Separation and quantitation of bacterial ribonucleoside triphosphates extracted with trifluoroacetic acid, by anion-exchange high-performance liquid chromatography

Pseudomonas aeruginosa aspartate transcarbamoylase. Characterization of its catalytic and regulatory properties

Aspartate transcarbamoylase from Pseudomonadaceae is a class A enzyme consisting of six copies of a 36-kDa catalytic chain and six copies of a 45-kDa polypeptide of unknown function. The 45-kDa polypeptide is homologous to dihydroorotase but lacks catalytic activity. Pseudomonas aeruginosa aspartate transcarbamoylase was overexpressed in Escherichia coli. The homogeneous His-tagged protein isolated in high yield, 30 mg/liter of culture, by affinity chromatography and crystallized. Attempts to dissociate the catalytic and pseudo-dihydroorotase (pDHO) subunits or to express catalytic subunits only were unsuccessful suggesting that the pDHO subunits are required for the proper folding and assembly of the complex. As reported previously, the enzyme was inhibited by micromolar concentrations of all nucleotide triphosphates. In the absence of effectors, the aspartate saturation curves were hyperbolic but became strongly sigmoidal in the presence of low concentrations of nucleotide triphosphates. The inhibition was unusual in that only free ATP, not MgATP, inhibits the enzyme. Moreover, kinetic and binding studies with a fluorescent ATP analog suggested that ATP induces a conformational change that interferes with the binding of carbamoyl phosphate but has little effect once carbamoyl phosphate is bound. The peculiar allosteric properties suggest that the enzyme may be a potential target for novel chemotherapeutic agents designed to combat Pseudomonas infection.

Reversed-phase and ion-pair separations of nucleotides, nucleosides and nucleobases: analysis of biological samples in health and disease

Methods for the assay of nucleotides, nucleosides and nucleobases in biological samples in health and disease are reviewed, with emphasis on reversed-phase and ion-pair reversed-phase techniques for their determination. Modes of extraction from biological samples are discussed with respect of the determination of in vivo concentrations. Advantages and limitations of ion-pair reversed-phase chromatography are discussed with examples from biochemistry and clinical chemistry. The capacity of the high-performance capillary electrophoresis is compared with that of ion-pair reversed-phase chromatography.

Characterization of temperature-sensitive cytidine triphosphate synthase mutations in bacteria by high-performance liquid chromatography

Cytidine triphosphate (CTP) synthase catalyzes the last step in pyrimidine ribonucleotide synthesis, namely the formation of CTP from UTP, ATP, and glutamine. Mutants devoid of CTP synthase activity require cytidine for growth and have been designated pyrG in an obligate cdd background. Using a ts mutation blocked in the conversion of UTP to CTP at 43 degrees C, it was demonstrated that the conversion occurs by growing cells at 33 degrees C or below where UTP and CTP pools are normal. Growth at 43 degrees C shuts off the enzyme, while UTP accumulates and CTP is decreased significantly. By now feeding exogenous cytidine the CTP pool can be restored to the level found at the permissive temperature. Intracellular nucleoside triphosphates (CTP and UTP) were separated on a Partisil SAX10 cartridge, using a linear gradient of low buffer (7 mM ammonium dihydrogenphosphate, pH 3.8) to high buffer (250 mM ammonium dihydrogenphosphate, pH 4.5 with 500 mM potassium chloride). Nucleoside triphosphates were also separated after enzymatic conversion of UTP to CTP in solution by cell extracts using ion-pair reversed-phase chromatography on a C18 cartridge eluted with a mixture of 95% buffer A (25 mM ammonium dihydrogenphosphate with 1 mM tetrabutylammonium phosphate, pH 7.0) and 5% buffer B (15% aqueous acetonitrile). Using the two different separation techniques, it was possible to monitor the level of UTP and CTP inside cells as well as the enzymatic conversion of UTP to CTP by the enzyme CTP synthase.

UTP/CTP ratio, an important regulatory parameter for ATCase expression

Intracellular nucleotides of Salmonella typhimurium were separated and quantified by high performance liquid chromatography (HPLC). Wild type and specially constructed strains of S. typhimurium, in which uridine and cytidine nucleotides could be manipulated independently, were used in this study. By varying growth conditions it was possible to create different concentrations of uridine and cytidine nucleotides in the cell. The specific activity of ATCase was determined for each condition. Generally, a direct correlation was found: at high nucleotide (UTP) concentrations, maximal repression of ATCase was usually seen; at low nucleotide (UTP) concentrations ATCase was derepressed. However, it was the ratio of the concentrations of UTP-to-CTP rather than either the concentration of UTP or CTP alone that best determined the extent of ATCase expression. This applied to all conditions in the present work as well as to all conditions in work hitherto reported by others. The ratio of UTP/CTP is proposed as a key regulatory parameter for pyr enzyme expression.