Modified methods for the determination of carbamyl aspartate

Isolation and preliminary characterization of single amino acid substitution mutants of aspartate carbamoyltransferase

In order to isolate functional Escherichia coli aspartate carbamoyltransferase (carbamoylphosphate:L-aspartate carbamoyltransferase, EC2.1.3.2) with single amino acid replacements, a series of pyrB nonsense mutants has been isolated. These nonsense mutants were induced by 2-aminopurine mutagenesis and selected by a combination of antibiotic treatments, direct enzyme assays, and suppressibility tests. Suppression of the pyrB nonsense mutation with various suppressors, which insert different amino acids, has resulted in the formation of a series of mutant aspartate carbamoyltransferases, each differing in one amino acid from the wild-type enzyme. After partial purification, kinetic studies revealed that some of the mutant enzymes had altered homotropic and heterotropic interactions. The mutants that had a tyrosine insert showed the most pronounced changes, followed by those with a serine insert. The mutants having a glutamine insert, howevr, were indistinguishable from the wild-type enzyme, supporting the conclusion that, because of the specificity of the mutagen, the glutamine insert had regenerated the wild-type enzyme.

Control of pyrimidine biosynthesis in the Ascaris ovary: regulatory properties of glutamine-dependent carbamoyl-phosphate synthetase and copurification of the enzyme with aspartate carbamoyltransferase and dihydroorotase

Glutamine-dependent carbamoyl-phosphate synthetase, the first enzyme of the de novo biosynthetic pathway for pyrimidine nucleotides, was purified about twenty-fold from 105 000 x g supernatant of the Ascaris ovary homogenate. The enzyme activity was feedback-inhibited by UDP and UTP while it was stimulated by 5-phosphoribosyl 1-pyrophosphate. Most of the catalytic and regulatory properties of the Ascaris synthetase were similar to those of the mammalian synthetase. A significant difference is that the Ascaris enzyme was more strongly inhibited by UDP than by UTP whereas the mammalian enzyme is more sensitive to UTP than to UDP. The Ascaris enzyme was also inhibited by other various nucleoside diphosphates, such as dUDP, dADP and CDP, generally more strongly than by the corresponding nucleoside triphosphates. Aspartate carbamoyltransferase and dihydroorotase, the second and third enzymes of the pathway, were also demonstrated in the supernatant fraction. These two enzymes were copurified with the synthetase and the relative activities of the three enzymes remained nearly constant (1:850-890:50-60) throughout the purification. In a sucrose gradient centrifugation, the enzymes cosedimented as a single peak with a sedimentation coefficient (s20,w) of about 32 S under the condition used. These results strongly suggest that the enzymes exist as a multienzyme complex similar to those found in higher animals. The activity of the carbamoyltransferase was insensitive to nucleotides and related compounds. These results indicate that the synthetase plays a key role in the control of pyrimidine biosynthesis in the Ascaris ovary.

Stabilization and purification of ornithine transcarbamylase from Neisseria gonorrhoeae

Ornithine transcarbamylase was stabilized in cell-free extracts by the presence of either carbamyl phosphate or glycerol. The enzyme was rapidly purified by a procedure consisting of ion-exchange chromatography and electrofocusing. The native molecular weight of the enzyme was determined by gel filtration to be 110,000. A subunit molecular weight of 36,000 was determined by polyacrylamide electrophoresis under dissociating conditions. These findings indicated a trimeric quaternary structure for the enzyme. The isoelectric point of the purified enzyme was 4.75, and no evidence of multiple forms of active enzyme was found in either crude or purified preparations. An inactive form of the enzyme appeared upon storage in the absence of stabilization buffer.

Synthesis and inactivation of carbamyl phosphate synthetase isozymes of Bacillus subtilis during growth and sporulation

Pyrimidine-repressible carbamyl phosphate synthetase P was synthesized in parallel with aspartate transcarbamylase during growth of Bacillus subtilis on glucose-nutrient broth. Both enzymes were inactivated at the end of exponential growth, but at different rates and by different mechanisms. Unlike the inactivation of aspartate transcarbamylase, the inactivation of carbamyl phosphate synthetase P was not interrupted by deprivation for oxygen or in a tricarboxylic acid cycle mutant. The arginine-repressible isozyme carbamyl phosphate synthetase A was synthesized in parallel with ornithine transcarbamylase during the stationary phase under these growth conditions. Again, both enzymes were subsequently inactivated, but at different rates and by apparently different mechanisms. The inactivation of carbamyl phosphate synthetase A was not affected in a protease-deficient mutatn the inactivation of ornithine transcarbamylase was greatly slowed.

Inhibition of urea synthesis by pent-4-enoate associated with decrease in N-acetyl-L-glutamate concentration in isolated rat hepatocytes

Pent-4-enoate at 0.1 to 1.0 mM strongly inhibited urea synthesis in isolated rat hepatocytes. Pent-4-enoate at the same concentrations markedly decreased concentrations of N-acetyl-L-glutamate, an essential activator of carbamoyl-phosphate synthase-I (EC 2.7.2.5), and the decrease was well in parallel with the inhibition of urea synthesis by pent-4-enoate. This compound also lowered cellular concentrations of acetyl-CoA, a substrate of acetylglutamate synthase (EC 2.3.1.1). Pent-4-enoate in a dose of 1 mM did not significantly affect cellular concentrations of ATP, and had no direct effect on acetylglutamate synthase activity. These results suggest that the inhibition of urea synthesis by pent-4-enoate is due to decrease in N-acetyl-L-glutamate concentration and that the decrease is probably brought about by decreased rate of its synthesis due to the lowered concentration of cellular acetyl-CoA.

Genetic evidence for the creation of a reinitiation site by mutation inside the yeast ura 2 gene

The ura 2 gene of yeast codes for two enzymatic activities which are translated from a unique messenger RNA in the order carbamoyl-phosphate synthetase (CPSase), aspartate transcarbamylase (ATCase) (Lacroute, 1968; Denis-Duphil and Kaplan, 1976). Nonsense mutations in the CPSPase region cause a complete loss in ATCase activity by a total polar effect, characteristic of eukaryotic mRNA translation, and due to the unique site of protein initiation present on each messenger (Shaffer et al., 1969). A triple nonsense mutant in the CPSase has been constructed by recombination and ATCase+ revertants have been selected from it. Among seventeen revertants obtained, three had a deletion covering the three nonsense mutations relieving thus the polar effect (Fink and Styles, 1974) but fourteen others examined had retained all the CPSase DNA including the three nonsense mutations; this can be explained in the present state of knowledge only by the creation by mutation of reinitiation site either for transcription or for translation in the region of the ura 2 gene distal to the last nonsense mutation.

Characterization of pyrimidine-repressible and arginine-repressible carbamyl phosphate synthetases from Bacillus subtilis

The number and properties of carbamyl phosphate synthetases in Bacillus subtilis have been uncertain because of conflicting genetic results and instability of the enzyme in extracts. The discovery of a previously unrecognized requirement of B. subtilis carbamyl phosphate synthetases for a high concentration of potassium ions for activity and stability permitted unequivocal demonstration that this bacterium elaborates two carbamyl phosphate synthetases. Carbamyl phosphate synthetase A was shown to be repressed by arginine, to have a molecular weight of about 200,000, and to be coded for by a gene that maps near argC4. This isozyme was insensitive to metabolites of the arginine and pyrimidine biosynthetic pathways. Carbamyl phosphate synthetase P was found to be repressed by uracil, to have a molecular weight of 90,000 to 100,000, and to be coded for by a gene that maps near the other pyr genes. This isozyme was activated by phosphoridine nucleotides. Other kinetic properties of the two isozymes were compared. Bacillus thus resembles eucaryotic microbes in producing two carbamyl phosphate synthetases, rather than the enteric bacteria, which produce a single carbamyl phosphate synthetase.

Glutamine-dependent carbamoyl-phosphate synthetase and control of pyrimidine biosynthesis in the parasitic helminth Schistosoma mansoni

1. Carbomoyl-phosphate synthetase in Schistosoma mansoni utilizes L-glutamine as well as ammonia as nitrogen donor but does not require N-acetyl-L-glutamate for the activity. 2. The enzyme activity was inhibited by UDP, UTP, ADP and AMP, among which UDP was the most effective. 3. Aspartate carbamoyltransferase and dihydroorotase were also found and copurified with the synthetase. 4. Relative activities among these three enzymes were 1:30-60:3-8 throughout the purification. 5. These results suggest that the synthetase plays a key role in the control of pyrimidine biosynthesis de novo.

Evidence for translational repression of arginine biosynthetic enzymes in Escherichia coli: altered regulation in a streptomycin-resistant mutant

The formation and repressibility of the arginine biosyntietic enzymes acetylornithine delta-aminotransferase (EC 2.6.1.11), acetylornithine deacetylase (EC 3.5.1.16), ornithine carbamoyltransferase (EC 2.1.3.3), and argininosuccinate lyase (EC 4.3.2.1) were studied in an Escherichia coli W derivative (strain 250-10) that carries (a) a mutant allele of the argR regulatory gene causing a diminished repression-derepression range and (b) a streptomycin resistance mutation. In comparison with the streptomycin-sensitive parent 250, all four enzymes (a) are formed as smaller proportions of the total protein (overall range, 12% to 71%), whether the conditions are repressive (arginine excess) or derepressive (arginine restriction), and (b) show increased repressibility ratios, the carbamoyltransferase giving the largest increase (from 5.7 to 25.0). These effects appear to depend on the concurrent expression of the regulatory-gene and streptomycin resistance mutations, as indicated by analogous experiments with canavanine-resistant mutants of 250-10 that have partial argR- character. The results provide evidence for translational repression in the arginine system, and are interpreted in terms of a functional interaction of a mutant arginine repressor with a mutant S12 ribosomal protein. The locale of translational repression may be near the site of S12, and this mode of regulation may involve initiational selectivity of groupwise recognizable arginine messenger RNA's.

A possible model for the structure of the Neurospora carbamoyl phosphate synthase-aspartate carbamoyl transferase complex enzyme

The pyrimidine-3 locus of Neurospora crassa specifies a multienzyme complex comprising pyrimidine-specific carbamoyl phosphate synthase (CPSpyr) and aspartate carbamoyl transferase (ACT). It appears to be divided into a translationally proximal CPS-specific region and a distal ACT-specific region. Levels of complementation for ACT activity between pairs of four pyr-3 CPS+ ACT- mutants showed a range from 12% to 68% of the wild-type level of the enzyme. This is interpreted as interallelic complementation, contradicting certain earlier suggestion of two dissimilar ACT subunits. Proteolysis of an extract from a heterokaryon formed from two of the above CPS+ ACT- alleles (alpha and beta) did not lead to loss of ACT activity, but led to the formation of a fragment with ACT activity with a similar molecular weight (92,000 daltons) to that produced in extracts of wild type strain. The pyr-3 polar mutant 43-174 which is enzymatically CPS+ ACT- and which fails to complement with any other CPS+ ACT- alleles, thus suggesting its location towards the proximal end of the ACT region, has CPS activity associated with a form of 180,000 daltons molecular weight. These findings are used to contruct a model for structure of the native enzyme complex.

Coordinate and non-coordinate accululation of aspartate transcarbamylase and dihydroorotase in synchronous Chlorella cells growing on different nitrogen sources

Regulation of the levels of aspartate transcarbamylase (carbamoylphosphate:L-aspartate carbamoyltransferase, EC 2.1.3.2) and dihydroorotase (L-5,6-dihydro-orotate amidohydrolase, EC 3.5.2.3) was studied in synchronous cultures of the eucaryotic microorganism Chlorella. Analytical polyacrylamide gel electrophoresis and sucrose density-gradient centrifugation studies revealed that these cells contain a single aspartate transcarbamylase and a dihydroorotase with apparent molecular weights of 160 000 and 80 000, respectively. In synchronous cells cultured in nitrate medium, these two enzymes accumulated in single step-patterns over different periods of the cell cycle. In contrast, these enzymes accumulated in a coordinate manner throughout the cell cycle in ammonium medium. Experiments with inhibitors of protein and RNA synthesis indicated that dihydroorotase is stable in vivo and suggested that cell cycle changes in the turnover rate of aspartate transcarbamylase might determine whether or not these enzymes accumulate in a coordinate manner. Although uracil and uridine could be absorbed and metabolized by the cells, synthesis of these two enzymes could not be repressed by culturing synchronous cells in medium, containing high concentrations (29-40 mM) of uracil or uridine, for an entire cell cycle.

Effect of the relA gene on derepression of amino acid biosynthetic enzymes in growing Escherichia coli depends on the pathway being derepressed

Derepression of an enzyme in the arginine biosynthetic pathway, but not of an enzyme in the tryptophan biosynthetic pathway, is inhibited during the stringent response produced by a partial deprivation of valyl transfer ribonucleic acid in a rel+ strain. In contrast, derepression of the tryptophan biosynthetic enzyme, but not of the arginine biosynthetic enzyme, was inhibited during the relaxed response produced in an isogenic relA strain by the partial deprivation of valyl transfer ribonucleic acid.

Ornithine transcarbamylase from Salmonella typhimurium: purification, subunit composition, kinetic analysis, and immunological cross-reactivity

Ornithine transcarbamylase (OTCase) was purified to hemogeneity from a derepressed strain of Salmonella typhimurium. The optimal pH for enzyme activity is 8.0. The molecular weight of the enzyme was calculated to be 116,000, based on measurements of the sedimentation coefficient by sucrose gradient ultracentrifugation and the Stokes radius by gel filtration. Polyacrylamide gel electrophoresis of cross-linked OTCase in the presence of sodium dodecyl sulfate showed that the enzyme is composed of three identical subunits. The molecular weight of the monomer was determined to be 39,000. Steady-state kinetics indicate that the reaction mechanism is sequential. The limiting Michealis constants for carbamylphosphate and ornithine were determined to be 0.06 and 0.2 mM, respectively. The dissociation constant for carbamylphosphate was 0.02 mM. Product and dead-end inhibition patterns are consistent with an ordered Bi Bi mechanism, in which carbamylphosphate is the first substrate added and phosphate is the last product released. OTCase activity was inhibited by arginine, but relatively high concentrations were required for significant inhibition. The inhibition by arginine might be physiologically significant in the regulation of carbamlphosphate utilization; a single carbamylphosphate synthetase is responsible for the synthesis of carbamylphosphate for both arginine and pyrimidines in S. typhimurium and the inhibition by argine might serve to divert carbamlphosphate to the synthesis of pyrimidines when arginine is present at high concentrations. The crossreaction of OTCases from different microorganisms with purified antibodies raised against the homogeneous OTCase from S. typhimurium was investigated. The results of immunotitration and immunodiffusion experiments revealed a high degree of identity between the enzymes form S. typhimurium and Esherichia coli B and W. In these three cases, a single gen (argl) encodes OTCase. Wild-type E. coli K-12 and strain 3000 X 111, which carry two OTCase genes (argI, argF), also revealed similar cross-reactivity, supporting the hypothesis that argF is the product of a relatively recent duplication. The activity of OTCase from Bacillus subtilis was partially inhibited by antibodies against the enzyme from S. typhimurium, indicating unusual conservation of primary structure among widely different taxonomic groups. OTCase from Saccharomyces cerevisiae, whose molecular weight and primary structure are similar to those of the enzyme from S. typhimurium, was without detectable cross-reactivity.

Biospecific-elution chromatography with 'imphilytes' as stationary phases

Six out of seven enzymes tested (four of them nicotinamide nucleotide-dependent dehydrogenases) showed differences in chromatographic behaviour in the presence and absence of their biospecific ligands, when chromatographed on immobilized amphipathic ampholytes ('imphilytes') as stationary phases. Some enzymes were adsorbed more tightly, others less tightly, in the presence of ligands. These results have implications for enzyme purification in general, and for some types of affinity chromatography in particular.

Stable mutants of mammalian cells that overproduce the first three enzymes of pyrimidine nucleotide biosynthesis

Upon exposure to 0.1 mM N-phosphonacetyl-L-aspartate (PALA), a transition state analog inhibitor of aspartate transcarbamylase, most cells of a simian virus 40 (SV40)-transformed Syrian hamster line are killed within a few days, but resistant mutants form spontaneously with frequency 2-5 X 10(-5) in a stochastic process not dependent upon the presence of the inhibitor. The resistant phenotype is stable for many months in the absence of PALA. Other cell lines also give resistant mutants, but with substantially lower frequencies. Serial selection with PALA at concentrations up to 25 mM has yielded clones with more than 100 times the original aspartate transcarbamylase activity. The activities of carbamyl-P synthetase and dihydroorotase, which co-purify with aspartate transcarbamylase as a three-enzyme complex, increase in parallel with aspartate transcarbamylase activity in each resistant clone tested, but there is no substantial change in the activities of the last three enzymes of the de novo pathway, which are not in this complex. In each of the three resistant clones tested, there is an increase in the number of aspartate transcarbamylase active sites, determined by titration with 3H-PALA, which closely parallels the increase in enzyme activity. In one resistant clone tested, there is no change in the Ki for PALA or the Km for carbamyl-P. The only mechanism detected for achieving resistance to PALA is an increase in the steady state amount of the three enzyme complex.

Arginine-sensitive phenotype of mutations in pyrA of Salmonella typhimurium: role of ornithine carbamyltransferase in the assembly of mutant carbamylphosphate synthetase

The phenotype of certain mutations in pyrA, the gene encoding carbamylphosphate synthetase (CPSase), is expressed only in the presence od exogenous arginine. In unsupplemented media, synthesis of carbamylphosphate and growth was almost normal; in arginine-containing media, synthesis of carbamylphosphate stopped, as did growth, as a consequence of starvation for pyrimidine. Genetic and biochemical evidence suggests that arginine exerts this inhibition by repressing the synthesis of ornithine carbamyltransferase (OTCase), the intracellular presence of which is required for assembly of the unequal subunits and proper functioning of the mutant CPSase. After the addition of arginine to a culture of the mutant, CPSase activity (glutamine dependent) characteristic of the intact holoenzyme progressively decreased, whereas activity (ammonia dependent) characteristic of the free large (alpha) subunit increased. Extracts of mutant cells contain free small (beta) subunits, as demonstrated directly by in vitro complementation using purified alpha subunits from wild type. The mutant enzyme from cultures grown in the presence of arginine had a markedly decreased affinity for adenosine 5'-triphosphate. Mutations in argR that cause depressed synthesis of OTCase suppressed the phenotype, and a certain mutation in argI, the gene encoding OTCase, enhanced it. In vitro experiments using purified enzyme confirm the stimulatory effect of OTCase on the activity of mutant CPSase.

The location of the feedback-specific region with the pyrimidine-3 locus of Neurospora crassa

The purimidine-3 locus of Neurospora crassa specifies two enzyme activities, pyrimidine-specific carbamyl phosphate synthetase (CPSpyr) and aspartate transcarbamylase (ATC). ATC is translationally distal. CPSpyr, but not ATC, is subject to feedback inhibition by uridine triphosphate (UTP). To investigate the location of the feedback-specific region within the locus, inhibition of a number of pyr-3 alleles by UTP was investigated. All CPS+ ATC- polar alleles, revertants of CPS- ATC- polar alleles, and 5-fluorouracil-resistant mutants had normal UTP response. The location of the feedback-specific region is in or close to the CPS-specific region.

Purification of properties of dihydroorotase, a zinc-containing metalloenzyme in Clostridium oroticum

Dihydroorotase +4,5-L-dihydro-orotate amidohydrolase [EC 3.5.2.3]), which catalyzes the reversible cyclization of N-carbamyl-L-aspartate to L-dihydroorotate, has been purified from orotate-grown Clostridium oroticum. The enzyme is homogeneous when subjected to polyacrylamide gel electrophoresis and is stable at pH 7.6 in 0.3 M NaCl containing 10 muM ZnSO4. The enzyme has a molecular weight of approximately 110,000. Sodium dodecyl sulfate gel electrophoresis, using three different buffer systems, indicated the enzyme is composed of two subunits, each having a molecular weight of 55,000. Dihydroorotase is shown by atomic absorption spectroscopy to be a zinc-containing metalloenzyme with 4 g-atoms of zinc per 110,000 g of protein. The pH optima for the conversion of N-carbamyl-L-aspartate to L-dihydroorotate and for L-dihydroorotate to N-carbamyl-L-aspartate are pH 6.0 and 8.2, respectively. The Km values for N-carbamyl-L-aspartate and for L-dihydroorotate are 0.13 and 0.07 mM, respectively. Inhibitor studies indicate that zinc may be involved in the catalytic activity of the enzyme.

Autosomal genetic control of the activity of a new variant ornithine transcarbamylase in chicken kidney

The mode of inheritance of the gene for chick kidney ornithine transcarbamylase (OTC), found previously as a genetic variant, was investigated. White Leghorn B line males homozygous for the allele for the variant OTC gene were selected using the California Gray breed, having a near-absolute deficiency of the enzyme. Then further crosses of the two breeds were made. The mean value of the OTC level of F1 progeny was about 170 units. Chicks from the backcross generation were divided into two groups, of high activity and low activity, in a ratio of 1:1. F2 chicks were divided into three groups: one-fourth of the chicks were classified as a "super high" group, one-half were "high," and the remaining one-fourth were "low"; the mean values for OTC level were 356.7, 196.4, and 15.6 units, respectively. From these results, it was suggested that the variant OTC represents a simple autosomal incompletely dominant trait.

Dual regulation by arginine of the expression of the Escherichia coli argECBH operon

The correlation between the level of messenger ribonucleic acid (mRNA) specific for the argECBH gene cluster (argECBH mRNA) measured by ribonucleic acid-deoxyribonucleic acid (RNA-DNA) hybridization and the rates of synthesis of N-acetylornithine deacetylase (argE enzyme) and of argininosuccinate lyase (argH enzyme) of Escherichia coli strain K-12 were determined for steady-state growth with and without added L-arginine and during the transition periods between these two states. During the transient period after arginine removal (transient derepression), the synthesis of enzymes argE and argH was initially three to five times greater than the steady-state derepressed rate finally reached 50 min later. The level of argECHB mRNA correlated well both quantitatively and temporally with the rates of enzyme synthesis during this transition. The level of in vivo charged arginyl-transfer RNA (tRNAarg), monitored simultaneously, was initially only 5 to 10% and gradually increased to a final level of 80% after 45 min. During the transient period after arginine addition (transient repression), the rates of synthesis of enzymes argE and argH decreased to almost zero and gradually reached steady-state repressed rates after about 180 min. The argECBH mRNA level remained constant at the steady-state repressed level throughout transient repression, revealing a discontinuity between the level of this mRNA and rates of enzyme synthesis. A similar discrepancy was noted during the transition after ornithine addition. In vivo charged tRNAarg remained constant at 80% during this transition. After removal of arginine, the zero-level transient enzyme synthesis developed after only 7.5 min of arginine deprivation and was maximum after 30 min. The results suggest an accumulation of a molecule regulated by arginine that plays a role in transient repression. Our data indicate that arginyl-tRNA synthetase is not this molecule since its synthesis was unaffected by arginine. The ratios of steady-state argE and argH enzyme synthesis without arginine to that with arginine were 12 and 20, respectively, whereas the similar ratio for argECBH mRNA was 2 to 3. The repressed level of argECBH mRNA was not affected by attempts to repress or derepress the ppc+ gene (carried on the DNA used for hybridization), and the repressed level of argECBH mRNA was lowered about 50% in cells carrying an internal argBH deletion. These data taken together indicate the presence of an excess of untranslated argECBH mRNA during both transient and steady-state repression by arginine. Thus, a second regulatory mechanism, not yet defined, appears to play an important role in arginine regulation of enzyme synthesis.

Pyrimidine biosynthetic enzymes of Salmonella typhimurium, repressed specifically by growth in the presence of cytidine

The repressive effects of exogenous cytidine on growing cells was examined in a specially constructed strain in which the pool sizes of endogenous uridine 5'-diphosphate and uridine 5'-triphosphate cannot be varied by the addition of uracil and/or uridine to the medium. Five enzymes of the pyrimidine biosynthetic pathway and one enzyme of the arginine biosynthetic pathway were assayed from cells grown under a variety of conditions. Cytidine repressed the synthesis of dihydroorotase (encoded by pyrC), dihydroorotate dehydrogenase (encoded by pyrD), and ornithine transcarbamylase (encoded by argI). Moreover, aspartate transcarbamylase (encoded by pyrB) became further derepressed upon cytidine addition, whereas no change occurred in the levels of the last two enzymes (encoded by pyrE and pyrF) of the pyrimidine pathway. Quantitative nucleotide pool determinations have provided evidence that any individual ribo- or deoxyribonucleoside mono-, di-, or triphosphate of cytosine or uracil is not a repressing metabolite for the pyrimidine biosynthetic enzymes. Other nucleotide derivatives or ratios must be considered.