Regulation of hamster carbamoyl-phosphate synthase II by 5-phospho-alpha-D-ribosyl 1-diphosphate and uridine 5'-triphosphate

Metabolic effects of thiopurine derivatives against human CCRF-CEM leukaemia cells

BACKGROUND and aims. To compare the metabolic effects induced by the anticancer drugs, 6-mercaptopurine (6-MP), 6-thioguanine (6-TG) and 6-methylmercaptopurine riboside (MMPR), which may inhibit the de novo biosynthesis of purine nucleotides or be mis-incorporated into DNA or RNA.

METHODS

Leukaemia cells were grown in culture, exposed to a thiopurine and cell extracts were analyzed for NTPs, dNTPs, drug metabolites and P-Rib-PP.

RESULTS

In leukaemia cells, 6-MP was converted to MPR-MP, thio-XMP, thio-GMP, thio-GDP and thio-GTP. Metabolites of 6-TG included thio-XMP, thio-GMP, thio-GDP and thio-GTP, while MMPR-MP was the only major metabolite of MMPR, MMPR (25 microM, 4 h) induced a 16-fold increase in P-Rib-PP and 6-MP (25 microM, 4 h) induced a delayed 5.2-fold increase. MPR-MP, thio-GMP and MMPR-MP are inhibitors of amido phosphoribosyltransferase from leukaemia cells with Ki values of 114 +/- 7.10 microM, 6.20 +/- 2.10 microM and 3.09 +/- 0.30 microM, respectively.

CONCLUSION

The nucleoside-5'-monophosphate derivatives of the 3 thiopurines inhibit amido phosphoribosyltransferase in growing leukaemia cells but there is also an initial inhibition of the further conversion of IMP in the pathway. In growing cells, MMPR acts solely as an inhibitor of de novo purine biosynthesis while 6-TG and to a lesser extent, 6-MP, are converted to significant concentrations of di- and tri-phosphate derivatives which may have other mechanisms of cytotoxicity.

Cytotoxic effects of inhibitors of de novo pyrimidine biosynthesis upon Plasmodium falciparum

The malarial parasite Plasmodium falciparum can only synthesize pyrimidine nucleotides via the de novo pathway which is therefore a suitable target for development of antimalarial drugs. New assay procedures have been developed using high-pressure liquid chromatography (HPLC) which enable concurrent measurement of pyrimidine intermediates in malaria. Synchronized parasites growing in erythrocytes were pulse-labeled with [14C]bicarbonate at 6-h intervals around the 48-h asexual life cycle. Analysis of malarial extracts by HPLC showed tht incorporation of [14C]bicarbonate into pyrimidine nucleotides was maximal during the transition from trophozoites to schizonts. The reaction, N-carbamyl-L-aspartate-->L-dihydroorotate (CA-asp-->DHO) catalyzed by malarial dihydroorotase is inhibited by L-6-thiodihydroorotate (TDHO) in vitro (Ki = 6.5 microM), and TDHO, as the free acid or methyl ester, induces a major accumulation of CA-asp in malaria. Atovaquone, a naphthoquinone, is a moderate inhibitor of dihydroorotate dehydrogenase in vitro (Ki = 27 microM) but induces major accumulations of CA-asp and DHO. Pyrazofurin induces accumulation of orotate and orotidine in malaria, consistent with inhibition of orotidine 5'-monophosphate (OMP) decarboxylase with subsequent dephosphorylation of the OMP accumulated. Although TDHO, atovaquone, and pyrazofurin arrest the growth of P. falciparum, only moderate decreases in UTP, CTP, and dTTP were observed. 5-Fluoroorotate also arrests the growth of P. falciparum with major accumulations of 5-fluorouridine mono-, di-, and triphosphates and the most significant inhibition of de novo biosynthesis of pyrimidine nucleotides.

Regulation of the mammalian carbamoyl-phosphate synthetase II by effectors and phosphorylation. Altered affinity for ATP and magnesium ions measured using the ammonia-dependent part reaction

We have measured the 'core' mammalian carbamoyl-phosphate synthetase II (CPSII) activity, using NH4Cl as the nitrogen-donating substrate and trapping carbamoyl phosphate as urea through its reaction with ammonium ions. When ATP and magnesium ion concentrations are close to those found in the cell, the substrate saturation curves for ammonia and bicarbonate are hyperbolic, giving Km (NH3) values of 166 microM at high ATP concentrations and 26 microM at low ATP concentrations, while the Km (bicarbonate) is 1.4 mM at both ATP concentrations used. These values for the Km (NH3) are lower than previously reported for CPS II, and closer to the values for the mitochondrial counterpart. The Km for ammonia and bicarbonate are not altered by phosphorylation of the multienzyme polypeptide CAD, which contains the first three enzyme activities of pyrimidine biosynthesis. The CPS II activity is lower with an excess of either ATP or magnesium ions, causing the apparently sigmoid dependence of activity upon ATP concentration to be enhanced at low concentrations of free magnesium ions. The feedback inhibitor, UTP, acts by stabilising a state with a low affinity for magnesium ions and for ATP. In the presence of the activator, 5-phosphoribosyl diphosphate (PRibPP), the enzyme has a higher affinity for magnesium ions and thus the ATP dependence of the activity is hyperbolic. Phosphorylation of CAD similarly activates the CPS II enzyme by increasing the affinity for magnesium ions and by pushing the equilibrium away from the low-affinity UTP-stabilised state. Using our improved assay procedure, we observe a very large activation by PRibPP of carbamoylphosphate synthesis at low concentrations of magnesium ions, and we find that unlike UTP, the activator PRibPP is able to act on the phosphorylated enzyme.

Nucleotide ligands protect the inter-domain regions of the multifunctional polypeptide CAD against limited proteolysis, and also stabilize the thermolabile part-reactions of the carbamoyl-phosphate synthase II domains within the CAD polypeptide

Improved methodologies are described which allow the measurement of the part-reactions, with glutamine or ammonia as nitrogen donor, of mammalian carbamoyl-phosphate synthase II (EC 6.3.5.5) through the incorporation of [14C]bicarbonate into either carbamoyl phosphate or carbamoylaspartate. The enzyme is part of the multifunctional polypeptide (CAD) which also comprises the pyrimidine-biosynthetic enzymes aspartate transcarbamoylase (EC 2.1.3.2) and dihydro-orotase (EC 3.5.2.3). The conformational stability of the carbamoyl-phosphate synthase was investigated through the inactivation of the part-reactions which occurred during incubation at 37 degrees C. The domain involved in the removal of the amide N from glutamine was more thermolabile than the ammonia-dependent synthase moiety. The former activity was stabilized in the presence of sodium aspartate or MgATP, whereas the latter was stabilized by MgATP and MgUTP. Binding of MgUTP and MgATP to CAD restricted the initial proteolysis by trypsin and elastase of one or both regions linking the carbamoyl-phosphate synthase domain to the other major domains. A model is described to account for both aspects of nucleotide binding to CAD; these stabilizing effects may be important in the cell, where similar concentrations of nucleotides are found.