Metabolite distribution in cells

CAD, A Multienzymatic Protein at the Head of de Novo Pyrimidine Biosynthesis

CAD is a 1.5 MDa particle formed by hexameric association of a 250 kDa protein that carries the enzymatic activities for the first three steps in the de novo biosynthesis of pyrimidine nucleotides: glutamine-dependent Carbamoyl phosphate synthetase, Aspartate transcarbamoylase and Dihydroorotase. This metabolic pathway is essential for cell growth and proliferation and is conserved in all living organisms. However, the fusion of the first three enzymatic activities of the pathway into a single multienzymatic protein only occurs in animals. In prokaryotes, by contrast, these activities are encoded as distinct monofunctional enzymes that function independently or by forming more or less transient complexes. Whereas the structural information about these enzymes in bacteria is abundant, the large size and instability of CAD has only allowed a fragmented characterization of its structure. Here we retrace some of the most significant efforts to decipher the architecture of CAD and to understand its catalytic and regulatory mechanisms.

The arg2 gene of Trichoderma virens: cloning and development of a homologous transformation system

The arg2 gene which encodes the small subunit of carbamoyl phosphate synthetase for Trichoderma virens has been cloned and used to develop a homologous transformation system. A genomic clone containing the arg2 gene was isolated from a cosmid library of T. virens based on complementation of an arginine auxotrophic mutant of this fungus. The predicted amino acid sequence of the arg2 gene shows 56-82% identity with homologous polypeptides from other fungi. It also contains an upstream open reading frame which encodes 24 amino acids. As is observed with other gene sequences encoding this polypeptide in filamentous fungi, the N-terminus of the predicted polypeptide showed characteristic features of a mitochondrial signal sequence. The arg2 gene was used for genetic transformation of T. virens in frequencies of up to 370 transformants/microgram of DNA. Heat-shock treatment of T. virens protoplasts increased the transformation frequency by fivefold, but more than 85% of the transformants were abortive. Both single-copy, homologous integration events and ectopic, non-homologous integration events were detected by Southern analyses of genomic DNA from transformed strains.

Potential signalling roles for UTP and UDP: sources, regulation and release of uracil nucleotides

Increasing evidence for receptors for uracil nucleotides has focused interest on specific signalling mechanisms involving UTP and UDP. At least three metabotropic P2 receptors are stimulated by uracil nucleotides with equal or greater potency than by adenine nucleotides, and there might be ionotropic receptors as well. Regulation of uridine and uracil nucleotide levels is important when considering the receptor-mediated effects of these compounds. Cells can synthesize uracil nucleotides de novo or by salvage of uridine. UTP made from salvage might be preferentially used for RNA synthesis in the nucleus, while UTP synthesized de novo seems to be used for UDP-sugar and CDP-phospholipid production. UTP from both pathways can enter a free UTP pool, from which UTP can be released from cells. UTP and UDP can stimulate pyrimidinoceptors, but metabolism by ecto-nucleotidases limits their effects. Alternatively, UTP might be a substrate for ecto-protein kinases, and this could contribute to its extracellular regulation. Cells can reclaim uridine, using nucleoside transport processes, following dephosphorylation of UTP, UDP and UMP. In this article Christopher Anderson and Fiona Parkinson discuss how understanding the processes that regulate uridine and uracil nucleotide concentrations will enhance our ability to manipulate UTP/UDP signalling pathways for pharmacological effect.

A reciprocal allosteric mechanism for efficient transfer of labile intermediates between active sites in CAD, the mammalian pyrimidine-biosynthetic multienzyme polypeptide

Carbamoyl phosphate is the product of carbamoyl phosphate synthetase (CPS II) activity and the substrate of the aspartate transcarbamoylase (ATCase) activity, each of which is found in CAD, a large 240-kDa multienzyme polypeptide in mammals that catalyses the first three steps in pyrimidine biosynthesis. In our study of the transfer of the labile intermediate between the two active sites, we have used assays that differentiate the synthesis of carbamoyl phosphate from the overall reaction of CPS II and ATCase that produces carbamoyl aspartate. We provided excess exogenous carbamoyl phosphate and monitored its access to the respective active sites through the production of carbamoyl phosphate and carbamoyl aspartate from radiolabelled bicarbonate. Three features indicate interactions between the folded CPS II and ATCase domains causing reciprocal conformational changes. First, even in the presence of approximately 1 mM unlabelled carbamoyl phosphate, when the aspartate concentration is high ATCase uses endogenous carbamoyl phosphate for the synthesis of radiolabelled carbamoyl aspartate. In contrast, the isolated CPS II forward reaction is inhibited by excess unlabelled carbamoyl phosphate. Secondly, the affinity of the ATCase for carbamoyl phosphate and aspartate is modulated when substrates bind to CPS II. Thirdly, the transition-state analogue phosphonacetyl-L-aspartate is a less efficient inhibitor of the ATCase when the substrates for CPS II are present. All these effects operate when CPS II is in the more active P state, which is induced by high concentrations of ATP and magnesium ions and when 5'-phosphoribosyl diphosphate (the allosteric activator) is present with low concentrations of ATP; these are conditions that would be met during active biosynthesis in the cell. We propose a phenomenon of reciprocal allostery that encourages the efficient transfer of the labile intermediate within the multienzyme polypeptide CAD. In this model, binding of aspartate to the active site of ATCase causes a conformational change at the active site of the liganded form of CPS II, which protects it from inhibition by its product, carbamoyl phosphate; reciprocally, the substrates for CPS II affect the active site of ATCase by increasing the affinity for its substrates, endogenous carbamoyl phosphate and aspartate, and thus impede access of exogenous carbamoyl phosphate or the transition-state analogue. Reciprocal allostery justifies the close association of the enzyme activities within the polypeptide and ensures that carbamoyl phosphate is efficiently synthesised and is dedicated to the second step of pyrimidine biosynthesis. These conditions fulfill those required for metabolic channeling in the cell.

Disruption of the gene encoding the 78-kilodalton subunit of the peripheral arm of complex I in Neurospora crassa by repeat induced point mutation (RIP)

We have used the procedure of sheltered RIP to generate mutants of the 78-kDa protein of the peripheral arm of Neurospora crassa complex I. The nuclei containing the mutations were initially isolated as one component of a heterokaryon but subsequent analysis showed that nuclei containing null alleles of the gene could be propagated as homokaryons. This demonstrates that the gene does not serve an essential function. Sequence analysis of one allele shows that 61 transition mutations were created resulting in 39 amino-acid changes including the introduction of four stop codons. Mutant strains grow at a slower rate than wild-type and exhibit a decrease in the production of conidia. Electron paramagnetic spectroscopy of mutant mitochondria suggest that they are deficient in Fe-S clusters N-1, N-3, and N-4.

Molecular analysis of the Trichosporon cutaneum DSM 70698 argA gene and its use for DNA-mediated transformations

Genomic clones capable of complementing a previously isolated arginine auxotrophic mutant strain of the filamentous yeast Trichosporon cutaneum DSM 70698 have been identified by DNA-mediated transformation, and a complementing 4,082-bp subfragment was sequenced. This analysis revealed an intact gene (arg4) showing a high degree of homology with the Saccharomyces cerevisiae CPA2 gene encoding the large subunit of carbamoyl-phosphate synthetase (CPS-A). The inferred amino acid sequence of the T. cutaneum argA-encoded protein contains 1,168 residues showing 62% identity with the sequence of the S. cerevisiae CPA2 protein, and the comparison of the two sequences uncovered a putative intron sequence of 81 nucleotides close to the 5' end of the coding region of the T. cutaneum argA gene. The presence of this intron was confirmed by nuclease protection studies and by direct DNA sequence analysis of a cDNA fragment which had been obtained by PCR amplification. The T. cutaneum intron shares the general characteristics of introns found in yeasts and filamentous fungi. A major transcript of around 4 kb was found in Northern (RNA) blots. The T. cutaneum argA coding region was expressed in Escherichia coli under the control of the regulatable tac promoter. A roughly 130-kDa protein which was found to cross-react with an anti-rat CPS antibody in Western blots (immunoblots) was observed. Two putative ATP-binding domains were identified, one in the amino-terminal half of the argA-encoded protein and the other in the carboxy-terminal half. These domains are highly conserved among the known CPS-A sequences from S. cerevisiae, E. coli, and the rat. From these results we conclude that the T. cutaneum argA gene encodes the large subunit of CPS. This is the first gene to be identified and analyzed in the T. cutaneum DSM 70698 strain.

Some approaches to the pharmacology of multisubstrate enzyme systems

Analytical and exploratory in vitro, in situ and in vivo, physio-pharmacotoxicology, from enzymology to population epidemiology, now embraces those approaches that correlate complex dynamic multisubstrate kinetics through conventional and more recent non-invasive quantitative methodologies. Basically, substrates may be classed as pertaining to fundamental energy turnovers (first-order cellular metabolic pathways or networks) and to iso- vs allosteric modulator systems (second-order metabolic control network). Pairs of substrates and cofactors set-up the third-order multienzyme-receptor patterns, which in intact, native in vivo structures establish and maintain the compartmentalized, dynamically superimposed overall coordination of local redox and phosphate potentials. Perturbations of the various levels of the metabolic hierarchy induced by drugs, as well their relaxations, can be readily submitted to non-invasive kinetic analysis. Both indirect and direct titrations of substrate levels, their modelling and statistical ad hoc evaluations of their interrelations can lead to the identification of the multiple sites involved in drug effects as structured at the different orders/levels of concomitant functional variations. Fractal geometries contribute towards defining the space- and time-related events.

In situ behavior of the pyrimidine pathway enzymes in Saccharomyces cerevisiae. 3. Catalytic and regulatory properties of carbamylphosphate synthetase: channeling of carbamylphosphate to aspartate transcarbamylase

The present work reports direct evidence for the channeling of carbamylphosphate from carbamylphosphate synthetase to aspartate transcarbamylase in the multifunctional protein that catalyzes the two first reactions of the pyrimidine pathway in Saccharomyces cerevisiae. This phenomenon is almost certainly related to the previously reported observation that the apparent in situ catalytic mechanism of aspartate transcarbamylase is altered by the association of this enzyme to carbamylphosphate synthetase. As a prerequisite of this investigation, the in situ catalytic and regulatory properties of carbamylphosphate synthetase were studied in the permeabilized cells of a strain that contains the wild-type multifunctional protein but is devoid of the carbamylphosphate synthetase specific for the arginine pathway.

Compartmentation of intracellular nucleotides in mammalian cells

The important role of nucleotides in cellular metabolism requires that serious consideration be given to the question of the homogeneity or inhomogeneity of nucleotide pools in cells. The purpose of this review is to summarize the existing evidence for compartmentation of nucleotide pools, discuss the limitations of this evidence, and to discuss the implications of compartmentation for the interpretation of nucleotide concentration measurements. Evidence for nucleotide compartmentation comes from the following types of evidence: compartmentation of RNA precursors; compartmentation of deoxynucleoside triphosphates; mitochondrial compartmentation; the existence of tightly bound nucleotides; pools derived from alternative synthetic routes; compartmentation in cyclic nucleotide metabolism; channeling in the synthesis of pyrimidine nucleotides; and others. The types of evidence adduced for compartmentation will be considered critically and in detail, and alternative explanations considered, as well. Implications of the data and hypotheses on nucleotide compartmentation for the interpretation of nucleotide pool measurements in various types of experiments will be discussed.

Encephalomyocarditis virus replication complexes preferentially utilizing nucleoside diphosphates as substrates for viral RNA synthesis. Nucleotide kinases specifically associated with the complex channel RNA precursor

Replication complexes (RC) of the encephalomyocarditis (EMC) virus were shown previously to contain components that exhibit marked preference for nucleoside diphosphates over nucleoside triphosphates (NTP) as substrates for viral RNA synthesis [Koonin and Agol (1983), Virology 129, 309-318]. These NDP-preferring components have now been found to posses the following properties. When RC preparations were fractionated by sucrose density gradient centrifugation, the fractions containing NDP-preferring components exhibited a considerably higher nucleotide kinase activity as compared to either the fractions containing NTP-preferring components or corresponding fractions from mock-infected cells. When NDP-preferring RC were incubated with ADP and three other NTP, very low concentrations of endogenously generated ATP ensured a greater rate of RNA synthesis than did much higher concentrations of exogenous ATP. When an equimolar mixture of differently labelled UDP and UTP was used as a substrate for NDP-preferring RC, the label from UDP predominated in the newly synthesized RNA, even though the UDP-derived UTP constituted a minor portion of the total UTP pool. When labelled UDP was diluted with unlabelled uridine nucleotides, unlabelled UTP proved to be far less efficient than unlabelled UDP in diminishing the specific radioactivity of UMP incorporated into RNA by NDP-preferring RC. These data are interpreted in the sense that the NTP generated by the built-in nucleotide kinase system are not freed into the external milieu but rather form a separate pool preferentially used for synthesis of viral RNA by NDP-preferring RC. It is suggested that this functional compartmentation of NTP may be significant for the replication of viral RNA in vivo.

The free amino acid pool in Leishmania tropica promastigotes

The principal constituents of the free amino acid pool in Leishmania tropica promastigotes include alanine, glycine, serine, threonine, alpha-aminobutyric acid and ethanolamine. Changes in levels of the pool constituents during starvation, nutrient supplementation and treatment with a metabolic inhibitor (sodium fluoride) were followed. Several enzymes capable of converting amino acids to Krebs cycle intermediates were identified. It is suggested that during prolonged starvation which is accompanied by enhanced proteolysis, these enzymes enable the cells to utilize endogenous amino acids as carbon and energy sources. This is compatible with previous reports that Leishmania promastigotes in culture could grow on amino acids as growth substrates when carbohydrates are totally lacking.

Inhibition by aphidicolin and dideoxythymidine triphosphate of a multienzyme complex of DNA synthesis from human cells

A multienzyme complex consisting of DNA polymerase and several DNA precursor-synthesizing enzymes was solubilized by gentle lysis of cultured human cells. This complex channelled the distal precursor [3H]dTMP into DNA. The patterns of inhibition of the complex by aphidicolin and dideoxythymidine triphosphate (ddTTP) suggested that the complex contained the replicative DNA polymerase, polymerase alpha. Inhibition by ddTTP was competitive with dTTP. This was exploited to estimate the effective concentration of [3H]dTTP at the site of DNA synthesis during channelling of [3H]dTMP into DNA. The estimated concentration (about 50 microM) was so high as to suggest that the solubilized complex was able to functionally compartmentalize DNA precursors.

Gel filtration of a complex of DNA polymerase and DNA precursor-synthesizing enzymes from a human lymphoblastoid cell line

A multienzyme complex containing at least DNA polymerase (EC 2.7.7.7), thymidine kinase (EC 2.7.1.21), dTMP kinase (EC 2.7.4.9) nucleoside diphosphokinase (EC 2.7.4.6) and thymidylate synthetase was separated from the corresponding free enzymes of DNA precursor synthesis by gel filtration of a gently lysed preparation of HPB-ALL cells (a human lymphoblastoid cell line). The isolated incorporated the distal DNA precursors [3H]thymidine or [3H]dTMP into an added DNA template at rates comparable to those observed using the immediate precursor [3H]dTTP. Measurement of the apparent overall concentrations of [3H]dTTP produced during incorporation of [3H]thymidine and of [3H]dTMP were so low as to suggest that these precursors were channelled into DNA by the operation of a kinetically linked complex of precursor-synthesizing enzymes and of DNA polymerase. The DNA polymerase inhibitor 1-beta-D-arabinofuranosylcytosine triphosphate reduced incorporation of distal precursors into DNA. However [3H]dTTP did not accumulate in the reaction mixture. This suggested that the DNA polymerase regulated the flow of substrates through the complex. The results in this paper constitute direct evidence for the existence of multienzyme complexes of DNA synthesis in mammalian cells.

Control of the ornithine cycle in Neurospora crassa by the mitochondrial membrane

In Neurospora crassa, the mitochondrial membrane separates ornithine used in arginine biosynthesis from ornithine used in the arginine degradative pathway in the cytosol. Ornithine easily exchanges across the mitochondrial membrane under conditions appropriate for synthesis of the immediate biosynthetic product, citrulline. Neither of the two mitochondrial enzymes required for the ornithine-to-citrulline conversion is feedback inhibitable in vitro. Nevertheless, when arginine is added to cells and cytosolic ornithine increases as arginine degradation begins, the rate of citrulline synthesis drops immediately to about 20% of normal (B. J. Bowman and R. H. Davis, Bacteriol. 130:285-291, 1977). We have studied this phenomenon in citrulline-accumulating strains carrying the arg-1 mutation. Citrulline accumulation is blocked when arginine is added to an arg-1 strain but not to an arg-1 strain carrying a mutation conferring insensitivity of intramitochondrial ornithine synthesis to arginine. Thus, ornithine is evidently unable to enter mitochondria in normal (feedback-sensitive) cells. Other experiments show that cytosolic ornithine enters mitochondria readily except when arginine or other basic amino acids are present at high levels in the cells. We conclude that in N. crassa, the mitochondrial membrane has evolved as a secondary site of feedback inhibition in arginine synthesis and that this prevents a wasteful cycling of catabolic ornithine back through the anabolic pathway. This is compared to the quite different mechanism by which the yeast Saccharomyces cerevisiae prevents a futile ornithine cycle.

Solubilization and partial characterization of a multienzyme complex of DNA synthesis from human lymphoblastoid cells

1. Gently lysed human lymphoblastoid cells (HPB-ALL) catalysed incorporation of the distal DNA precursors [3H]thymidine, [3H]dTMP, and [3H]dUMP into DNA. Measurement of the [3H]dTTP, produced during the incorporation reactions, provided kinetic evidence for the channelling of the distal precursors into DNA by a multienzyme complex consisting of precursor-synthesizing enzymes and of DNA polymerase. 2. In the presence of the DNA polymerase inhibitor, 1-beta-D-arabinofuranosylcytosine triphosphate, incorporation of [3H]dTMP into DNA was abolished. However, there was no accumulation of [3H]dTTP, suggesting that the flow of substrates through the complex was regulated by the activity of DNA polymerase. 3. Multienzyme complexes were detected by chromatography of a lysate on a column of Sepharose 6B. These complexes retained the ability to channel [3H]thymidine and [3H]dTMP into DNA.

Cloning of a yeast gene coding for arginine-specific carbamoyl-phosphate synthetase

Several recombinant plasmids containing cpaII, the gene that encodes the large subunit of yeast arginine-specific carbamoyl-phosphate synthetase [carbamoyl-phosphate synthetase (glutamine-hydrolyzing), carbon-dioxide: L-glutamine amido-ligase (ADP-forming, carbamate-phosphorylating), EC 6.3.3.5], have been isolated. The plasmids were selected by transformation of a yeast strain with a mutation in the structural gene of the large subunit of carbamoyl-phosphate synthetase. By using a recombinant pool with inserts of yeast nuclear DNA of 5-20 kilobase pairs, we obtained 13 transformants. Of five transformants studied, three have been found to have stable plasmid inserts. These plasmids could be amplified in Escherichia coli and transferred back into the yeast carbamoyl-phosphate synthetase-deficient strains with concomitant complementation of the nuclear mutation. Plasmids pJL2/T1 and pJL2/T5 contain identical nuclear DNA inserts of 5.9 kilobase pairs. Although the insert of plasmid pJL2/T3 is also 5.9 kilobase pairs long, the sequence overlap with pJL2/T1 and pJL2/T5 is only 4.5 kilobase pairs long. The T3 insert has an orientation in the vector opposite to that of the T1 and T5 inserts. The recombinant plasmids with the yeast cpaII gene fail to cross-hybridize with a cloned fragment of E. coli DNA containing the carA and carB genes for the bacterial carbamoyl-phosphate synthetase.

Alternative metabolic fates of thymine nucleotides in human cells

Three types of experiments have been used to study the metabolism of thymine nucleotides by human cells. (1) Cells were labelled continuously with [3H]thymidine and the incorporation of label into DNA compared with the specific radioactivities of pools of individual thymine nucleotides separated by chromatography on polyethylene-imine-cellulose. (2) Cellular thymine nucleotides were labelled with [3H]thymidine at 13 degrees C, followed by incubation at 37 degrees C in unlabelled medium. Incorporation of label into DNA and loss of label from the nucleotide pools were monitored during the 'chase' period at 37 degrees C. (3) The experiments described in (2) above were repeated in the presence of the DNA-synthesis inhibitor cytosine arabinoside, in order to demonstrate more clearly and to quantify degradative pathways for thymine nucleotides. In phytohaemagglutinin-stimulated lymphocytes and in bone-marrow cells, only a proportion (25-60%) of labelled thymine nucleotide was incorporated into DNA, the rest being rapidly degraded and lost from the cell. In contrast, an established cell line (HPB-ALL) from a patient with acute lymphoblastic leukaemia of thymic origin incorporated 100% of its exogenously labelled thymine nucleotides into DNA. These results indicated that alternative metabolic routes are open to thymine nucleotides in human cells. In lymphocytes from patients with megaloblastic anaemia and in normal lymphocytes treated with methotrexate, the utilization of labelled thymine nucleotides for DNA synthesis was more efficient than in controls. These results offer an explanation for the observation of a normal pool of thymidine triphosphate in the cells of patients with untreated megaloblastic anaemia even though the amount of this compound available for DNA synthesis appears to be decreased.

Independent localization and regulation of carbamyl phosphate synthetase A polypeptides of Neurospora crassa

Carbamyl phosphate synthetase A is a two-polypeptide, mitochondrial enzyme of arginine synthesis in Neurospora. The large subunit is encoded in the arg-3 locus and can catalyze formation of carbamyl-P with ammonia as the N donor. The small subunit is encoded in the unlinked arg-2 locus and imparts to the holoenzyme the ability to use glutamine, the biological substrate, as the N donor. By using nonsense mutations of arg-3, it was shown that the small subunit of the enzyme enters the mitochrondrion independently and is regulated in the same manner as it is in wild type. Similarly, arg-2 mutations, affecting the small subunit, have no effect on the localization or the regulation of the large subunit. The two subunits are regulated differently. Like most polypeptides of the pathway, the large subunit is not repressible and derepresses 3- to 5-fold upon arginine-starvation of mycelia. In contrast, the glutamine-dependent activity of the holoenzyme is fully repressible and has a range of variation of over 100-fold. In keeping with this behavior, it is shown here that the small polypeptide, as visualized on two-dimensional gels, is also fully repressible. We conclude that the two subunits of the enzyme are localized independently, controlled independently and over different ranges, and that aggregation kinetics cannot alone explain the unusual regulatory amplitude of the native, two-subunit enzyme. The small subunit molecular weight was shown to be approximately 45,000.

The unsteady model. An alternative approach to nonlinear pharmacokinetics

In this paper an alternative approach to nonlinear pharmacokinetics modelling has been developed from first principles. This "Unsteady Model" is able to describe the situation whereby one or more independent drugs and their metabolites compete for free attachment sites in liver, tissue, red blood cells etc. The Unsteady model collapses to well-known existing models viz: Michaelis-Menten, Langmuir tissue binding and the linear compartment models, under certain well-defined, limiting conditions. The Unsteady model and its collapse to existing models is derived mathematically and illustrated with simulations. For several reasons the Unsteady model would appear to have many advantages and wide application.

Regulation of carbamylphosphate synthesis in Serratia marcescens

Serratia marcescens HY possessed a single carbamylphosphate synthase (CPSase) which was subject to cumulative repression by arginine and a pyrimidine. CPSase did not appear to be a part of a multifunctional enzyme complex as is the case for other enzymes of pyrimidine biosynthesis in this organism. CPSase was purified to homogeneity. The molecular weight of the enzyme was estimated to be 167,000 by sucrose density gradient ultracentrifugation. The double-reciprocal plot for magnesium adenosine triphosphate was linear, yielding a Km value of 2.5 mM. The enzyme utilized either glutamine (Km, 0.1 mM) or NH3 (Km, 10.5 mM) as a nitrogen donor in the reaction. CPSase activity was subject to activation by ornithine and feedback inhibition by uridine monophosphate, as is the case for other enteric bacteria. Carbamate kinase activity, detected in crude extracts of S. marcescens, was shown to be due to a constitutive acetate kinase. The absence of carbamate kinase from S. marcescens HY is consistent with the inability of this organism to utilize arginine as a source of energy under anaerobic conditions.

Carbamyl phosphate synthetase A of Neurospora crassa

Carbamyl phosphate synthetase A of Neurospora crassa was partially purified from mitochondrial extracts. It is an extremely unstable enzyme (t 1/2 = 45 min at 25 detrees C) made up of two unequal subunits. The native enzyme has a molecular weight of approximately 175,000, and the large subunit has a molecular weight of about 125,000. Both the native enzyme and its large subunit are quite asymmetric, as revealed by slow sedimentation in sucrose gradents (7.3S and 6.6S, respectively). The small subunit has not been identified physically as a separate entity. The denaturation of the native, glutamine-dependent activity is correlated with dissociation of subunits, the larger of which retains a more stable, ammonia-dependent activity. Neither substrates nor any other agents except glycerol or polyethylene glycol appreciably stabilized the glutamine-dependent activity. Kinetic studies showed the native enzyme to have a Km for glutamine of about 0.16 mM, and a Km for NH4Cl of about 16 mM, at the optimal pH, 8.0. The enzyme, using either N donor, has a K+ requirement for activity, for which NH4+ can substitute. The glutamine leads to glutamate reaction, which requires the small subunit, also requires the large subunit and all reaction substrates for optimal activity. Other evidences of subunit interaction are the greater activity of the native enzyme, as opposed to the large subunit, with low concentrations of adenosine 5'-triphosphate-Mg2+, and in the stimulation of the ammonia-dependent activity of the native enzyme by glycine. Curiously, although the enzyme's role in biosynthesis is confined to the arginine pathway, it is completely indifferent to arginine or its precursors as feedback effectors or activators. The enzyme is compared with carbamyl phosphate synthetases of other organisms.

The 'enzyme-probe' method for characterizing metabolite pools. The use of NAD-glycohydrolase in human erythrocyte sonicate as a model system

1. An approach for testing the homogeneity of metabolite pools is described. An alien enzyme that can attack the metabolite in question is introducted into the system studied. By analyzing the time-course of decomposition of the metabolite it can be decided whether the pool is homogeneous in respect of reactivity towards the probe-enzyme or can be divided into fractions of different reactivities. 2. The information obtainable from such experiments is illustrated by the case of human erythrocyte sonicate as model system with NAD-glycohydrolase as probe-enzyme. The nicotinamide adenine dinucleotide pool in the concentrated sonicate could be resolved into three fractions (I, II and III) with half-lives of about 1, 7 and 240 min, respectively. Fraction I is free NAD, fraction II is NAD bound to glyceraldehyde-3-phosphate dehydrogenase, and fraction III is coenzyme strongly bound to some, so far unidentified, protein. Sonicate glycolysis seems to require only fraction II and is unable to use fraction III under the experimental conditions applied. 3. The scope of application of the enzyme-probe method is discussed.

Arginine metabolism in Saccharomyces cerevisiae: subcellular localization of the enzymes

Subcellular localization of enzymes of arginine metabolism in Saccharomyces cerevisiae was studied by partial fractionation and stepwise homogenization of spheroplast lysates. These enzymes could clearly be divided into two groups. The first group comprised the five enzymes of the acetylated compound cycle, i.e., acetylglutamate synthase, acetylglutamate kinase, acetylglutamyl-phosphate reductase, acetylornithine aminotransferase, and acetylornithine-glutamate acetyltransferase. These enzymes were exclusively particulate. Comparison with citrate synthase and cytochrome oxidase, and results from isopycnic gradient analysis, suggested that these enzymes were associated with the mitochondria. By contrast, enzymatic activities going from ornithine to arginine, i.e., arginine pathway-specific carbamoylphosphate synthetase, ornithine carbamoyltransferase, argininosuccinate synthetase, and argininosuccinate lyase, and the two first catabolic enzymes, arginase and ornithine aminotransferase, were in the "soluble" fraction of the cell.

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.

Gene expression of a region of chromosome 17 during murine spermatogenesis

The presence of H-2-linked gene products on spermatozoa and their time of appearance during spermatogenesis was determined. Thymus leukaemia antigen specificities 1, 2 and 3 could not be detected on spermatozoa by absorption of the antisera. Immunofluorescent studies with anti-Slp sera did not reveal any specific reactivity with target spermatozoa. In contrast, H-2D antigens were present on somatic as well as germ line components in testes so the time of their first appearance during spermatogenesis could not be precisely specified. Cell separation experiments indicate that H-2D antigens are present on pachytene spermatocytes and increased in quantity on spermatids. The sperm-specific isoenzyme of phosphoglycerate kinase, Pgk-2, appears at a later stage of spermatogenesis than do the H-2 region antigens.

Isolation of putrescine-requiring mutants of Neurospora crassa

Two auxotrophs of Neurospora crassa have been isolated that give a positive growth response to putrescine, spermidine or spermine. One of the mutants is deficient in ornithine decarboxylase activity and has been designated put-I. Both mutants map on linkage group VR, fail to complement and are infertile when crossed to one another, indicating that they are probably alleles. A putrescine auxotroph is incapable of suppressing a pro-4 mutant. The isolation of the mutants confirms that putrescine is an essential factor the normal growth of the organism, and is synthesized via a single pathway in Neurospora.

Glutamine utilization in both the arginine-specific and pyrimidine-specific carbamoyl phosphate synthase enzymes of eurospora crassa

As glutamine-dependent carbamoyl phasphate synthetase (CPS) activity in some organisms is composed of a glutaminase and an ammonium-dependent CPS, CPS- mutants in Neurospora crassa were examined for glutamine- and ammonium-dependent CPS activities. No evidence was found that the genetic location of these two functions were separable. This is discussed with reference to the close genetic proximity of the CPSpyr and aspartate carbamoyltransferase (ACT) structural gene (pyr-3) and the arg-2 gene which appears to specify a subunit responsible for glutamine utilisation in CPSarg.