A method for assaying orotate phosphoribosyltransferase and measuring phosphoribosylpyrophosphate

A Facile Fluorometric Assay of Orotate Phosphoribosyltransferase Activity Using a Selective Fluorogenic Reaction for Orotic Acid

Orotate phosphoribosyltransferase (OPRT) exists as a bifunctional enzyme, uridine 5'-monophosphate synthase, in mammalian cells and plays an important role in pyrimidine biosynthesis. Measuring OPRT activity has been considered important for understanding biological events and development of molecular-targeting drugs. In this study, we demonstrate a novel fluorescence method for measuring OPRT activity in living cells. The technique utilizes 4-trifluoromethylbenzamidoxime (4-TFMBAO) as a fluorogenic reagent, which produces selective fluorescence for orotic acid. To perform the OPRT reaction, orotic acid was added to HeLa cell lysate, and a portion of the enzyme reaction mixture was heated at 80 °C for 4 min in the presence of 4-TFMBAO under basic conditions. The resulting fluorescence was measured using a spectrofluorometer, which reflects the consumption of orotic acid by the OPRT. After optimization of the reaction conditions, the OPRT activity was successfully determined in 15 min of enzyme reaction time without further procedures such as purification of OPRT or deproteination for the analysis. The activity obtained was compatible with the value measured by the radiometric method with [³H]-5-FU as the substrate. The present method provides a reliable and facile measurement of OPRT activity and could be useful for a variety of research fields targeting pyrimidine metabolism.

Pentose phosphate metabolism in Leishmania mexicana

The metabolism of pentose phosphates was studied in Leishmania mexicana promastigotes. Each of the enzymes of the classical pentose phosphate pathway (PPP) has been identified and specific activities measured. Functioning of the PPP was demonstrated in non-growing cells by measuring the evolution of 14CO2 from [1-14C]D-glucose and [6-14C]D-glucose under normal conditions and also under selective stimulation of the PPP by exposure to methylene blue. The proportion of glucose which passes through the PPP increases in the latter condition, thus suggesting a protective role against oxidant stress. The incorporation into nucleic acids of ribose 5-phosphate provided via either glucose or free ribose was also determined. Results indicate that the PPP enables glucose to serve as a source of ribose 5-phosphate in nucleotide biosynthesis. Moreover, free ribose is incorporated efficiently, implying the presence of a ribose uptake system and also of ribokinase. Ribose was shown to be accumulated by a carrier mediated process in L. mexicana promastigotes and ribokinase activity was also measured in these cells.

Dihydroorotate (dhout) and orotate (orout) utilizer mutants in yeast: identification of the dhout mutation and allelism of the DHO and URE2 genes

We induced by UV mutagenesis a series of yeast mutants that were able to utilize dihydroorotic (dhout) and orotic acid (orout) as precursors for pyrimidine biosynthesis. These recessive mutations defined three complementation groups named dhout, orout1 and orout2. The wild-type allele of the gene responsible for dihydroorotate utilization was cloned using the sensitivity of the dhout mutant to 5-fluoroorotate. The DHO gene was sequenced and found to be identical to the URE2 gene. The dhout mutation resulted from the introduction of a stop codon instead of a glutamine at position 59, which led to the production of a truncated Ure2p. Therefore, the URE2 and DHO genes are alleles in yeast.

A library of monoclonal antibodies to Escherichia coli K-12 pyruvate dehydrogenase complex. A biochemical analysis and their ability to inhibit the enzyme complex

A library of monoclonal antibodies to K-12 Escherichia coli pyruvate dehydrogenase complex (PDHc) and its pyruvate decarboxylating (EC 1.2.4.1; E1) subunit is reported. 21 monoclonal antibodies were generated, and 20 were investigated, of which 9 were elicited to PDHc and 11 to pure E1 subunit; 19 were of the IgG1 isotype and one of the IgG3 isotype. According to an enzyme immunoassay, all 20 of the monoclonal antibodies bound the PDHc, and 17 bound the E1 subunit. According to Western blot analysis, 14 of the 19 monoclonal antibodies bound to the E1 subunit. The monoclonal antibodies inhibited PDHc from 0 to > 98%. The six monoclonal antibodies that displayed greater than 30% inhibition of E. coli PDHc were unable to inhibit porcine heart PDHc nor did they bind porcine heart PDHc according to dot blot analysis. Radiolabeling gave binding constants ranging from 5 to 10 x 10(8) M-1 on these six monoclonal antibodies, with greater than 80% of maximal inhibition achieved in less than 1 min. One of the six, 18A9, gave > 98% inhibition, required two antibodies/E1 subunit for maximum inhibition, and was shown to be a non-competitive inhibitor. Monoclonal antibody 15A9 was shown to counteract GTP-induced inhibition, while 1F2 influenced the conformation of E1, allowing two antibodies, which did not previously bind E1, to bind to it. A new mechanism-based kinetic assay is presented that is specific for the E1 component of 2-keto acid dehydrogenases. This assay confirmed that the three most strongly inhibitory monoclonal antibodies specifically inhibited the E1 function while antibody 1F2 led to enhanced activity, suggesting an induced conformational change in PDHc or in E1.

An immunochemical assay model system for the sensitive detection of pyruvate dehydrogenase complex (PDHc) and its decarboxylating subunit pyruvate dehydrogenase (E1)

An immunochemical enzyme immunoassay model system was developed and compared for maximum sensitivity with a radioimmunoassay method and the classic enzyme activity method for the detection of pyruvate dehydrogenase complex (PDHc) and its decarboxylating subunit, pyruvate dehydrogenase (E1), isolated from Escherichia coli. Cross-linked large molecular weight antibody-enzyme conjugate systems are compared with heterobifunctional singular antibody conjugates substituted with high levels of horseradish peroxidase. Both polyclonal and monoclonal antibodies generated to the Escherichia coli PDHc and E1 antigens were used to develop a double-antibody sandwich microtiter plate enzyme-linked immunosorbent assay. It is demonstrated that a double sandwich immunochemical assay system can be quantitative for PDHc, can detect PDHc in crude cell lysates and has levels of sensitivity of 2.0.10(-16) mol for the detection of PDHc. This assay model system provides specific antibody selection criteria and coupling methods needed to select specific antisera that cross-react with human PDHc. This rapid and sensitive immunochemical assay method clearly demonstrates that sensitive mass assay systems can be developed for the detection of PDHc. Different from Western blot, this methodology could be used to generate mass assays which could be applied to the rapid detection of mammalian antigens (employing the corresponding antibodies) implicated in a number of pyruvate dehydrogenase deficiencies associated with human disorders.

Anti-proliferative activity of L-651,582 correlates with calcium-mediated regulation of nucleotide metabolism at phosphoribosyl pyrophosphate synthetase

L-651,582, 5-amino-[4-(4-chlorobenzoyl)-3,5-dichlorobenzyl]-1, 2,3-triazole-4-carboxamide, is an antiproliferative and antiparasitic agent which inhibits nucleotide metabolism in mammalian cells. The drug equivalently inhibited 3H-hypoxanthine, 14C-adenine, and 14C-formate incorporation into nucleotide pools in Madin-Darby bovine kidney (MDBK) cells, suggesting depletion of the supply of phosphoribosyl pyrophosphate, (PRPP), required for each of these independent pathways. Inhibition of nucleotide metabolism correlated with inhibition of proliferation for three cell types with differing sensitivities toward the drug. L-651,582 inhibited incorporation of 3H-hypoxanthine into nucleotide pools with either glucose, uridine, or ribose as carbon source suggesting a block at PRPP synthetase, rather than a block in a pathway supplying ribose-5-phosphate. PRPP synthetase was not inhibited directly by the compound, indicating regulation of the enzyme in intact cells. Drug treatment did not kill cells but reduced the fraction of cells in S and G2/M while increasing the population in G1. Inhibition of uptake of 45Ca was demonstrated at concentrations identical to those required for inhibition of nucleotide metabolism or proliferation. Inhibition of cellular PRPP biosynthesis rates were also observed using EGTA to lower calcium levels. These data suggest a previously unrecognized link between calcium entry, the regulation of nucleotide biosynthesis at PRPP synthetase, and the rate of proliferation of mammalian cells.