Subunit structure of the orotate phosphoribosyltransferase--orotidylate decarboxylase complex from human erythrocytes

Pyrimidine metabolism in schistosomes: A comparison with other parasites and the search for potential chemotherapeutic targets

Schistosomes are responsible for the parasitic disease schistosomiasis, an acute and chronic parasitic ailment that affects >240 million people in 70 countries worldwide. It is the second most devastating parasitic disease after malaria. At least 200,000 deaths per year are associated with the disease. In the absence of the availability of vaccines, chemotherapy is the main stay for combating schistosomiasis. The antischistosomal arsenal is currently limited to a single drug, Praziquantel, which is quite effective with a single-day treatment and virtually no host-toxicity. Recently, however, the question of reduced activity of Praziquantel has been raised. Therefore, the search for alternative antischistosomal drugs merits the study of new approaches of chemotherapy. The rational design of a drug is usually based on biochemical and physiological differences between pathogens and host. Pyrimidine metabolism is an excellent target for such studies. Schistosomes, unlike most of the host tissues, require a very active pyrimidine metabolism for the synthesis of DNA and RNA. This is essential for the production of the enormous numbers of eggs deposited daily by the parasite to which the granulomas response precipitates the pathogenesis of schistosomiasis. Furthermore, there are sufficient differences between corresponding enzymes of pyrimidine metabolism from the host and the parasite that can be exploited to design specific inhibitors or "subversive substrates" for the parasitic enzymes. Specificities of pyrimidine transport also diverge significantly between parasites and their mammalian host. This review deals with studies on pyrimidine metabolism in schistosomes and highlights the unique characteristic of this metabolism that could constitute excellent potential targets for the design of safe and effective antischistosomal drugs. In addition, pyrimidine metabolism in schistosomes is compared with that in other parasites where studies on pyrimidine metabolism have been more elaborate, in the hope of providing leads on how to identify likely chemotherapeutic targets which have not been looked at in schistosomes.

A nonradioactive high-performance liquid chromatographic microassay for uridine 5'-monophosphate synthase, orotate phosphoribosyltransferase, and orotidine 5'-monophosphate decarboxylase

A novel nonradioactive, microassay method has been developed to determine simultaneously the two enzymatic activities of orotate phosphoribosyltransferase (OPRTase) and orotidine 5'-monophosphate decarboxylase (ODCase), either as a bifunctional protein (uridine 5'-monophosphate synthase, UMPS) or as separate enzymes. Substrates (orotate for OPRTase or orotidine 5'-monophosphate for ODCase) and a product (UMP) of the enzymatic assay were separated by high-performance liquid chromatography (HPLC) using a reversed-phase column and an ion-pairing system; the amount of UMP was quantified by dual-wavelength uv detection at 260 and 278 nm. This HPLC assay can easily detect picomole levels of UMP in enzymatic reactions using low specific activity UMPS of mammalian cell extracts, which is difficult to do with the other nonradioactive assays that have been described. The HPLC assay is suitable for use in protein purification and for kinetic study of these enzymes.

Sequence analysis of the DdPYR5-6 gene coding for UMP synthase in Dictyostelium discoideum and comparison with orotate phosphoribosyl transferases and OMP decarboxylases

A Dictyostelium discoideum DNA fragment that complements the ura3 and the ura5 mutants of Saccharomyces cerevisiae has been sequenced. It contains an open reading frame of 478 codons capable of encoding a polypeptide of molecular weight 52475. This gene, named DdPYR5-6, encodes a bifunctional protein composed of the orotate phosphoribosyl transferase (OPRTase) and the orotidine-5'-phosphate decarboxylase (OMPdecase) domains described for UMP synthase in mammals. The existence of separate domains for the two activities was suspected because deletion of the N-terminal coding segment of the gene eliminated the ura5 but not the ura3 complementing activity. We have now confirmed that the two parts of the open reading frame share homology with known OPRTase and OMPdecase sequences. Several blocks of sequence are conserved among OPRTase from bacteria, fungi and slime mold and one of them corresponds to the consensus sequence for phosphoribosylbinding sites. The OMPdecase domain shows extensive similarity with the yeast and Neurospora crassa enzymes, suggesting that they have evolved from an ancestral gene which was fused to the OPRTase gene in D. discoideum. It is less related to the bacterial enzyme but all these sequences present conserved blocks of homology which could identify the active site. The codon usage is strongly biased in a manner similar to that found for other D. discoideum genes. The flanking DNA contains homopolymers of A and T and alternating sequences that are characteristic of the gene organization in D. discoideum.

Purine-mediated growth inhibition caused by a pyrE mutation in Escherichia coli K-12

A purine-sensitive phenotype results from a previously described mutation in the structural gene (pyrE) for orotate phosphoribosyltransferase (OPT) in Escherichia coli K-12. OPT from both the mutant and the wild-type was partially inhibited by adenine and adenosine, although other purine derivatives were not effective for this inhibition. The Km values of the mutant OPT were 580 and 760 microM for orotate and 5'-phosphoribosyl-1'-pyrophosphate (PRib-PP), respectively, whereas the corresponding values for the wild-type OPT were 40 and 60 microM. The intracellular level of PRib-PP was decreased to less than 15% of the normal level when purine derivatives were added to exponentially growing cultures of both the parent and mutant strains. However, this decrease of the PRib-PP level was not found in strains derived from the mutant, in which the purine-sensitive phenotype was suppressed by a secondary mutation. The purine-sensitive phenotype was caused by retardation of the pyrimidine de novo pathway, when the intracellular level of PRib-PP was diminished by exogenously supplied purine derivatives.

Enzymes of uridine 5'-monophosphate biosynthesis in Schistosoma mansoni

In Schistosoma mansoni, the major product of in vitro orotate metabolism was orotidine 5'-monophosphate (OMP), whereas in mouse liver it was UMP. In contrast to mammalian cells, OMP appeared not to be 'channeled' from orotate phosphoribosyltransferase to OMP decarboxylase in S. mansoni, resulting in substantial degradation of OMP to orotidine. Significant differences were observed in the inhibitor specificity of phosphoribosyltransferase between S. mansoni and mouse liver, indicating that this enzyme may be a potential chemotherapeutic target in S. mansoni. Two distinct phosphoribosyltransferases were found in S. mansoni. One enzyme, having the higher molecular weight, utilized orotate, 5-fluorouracil and uracil as substrates, while the other only orotate. Both enzymes were inhibited by 5-azaorotic acid (oxonic acid) but only the 'orotate-specific' enzyme was inhibited by 4,6-dihydroxypyrimidine. OMP decarboxylase activity co-eluted with both phosphoribosyltransferases from Sephadex G-100 gel chromatography. We suggest that phosphoribosyltransferase in S. mansoni plays a role in both de novo UMP biosynthesis as well as in the salvage of uracil and uridine.

Structural gene coding for multifunctional protein carrying orotate phosphoribosyltransferase and OMP decarboxylase activity is located on long arm of human chromosome 3

In humans, deficiency in the last two enzymes of UMP biosynthesis, orotate phosphoribosyltransferase (OPRT) and OMP decarboxylase results in the inborn error of metabolism hereditary orotic aciduria, type 1. In this manuscript, we present immunologic, molecular, biochemical, and genetic evidence that the gene coding for this set of enzymatic activities is located on the long arm of human chromosome 3. The evidence presented here is consistent with both these activities being carried on the same multifunctional protein in mammalian cells. These studies allow further genetic analysis of human chromosome 3, confirming that human markers ACY-1, previously assigned to 3p21, and beta-gal, previously assigned by others to the region 3(p21-q21), must be in the region 3 (cen-p21) and confirming the regional assignment of a human DNA segment, D3S1, to 3q12. The significance of these studies to genetic analysis of genes on human chromosome 3, some of which appear to play a role in some forms of malignancy, is discussed.

Potential for ambiguity in the interpretation of immunochemical studies on multienzymic proteins. The case of uridylate synthase

Rabbit antibodies directed against homogeneous uridylate synthase multienzyme from mouse Ehrlich ascites carcinoma precipitate both the orotidine-5'-monophosphate decarboxylase (EC 4.1.1.23) and orotate phosphoribosyltransferase (EC 2.4.2.10) activities of mouse and human erythrocyte uridylate synthase. When the partially purified human enzyme is used as antigen the two activities coprecipitate with the same apparent titer; however, when the mouse carcinoma protein was studied under the same conditions the decarboxylase activity immunoprecipitated with significantly higher avidity than did the transferase activity. Since the mouse multienzyme has been shown to be a single polypeptide that contains both activities (McClard, R.W., Black, M.J., Livingstone, L.R. and Jones, M.E. (1980) Biochemistry 19, 4699-4706), these results were, at face value, surprising. However, when the mouse orotate phosphoribosyltransferase activity (which is largely lost upon dilution into the immunoassay medium) was stabilized with 5-phosphoribosyl 1-pyrophosphate, both enzyme activities displayed the same apparent antibody titer. The immunochemical studies indicate that the antibodies, as a population, preferentially bind to a form or forms of the enzyme which contain(s) denatured transferase domains. A calculation based on a simple model yields a value of approximately 100 for the relative selectivity of the antibody for the denatured form of uridylate synthase. These results illustrate an ambiguity that is inherent in the interpretation of immunochemical studies on such multienzymic proteins; that is, it is possible to conclude incorrectly that two enzyme activities are not functionally associated if one of the catalytic domains is particularly unstable and thereby displays greater immunoreactivity for the specific antiserum.

Genetic complementation and enzyme correlates at the locus encoding the last two steps of de novo pyrimidine biosynthesis in Drosophila melanogaster

Eight mutations of the rudimentary-like (r-l) locus were isolated following mutagenesis with ethylmethanesulfonate and inter se crosses revealed three basic complementation groups, using the wing phenotype as an index of complementation. One group consists of three entirely noncomplementing mutants that each specify severe reductions in levels of both r-l-encoded enzymes, orotate phosphoribosyltransferase (OPRTase) and orotidylate decarboxylase (ODCase). The other two groups consist of complementing mutants, such that any member of one group fully complements all members of the other group. One of these groups consist of two mutants that each specify severely reduced OPRTase, but normal ODCase. The other group consists of three mutants that specify severe OPRTase and ODCase reductions in homoallelic flies, but that appear to contribute OPRTase in certain heteroallelic genotypes. It is concluded that the reciprocal and complementing enzymatic phenotypes of mutants in these two groups account for most instances of genetic trans complementation among r-l mutants. These findings are discussed relative to extant information on OPRTase and ODCase in animals and an hypothesis is developed that the r-l locus encodes a single polypeptide product that contains both enzyme activities.

Structural features of the phosphoribosyltransferases and their relationship to the human deficiency disorders of purine and pyrimidine metabolism

Similarities in the physical and chemical properties of the phosphoribosyltransferase family of enzymes suggest that they may share common structural features as observed in other functionally related proteins. The unusually high incidence of structural gene mutations of these enzymes in man are associated with several metabolic diseases of purine and pyrimidine metabolism. It is proposed that these disorders are the consequence of structural mutations to an architectural domain common to all of the phosphoribosyltransferases.

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.

Identification of a small genetic region that encodes orotate phosphoribosylatransferase and orotidylate decarboxylase in Droxophila melanogaster

A variety of tests indicate that a small genetic region in chromosome 3R encodes at least a part of the bienzyme complex containing the last two enzymes of the pyrimidine biosynthetic pathway in Drosophila melanogaster, Complex U. Electrophoretic variants of the last pathway enzyme map to this region and both Complex U activities are proportionally reduced in flies heterozygous for specific deficiencies of the region. Complex U mutants have been isolated that lack both activities. Characteristics of these mutants are discussed relative to other pyrimidine pathway mutants known in Drosophila melanogaster and in mammals.

Isolation and characterization of 5-fluorouracil-resistant mutants of Chinese hamster ovary cells deficient in the activities of orotate phosphoribosyltransferase and orotidine 5'-monophosphate decarboxylase

A rapid, simple method for isolation of large numbers of Chinese hamster ovary cell (CHO-K1) mutants deficient in the final two enzymes of UMP biosynthesis, orotate phosphoribosyltransferase (EC 2.4.2.10), and OMP decarboxylase (EC 4.1.1.23) is described. The method takes advantage of the fact that CHO-K1 cells require orotate phosphoribosyltransferase to activate the pyrimidine analog 5-fluorouracil to its active form; hence, mutants lacking thhe relevance of these observations to the metabolism of this cancer chemotheraputic agent, to the study of the structure of the protein(s) involved in catalyzing the last two steps of UMP biosynthesis, to the study of the structure of the gene(s) coding for this protein, and to analysis of the human genetic disease orotic aciduria is discussed.

The genes for and regulation of the enzyme activities of two multifunctional proteins required for the de novo pathway for UMP biosynthesis in mammals

UMP biosynthesis requires six enzyme activities. Five of these enzyme centers are clustered into two multienzymatic proteins which are known to, or appear to, sequester the intermediates carbamyl approximately P, carbamyl aspartate and orotidylic acid. The advantages of sequestering these intermediates appear to be a conservation of energy, since two intermediates, carbamyl approximately P and orotidylate, might otherwise be rapidly degraded in mammalian cells. Carbamyl-aspartate appears not to be degraded rapidly in mammalian cells but it can pass into the blood and could possible disrupt brain metabolism by action as an acetylaspartate analog, if it passes the blood-brain barrier. For this, and possible for other reasons, there may be advantages to the fact that these intermediates are not other reasons, there may be advantages to the fact that these intermediates are not readily released from Complex A and U. In addition, these multienzymatic proteins may have other kinetic advantages, some of which have been discussed above. Studies with intact cells illustrate that azauridine, a chemical designed originally as an antineoplastic drug, produces a "ripple" effect when it inhibits the last enzyme of this pathway which leads to a sequential accumulation of pools of the various intermediates or their metabolites. This same agent increases the amount of some of the enzymes of this biosynthetic pathway in cells exposed to this drug. Both of these effects can negate the effectiveness of this potential antineoplastic drug. Sophisticated drug design may depend on whole-cell studies, such as those discussed here, in addition to the classic studies on the inhibition of a single enzyme center to select drugs that may be without significant side effects when they are finally tested in animals.

Resistance to 5-fluoroorotic acid and pyrimidine auxotrophy: a new bidirectional selective system for mammalian cells

We have isolated a clone of murine erythroleukemic cells which will yield a population when a single cell is suspended in a medium composed of dialyzed sera and small molecules. We report that it is feasible--in one experiment--to screen more than 10 of these cells for growth under selective media containing 10(-4) M 5-fluoroorotic acid and 10(-4) M uridine. Cells capable of sustained growth in such media were eventually recovered. Clones of these cells, unlike clones of the parental population, required uridine for growth and contained only 0.39% as much orotate phosphoribosyltransferase, less than 0.5% as much 5-fluoroorotate phosphoribosyltransferase, and 0.013% as much orotidine-5'-monophosphate decarboxylase activity as the parental clones. The parental and variant clones had similar levels of activity for six other enzymes that participate in pyrimidine metabolism.