specific radioactivity in the product 14CO2 is a sensitive and accurate measure of the amount of orotic acid present in the sample. Orotidine can also be determined after hydrolytic cleavage to orotic acid. The method was used to measure orotic acid and orotidine in urine samples from newborns, healthy controls and patients with gout or deficiency of hypoxanthine-guanine phosphoribosyltransferase receiving allopurinol. Urinary excretion of orotic acid and orotidine in newborns was similar whether the infants were breast-fed or received milk powder. The excretion of orotidine was increased in all patients receiving allopurinol. After allopurinol administration orotic acid excretion was increased in gouty patients but close to normal values in patients with deficiency of hypoxanthine-guanine phosphoribosyltransferase. The results are discussed in relation to the mechanism by which allopurinol inhibits pyrimidine metabolism.

Chemical evolution from hydrogen cyanide: photochemical decarboxylation of orotic acid and orotate derivatives

Irradiation of solutions at pH 7 to pH 8.5 of orotic acid, orotidine, and orotidine 5'-phosphate with light at 254 nanometers yields the corresponding uracil derivative via the singlet excited state. This reaction completes a plausible prebiotic synthesis of uracil and its derivatives starting from HCN as the only carbon source.

Increased intracellular phosphoribosylpyrophosphate and accelerated orotic acid decarboxylation in a mouse cell line resistant to purine and pyrimidine ribonucleosides

A line of mouse fibroblasts (A9AU-1), originally selected for growth in the presence of 6-azauridine, has been found to be resistant to cytotoxic concentrations of adenosine, guanosine, and thymidine. A9AU-1 cells convert orotic acid to uridine 5'-monophosphate at twice the rate of the A9P line from which the A9AU-1 clone was selected. The resistant cells also excrete purines, synthesized de novo, into the medium at an increased velocity. The average intracellular 5-phosphoribosyl-1-pyrophosphate (PRPP) concentration of the resistant line is 45% higher than that of the parental line. The elevated PRPP concentration is likely to be responsible for both the apparent acceleration of pyrimidine synthesis and the increased excretion of purines into the growth medium; it might also account, by one of the several possible mechanisms, for the resistance of the cells to cytotoxic concentrations of the various nucleosides.