Fardos N.M. Naguib

Circadian rhythm of hepatic uridine phosphorylase activity and plasma concentration of uridine in mice

The activity of hepatic uridine phosphorylase (EC 2.4.2.3.) in male mice (24-29 g) maintained in standardized conditions of 12 hr light (0600-1800 hr) alternating with 12 hr darkness (1800-0600 hr), food and water ad lib., exhibited a circadian rhythm (P less than 0.0001, Cosinor analysis). The peak of enzyme activity (559 +/- 25 pmol/min/mg protein) occurred at 15 hr after light onset (HALO) with the nadir (139 +/- 25 pmol/min/mg protein) at 3 HALO when samples were taken every 4 hr. Female mice showed essentially the same pattern. A circadian rhythm (P less than 0.0001, Cosinor analysis) was also observed when the light-dark cycle was shifted (reverse cycle) so that the lights went on at 2200 hr and off at 1000 hr. Under the reverse cycle condition, there was a corresponding shift in the enzyme activity with a lag period of 3.5 hr in the time of maximum and minimum enzyme activities (i.e. the peak at 11 HALO and the nadir at 23 HALO) after a 2-week adaptation period. The lag period was reduced to 1 hr after 4 weeks of adaptation, and no further change was observed after 6 weeks of adaptation. The plasma concentration of uridine also exhibited a circadian rhythm (P less than 0.0001, Cosinor analysis) with peak concentration (10 microM) occurring at 2 HALO and a nadir (5 microM) at 14 HALO. The circadian rhythm observed in the plasma concentration of uridine is the inverse of that for uridine phosphorylase activity. These results demonstrate that hepatic uridine phosphorylase plays an important role in the regulation of the uridine level in the blood which, in turn, may be involved in the humoral control of sleep by uridine. This may also be of clinical significance in enhancing the antitumor efficacy of the 5-fluorinated pyrimidines by modulating the time of their administration.

Structure-activity relationship of ligands of dihydrouracil dehydrogenase from mouse liver☆

One hundred and five nucleobase analogues were screened as inhibitors of dihydrouracil dehydrogenase (DHUDase, EC 1.3.1.2) from mouse liver. 5-Benzyloxybenzyluracil, 1-deazauracil (2,6-pyridinediol), 3-deazauracil (2,4-pyridinediol), 5-benzyluracil, 5-nitrobarbituric acid and 5,6-dioxyuracil (alloxan) were identified as potent inhibitors of this activity, with apparent Ki values of 0.2, 0.5, 2.1, 3.4, 3.8 and 6.6 microM respectively. Both 5-benzyloxybenzyluracil and 1-deazauracil were also potent inhibitors of DHUDase from human livers. These findings along with an extensive review of literature allowed the formulation of a structure-activity relationship. The binding to DHUDase required intact C2 and C4 oxo groups. Replacement of N1 or N3 by an endocyclic carbon enhanced binding. In contrast, replacement of C5 or C6 by an endocyclic nitrogen abolished binding. Addition of a charged group to C5 and/or C6, and of a hydrophobic group to C5 but not C6 improved the binding.

Synthesis And Biological Activity Op Hydpdxymetflyl Analogs op 5-Benzylacyclouridine and 5-Benzyloxybenzylacycloaridine

Abstract Hydroxymethyl analogs of 5-benzylacyclouridine (BAU) and 5-benzyloxybenzylacyclouridine (BBAU) were synthesized by the condensation of appropriately blocked 2-(chloromethyl)glycerols with substituted 2, 4-dimethoxypyrimidines. The HM derivatives were found to be potent inhibitors of the enzyme uridine phosphorylase and to potentiate significantly the growth-inhibiting action of FdUrd in cell culture.

Synthesis of 5-Benzyl and 5-Benzyloxybenzyl 2,2′-Anhydrouridines and Related Nucleoside Analogs as Inhibitors of Uridine Phosphorylase

Abstract Furanosyl analogs of BAU (5-benzylacyclouridine) and BBAU (5-benzyloxybenzylacyclouridine), two potent inhibitors of uridine phosphorylase, were synthesized and evaluated as potential cancer chemotherapeutic agents. The analogs included ribosides, 2,2′-anhydro nucleosides, arabinosides and deoxyribosides. The anhydrouridine intermediates were potent inhibitors of uridine phosphorylase and good potentiators of FdUrd activity in human tumor cells in culture.

AS Triazine Acyclonucleosides: Potential Inhibitors of Pyrimidine Enzymes1

Abstract Seven as-triazine-3,5-dione acyclonucleosides were synthesized and evaluated as inhibitors of orotate phosphoribosyltransferase (OPRTase, EC 2.4.2.10), orotidine 5′-monophosphate decarboxylase (ODCase, EC 4.1.2.23), uridine phosphorylase (UrdPase, EC 2.4.2.3), and thymidine phosphorylase (dThdPase, EC 2.4.2.4).

Tight binding inhibitors—IX: Kinetic parameters of dihydrofolate reductase inhibited by methotrexate, an example of equilibrium study

Equilibrium studies in the presence of methotrexate (MTX), based on the new theories of tight-binding inhibitors and on classical initial velocity analysis, indicated that the reaction mechanism of dihydrofolate reductase Lactobacillus casei MTX/R is consistent with a rapid equilibrium random bi-bi and that MTX inhibits the enzyme competitively with respect to dihydrofolate but noncompetitively with respect to NADPH. The kinetic parameters determined at pH 7.3 and 23° were: Km for DHF, 9.8 ± 1.3 μM; Km for NADPH, 6.0 ± 1.2 μM; Kd for E·DHF, 5.7 ± 0.7 μM; Kd for E·NADPH, 0.037 ± 0.028 μM; Kd for E·MTX, 1.20 ± 0.15 nM; Kd for E·NADPH·MTX → E·NADPH + MTX, 0.19 ± 0.04 nM; and Kd for E·NADPH·MTX → E·MTX + NADPH, 7.6 ± 5.9 nM; the molar equivalency factor was 3.33 ± 0.44 nM per unit/liter of the enzyme, and the catalytic number was 300 min−.

Effects of N,N-dimethylformamide and sodium butyrate on enzymes of pyrimidine metabolism in cultured human tumor cells

Effects of a 7-day treatment with the maturational agents DMF and sodium butyrate on enzymes of pyrimidine metabolism, growth rate and cell maturation were assessed in 5 human tumor cell lines, ARH-77 (myeloma), K-562 (chronic myeloid leukemia), KG-1 (myeloid leukemia), HL-60 (promyelocytic leukemia) and RWLy-1 (non-Hodgkins lymphoma). DMF lengthened the doubling times of all five cell lines while sodium butyrate lengthened only those of K-562, HL-60 and RWLy-1. Full maturation was induced only in HL-60 by either agent and in K-562 by butyrate. Exposure resulted in a decreased activity of the anabolic enzyme orotate phosphoribosyltransferase (EC 2.4.2.10) and increased activities of the catabolic enzymes thymidine phosphorylase (EC 2.4.2.4) and dihydrouracil dehydrogenase (EC 1.3.1.2). Changes in the amphibolic enzyme, uridine phosphorylase (EC 2.4.2.3) did not follow any apparent pattern. This study indicates that the pattern of pyrimidine metabolism differs between the differentiated and slowly growing, and undifferentiated rapidly growing counterpart of several human tumors, suggesting that enzymes of pyrimidine metabolism can be used as markers for cellular growth and/or maturity.