Compound NSC84167 selectively targets NRF2-activated pancreatic cancer by inhibiting asparagine synthesis pathway

Nuclear factor erythroid 2-related factor 2 (NRF2) is aberrantly activated in about 93% of pancreatic cancers. Activated NRF2 regulates multiple downstream molecules involved in cancer cell metabolic reprogramming, translational control, and treatment resistance; however, targeting NRF2 for pancreatic cancer therapy remains largely unexplored. In this study, we used the online computational tool CellMinerTM to explore the NCI-60 drug databases for compounds with anticancer activities correlating most closely with the mRNA expression of NQO1, a marker for NRF2 pathway activity. Among the >100,000 compounds analyzed, NSC84167, termed herein as NRF2 synthetic lethality compound-01 (NSLC01), was one of the top hits (r = 0.71, P < 0.001) and selected for functional characterization. NSLC01 selectively inhibited the viabilities of four out of seven conventional pancreatic cancer cell lines and induced dramatic apoptosis in the cells with high NRF2 activation. The selective anticancer activity of NSLC01 was further validated with a panel of nine low-passage pancreatic patient-derived cell lines, and a significant reverse correlation between log(IC50) of NSLC01 and NQO1 expression was confirmed (r = -0.5563, P = 0.024). Notably, screening of a panel of nine patient-derived xenografts (PDXs) revealed six PDXs with high NQO1/NRF2 activation, and NSLC01 dramatically inhibited the viabilities and induced apoptosis in ex vivo cultures of PDX tumors. Consistent with the ex vivo results, NSLC01 inhibited the tumor growth of two NRF2-activated PDX models in vivo (P < 0.01, n = 7-8) but had no effects on the NRF2-low counterpart. To characterize the mechanism of action, we employed a metabolomic isotope tracer assay that demonstrated that NSLC01-mediated inhibition of de novo synthesis of multiple amino acids, including asparagine and methionine. Importantly, we further found that NSLC01 suppresses the eEF2K/eEF2 translation elongation cascade and protein translation of asparagine synthetase. In summary, this study identified a novel compound that selectively targets protein translation and induces synthetic lethal effects in NRF2-activated pancreatic cancers.

Effects of nitrogen combined with zinc application on glutamate, glutamine, aspartate and asparagine accumulation in two winter wheat cultivars

Zinc (Zn) deficiency remarkably depresses the protein concentration in the grain of winter wheat. Cultivar 'Pingan 8' showed lower Zn concentrations in the grain than did cultivar 'Yangao 006' after nitrogen (N) combined with Zn application. However, little is known about how amino acids are influenced by Zn combined with N application or about the differences in amino acid accumulation between the two winter wheat cultivars. A pot experiment was conducted to characterize amino acid accumulation in the low Zn-accumulating cultivar 'Pingan 8' and the high Zn-accumulating cultivar 'Yangao 006' at various growth stages (seedling, jointing, grain filling and maturity) as influenced by N and Zn supply. The N (N_0.2) combined with Zn (Zn₁₀) application significantly increased grain yields and the concentrations of N, Zn and crude protein in the grain of both wheat cultivars. N combined with Zn application significantly increased the concentrations of glutamate (Glu) and asparagine (Asn) but decreased the concentrations of glutamine (Gln) and aspartate (Asp) in cultivar 'Yangao 006'; the N combined with Zn application decreased the concentrations of Glu and Gln but increased the concentrations of Asp and Asn in cultivar 'Pingan 8' at the jointing, grain filling and mature stages. Correlation analysis results showed that there were significant relationships between grain yields, spike number, grain number and Zn, N, crude protein, Glu, Gln, Asp and Asn concentrations in the shoots and grain of winter wheat at different growth stages. These results demonstrate that N combined with Zn application enhanced protein synthesis by altering amino acid accumulation in both winter wheat cultivars. Cultivar 'Pingan 8' had lower Gln, Asp and Asn concentrations and higher Glu concentrations than did cultivar 'Yangao 006' after the N_0.05 treatment but had higher Glu, Gln, Asp, and Asn concentrations and lower Glu concentrations than did cultivar 'Yangao 006' after the N_0.2 treatment. These results revealed that the difference in amino acid concentrations between the two cultivars was related to the N application level.

Characterization of a novel variant in siblings with Asparagine Synthetase Deficiency

Asparagine Synthetase Deficiency (ASD) is a recently described inborn error of metabolism caused by bi-allelic pathogenic variants in the asparagine synthetase (ASNS) gene. ASD typically presents congenitally with microcephaly and severe, often medically refractory, epilepsy. Development is generally severely affected at birth. Tone is abnormal with axial hypotonia and progressive appendicular spasticity. Hyperekplexia has been reported. Neuroimaging typically demonstrates gyral simplification, abnormal myelination, and progressive cerebral atrophy. The present report describes two siblings from consanguineous parents with a homozygous Arg49Gln variant associated with a milder form of ASD that is characterized by later onset of symptoms. Both siblings had a period of normal development before onset of seizures, and development regression. Primary fibroblast studies of the siblings and their parents document that homozygosity for Arg49Gln blocks cell growth in the absence of extracellular asparagine. Functional studies with these cells suggest no impact of the Arg49Gln variant on basal ASNS mRNA or protein levels, nor on regulation of the gene itself. Molecular modelling of the ASNS protein structure indicates that the Arg49Gln variant lies near the substrate binding site for glutamine. Collectively, the results suggest that the Arg49Gln variant affects the enzymatic function of ASNS. The clinical, cellular, and molecular observations from these siblings expand the known phenotypic spectrum of ASD.

Silencing of vacuolar invertase and asparagine synthetase genes and its impact on acrylamide formation of fried potato products

Acrylamide is produced in a wide variety of carbohydrate-rich foods during high-temperature cooking. Dietary acrylamide is a suspected human carcinogen, and health concerns related to dietary acrylamide have been raised worldwide. French fries and potato chips contribute a significant proportion to the average daily intake of acrylamide, especially in developed countries. One way to mitigate health concerns related to acrylamide is to develop potato cultivars that have reduced contents of the acrylamide precursors asparagine, glucose and fructose in tubers. We generated a large number of silencing lines of potato cultivar Russet Burbank by targeting the vacuolar invertase gene VInv and the asparagine synthetase genes StAS1 and StAS2 with a single RNA interference construct. The transcription levels of these three genes were correlated with reducing sugar (glucose and fructose) and asparagine content in tubers. Fried potato products from the best VInv/StAS1/StAS2-triple silencing lines contained only one-fifteenth of the acrylamide content of the controls. Interestingly, the extent of acrylamide reduction of the best triple silencing lines was similar to that of the best VInv-single silencing lines developed previously from the same potato cultivar Russet Burbank. These results show that an acrylamide mitigation strategy focused on developing potato cultivars with low reducing sugars is likely to be an effective and sufficient approach for minimizing the acrylamide-forming potential of French fry processing potatoes.

Asparagine synthetase1, but not asparagine synthetase2, is responsible for the biosynthesis of asparagine following the supply of ammonium to rice roots

Asparagine is synthesized from glutamine by the reaction of asparagine synthetase (AS) and is the major nitrogen form in both xylem and phloem sap in rice (Oryza sativa L.). There are two genes encoding AS, OsAS1 and OsAS2, in rice, but the functions of individual AS isoenzymes are largely unknown. Cell type- and NH4(+)-inducible expression of OsAS1 as well as analyses of knockout mutants were carried out in this study to characterize AS1. OsAS1 was mainly expressed in the roots, with in situ hybridization showing that the corresponding mRNA was specifically accumulated in the three cell layers of the root surface (epidermis, exodermis and sclerenchyma) in an NH4(+)-dependent manner. Conversely, OsAS2 mRNA was abundant in leaf blades and sheathes of rice. Although OsAS2 mRNA was detectable in the roots, its content decreased when NH4(+) was supplied. Retrotransposon-mediated knockout mutants lacking AS1 showed slight stimulation of shoot length and slight reduction in root length at the seedling stage. On the other hand, the mutation caused an approximately 80-90% reduction in free asparagine content in both roots and xylem sap. These results suggest that AS1 is responsible for the synthesis of asparagine in rice roots following the supply of NH4(+). Characteristics of the NH4(+)-dependent increase and the root surface cell-specific expression of OsAS1 gene are very similar to our previous results on cytosolic glutamine synthetase1;2 and NADH-glutamate synthase1 in rice roots. Thus, AS1 is apparently coupled with the primary assimilation of NH4(+) in rice roots.

Genetic variation and possible SNP markers for breeding wheat with low-grain asparagine, the major precursor for acrylamide formation in heat-processed products

BACKGROUND

In products made from wheat (Triticum aestivum) flour, acrylamide formation is almost exclusively determined by the level of free asparagine in the grain. Genetic variability for grain asparagine content was evaluated in order to assess the potential for acrylamide mitigation by breeding.

RESULTS

Free asparagine levels in the grains of 92 varieties varied from 137 to 471 mg kg⁻¹, representing an approximate threefold difference between the low- and high-asparagine genotypes. Heritability was low, with a value of 32%, indicating that breeding cultivars with inherently low grain asparagine would be a challenge. A genome-wide scan with single-nucleotide polymorphism (SNP) markers identified nine SNPs that were significantly (P < 0.001) associated with variation in free asparagine. The significant SNPs were localized on chromosome 5A, and explained between 14% and 24% of the observed variation. These putative SNPs are candidates for further studies to develop molecular markers.

CONCLUSION

Significant genetic variation exists for reducing acrylamide precursors in wheat flour, indicating that breeding and genetics could play an important role in mitigating the acrylamide risk in wheat products. The study identified a region on chromosome 5A that could provide a basis for further research to develop functional markers.

Molecular characterization of PVAS3: an asparagine synthetase gene from common bean prevailing in developing organs

In common bean, asparagine synthetase (AS; EC 6.3.5.4) is encoded by three members of a multigene family called PVAS1, PVAS2 and PVAS3. Two of these genes, PVAS1 and PVAS2, have been extensively studied, but little is known about PVAS3, remaining unclear whether PVAS3 function is redundant to the other AS or if it plays a specific role in Phaseolus vulgaris metabolism. In this work, we used a molecular approach to characterize PVAS3 expression and to gain some knowledge about its physiological function. We showed that, in contrast to PVAS1 and PVAS2, PVAS3 was expressed in all organs analyzed. Interestingly, PVAS3 was the AS gene most highly expressed in nodules, leaves and pods at the earliest stages of development, and its expression decreased as these organs developed. Expression of PVAS3 parallels the accumulation of AS protein and the asparagine content during the earliest stages of nodule, leaf and pod development, suggesting an important role for PVAS3 in the synthesis of asparagine in that period. Furthermore, PVAS3 was not repressed by light, as most class-II AS genes. Surprisingly, fertilization of nodulated plants with nitrate or ammonium, conditions that induce PVAS1 and PVAS2 and the shift from ureides to amide synthesis, repressed the expression of PVAS3 in nodules and roots. The possible implications of this regulation are discussed.

Changes in the C/N balance caused by increasing external ammonium concentrations are driven by carbon and energy availabilities during ammonium nutrition in pea plants: the key roles of asparagine synthetase and anaplerotic enzymes

An understanding of the mechanisms underlying ammonium (NH(4)(+)) toxicity in plants requires prior knowledge of the metabolic uses for nitrogen (N) and carbon (C). We have recently shown that pea plants grown at high NH(4)(+) concentrations suffer an energy deficiency associated with a disruption of ionic homeostasis. Furthermore, these plants are unable to adequately regulate internal NH4(+) levels and the cell-charge balance associated with cation uptake. Herein we show a role for an extra-C application in the regulation of C-N metabolism in NH(4)(+) -fed plants. Thus, pea plants (Pisum sativum) were grown at a range of NH(4)(+) concentrations as sole N source, and two light intensities were applied to vary the C supply to the plants. Control plants grown at high NH(4)(+) concentration triggered a toxicity response with the characteristic pattern of C-starvation conditions. This toxicity response resulted in the redistribution of N from amino acids, mostly asparagine, and lower C/N ratios. The C/N imbalance at high NH(4)(+) concentration under control conditions induced a strong activation of root C metabolism and the upregulation of anaplerotic enzymes to provide C intermediates for the tricarboxylic acid cycle. A high light intensity partially reverted these C-starvation symptoms by providing higher C availability to the plants. The extra-C contributed to a lower C4/C5 amino acid ratio while maintaining the relative contents of some minor amino acids involved in key pathways regulating the C/N status of the plants unchanged. C availability can therefore be considered to be a determinant factor in the tolerance/sensitivity mechanisms to NH(4)(+) nutrition in plants.

Two-codon T-box riboswitch binding two tRNAs

T-box riboswitches control transcription of downstream genes through the tRNA-binding formation of terminator or antiterminator structures. Previously reported T-boxes were described as single-specificity riboswitches that can bind specific tRNA anticodons through codon-anticodon interactions with the nucleotide triplet of their specifier loop (SL). However, the possibility that T-boxes might exhibit specificity beyond a single tRNA had been overlooked. In Clostridium acetobutylicum, the T-box that regulates the operon for the essential tRNA-dependent transamidation pathway harbors a SL with two potential overlapping codon positions for tRNA(Asn) and tRNA(Glu). To test its specificity, we performed extensive mutagenic, biochemical, and chemical probing analyses. Surprisingly, both tRNAs can efficiently bind the SL in vitro and in vivo. The dual specificity of the T-box is allowed by a single base shift on the SL from one overlapping codon to the next. This feature allows the riboswitch to sense two tRNAs and balance the biosynthesis of two amino acids. Detailed genomic comparisons support our observations and suggest that "flexible" T-box riboswitches are widespread among bacteria, and, moreover, their specificity is dictated by the metabolic interconnection of the pathways under control. Taken together, our results support the notion of a genome-dependent codon ambiguity of the SLs. Furthermore, the existence of two overlapping codons imposes a unique example of tRNA-dependent regulation at the transcriptional level.

Asparagine synthetase: regulation by cell stress and involvement in tumor biology

Asparagine synthetase (ASNS) catalyzes the conversion of aspartate and glutamine to asparagine and glutamate in an ATP-dependent reaction. The enzyme is ubiquitous in its organ distribution in mammals, but basal expression is relatively low in tissues other than the exocrine pancreas. Human ASNS activity is highly regulated in response to cell stress, primarily by increased transcription from a single gene located on chromosome 7. Among the genomic elements that control ASNS transcription is the C/EBP-ATF response element (CARE) within the promoter. Protein limitation or an imbalanced dietary amino acid composition activate the ASNS gene through the amino acid response (AAR), a process that is replicated in cell culture through limitation for any single essential amino acid. Endoplasmic reticulum stress also increases ASNS transcription through the PERK-eIF2-ATF4 arm of the unfolded protein response (UPR). Both the AAR and UPR lead to increased synthesis of ATF4, which binds to the CARE and induces ASNS transcription. Elevated expression of ASNS protein is associated with resistance to asparaginase therapy in childhood acute lymphoblastic leukemia and may be a predictive factor in drug sensitivity for certain solid tumors as well. Activation of the GCN2-eIF2-ATF4 signaling pathway, leading to increased ASNS expression appears to be a component of solid tumor adaptation to nutrient deprivation and/or hypoxia. Identifying the roles of ASNS in fetal development, tissue differentiation, and tumor growth may reveal that ASNS function extends beyond asparagine biosynthesis.

Low-acrylamide French fries and potato chips

Acrylamide is produced in starchy foods that are baked, roasted or fried at high temperatures. Concerns about the potential health issues associated with the dietary intake of this reactive compound led us to reduce the accumulation of asparagine, one of its main precursors, in the tubers of potato (Solanum tuberosum). This metabolic change was accomplished by silencing two asparagine synthetase genes through 'all-native DNA' transformation. Glasshouse-grown tubers of the transformed intragenic plants contained up to 20-fold reduced levels of free asparagine. This metabolic change coincided with a small increase in the formation of glutamine and did not affect tuber shape or yield. Heat-processed products derived from the low-asparagine tubers were also indistinguishable from their untransformed counterparts in terms of sensory characteristics. However, both French fries and potato chips accumulated as little as 5% of the acrylamide present in wild-type controls. Given the important role of processed potato products in the modern Western diet, a replacement of current varieties with intragenic potatoes could reduce the average daily intake of acrylamide by almost one-third.

Mechanistic and structural basis of stereospecific Cbeta-hydroxylation in calcium-dependent antibiotic, a daptomycin-type lipopeptide

Non-ribosomally synthesized lipopeptide antibiotics of the daptomycin type are known to contain unnatural beta-modified amino acids, which are essential for bioactivity. Here we present the biochemical and structural basis for the incorporation of 3-hydroxyasparagine at position 9 in the 11-residue acidic lipopeptide lactone calcium-dependent antibiotic (CDA). Direct hydroxylation of l-asparagine by AsnO, a non-heme Fe(2+)/alpha-ketoglutarate-dependent oxygenase encoded by the CDA biosynthesis gene cluster, was validated by Fmoc derivatization of the reaction product and LC/MS analysis. The 1.45, 1.92, and 1.66 A crystal structures of AsnO as apoprotein, Fe(2+) complex, and product complex, respectively, with (2S,3S)-3-hydroxyasparagine and succinate revealed the stereoselectivity and substrate specificity of AsnO. The comparison of native and product-complex structures of AsnO showed a lid-like region (residues F208-E223) that seals the active site upon substrate binding and shields it from sterically demanding peptide substrates. Accordingly, beta-hydroxylated asparagine is synthesized prior to its incorporation into the growing CDA peptide. The AsnO structure could serve as a template for engineering novel enzymes for the synthesis of beta-hydroxylated amino acids.

A single tRNA base pair mediates bacterial tRNA-dependent biosynthesis of asparagine

In many prokaryotes and in organelles asparagine and glutamine are formed by a tRNA-dependent amidotransferase (AdT) that catalyzes amidation of aspartate and glutamate, respectively, mischarged on tRNAAsn and tRNAGln. These pathways supply the deficiency of the organism in asparaginyl- and glutaminyl-tRNA synthtetases and provide the translational machinery with Asn-tRNAAsn and Gln-tRNAGln. So far, nothing is known about the structural elements that confer to tRNA the role of a specific cofactor in the formation of the cognate amino acid. We show herein, using aspartylated tRNAAsn and tRNAAsp variants, that amidation of Asp acylating tRNAAsn is promoted by the base pair U1-A72 whereas the G1-C72 pair and presence of the supernumerary nucleotide U20A in the D-loop of tRNAAsp prevent amidation. We predict, based on comparison of tRNAGln and tRNAGlu sequence alignments from bacteria using the AdT-dependent pathway to form Gln-tRNAGln, that the same combination of nucleotides also rules specific tRNA-dependent formation of Gln. In contrast, we show that the tRNA-dependent conversion of Asp into Asn by archaeal AdT is mainly mediated by nucleotides G46 and U47 of the variable region. In the light of these results we propose that bacterial and archaeal AdTs use kingdom-specific signals to catalyze the tRNA-dependent formations of Asn and Gln.

Role of asparagine and asparagine synthetase genes in sunflower (Helianthus annuus) germination and natural senescence

Sunflower (Helianthus annuus) contains three active asparagine synthetase (EC 6.3.5.4, AS) genes: HAS1, HAS1.1 and HAS2. Asparagine content and AS gene expression were determined during germination and leaf and cotyledon natural senescence to assess the role of asparagine as well as the extent of participation of each AS gene in different nitrogen mobilizing processes. Asparagine accumulated in the dry seed and was the predominant amide throughout germination. During cotyledon senescence, the asparagine level was slightly higher than that of glutamine. The opposite was true for leaf senescence. According to transcript accumulation data, most of the asparagine newly synthesized for germination and cotyledon expansion was due to HAS2 activity, with little contribution of the other AS genes. However, all three genes work together to synthesize asparagine for leaf senescence. The absence of significant AS gene expression in cotyledon senescence differentiates leaf and cotyledon senescence, and suggests a cotyledon-specific regulation.

Dermcidin expression in hepatic cells improves survival without N-glycosylation, but requires asparagine residues

Proteolysis-inducing factor, a cachexia-inducing tumour product, is an N-glycosylated peptide with homology to the unglycosylated neuronal survival peptide Y-P30 and a predicted product of the dermcidin gene, a pro-survival oncogene in breast cancer. We aimed to investigate whether dermcidin is pro-survival in liver cells, in which proteolysis-inducing factor induces catabolism, and to determine the role of potentially glycosylated asparagine residues in this function. Reverse cloning of proteolysis-inducing factor demonstrated ∼100% homology with the dermcidin cDNA. This cDNA was cloned into pcDNA3.1+ and both asparagine residues removed using site-directed mutagenesis. In vitro translation demonstrated signal peptide production, but no difference in molecular weight between the products of native and mutant vectors. Immunocytochemistry of HuH7 cells transiently transfected with V5-His-tagged dermcidin confirmed targeting to the secretory pathway. Stable transfection conferred protection against oxidative stress. This was abrogated by mutation of both asparagines in combination, but not by mutation of either asparagine alone. These findings suggest that dermcidin may function as an oncogene in hepatic as well as breast cells. Glycosylation does not appear to be required, but the importance of asparagine residues suggests a role for the proteolysis-inducing factor core peptide domain.

Asparagine synthetase chemotherapy

Modern clinical treatments of childhood acute lymphoblastic leukemia (ALL) employ enzyme-based methods for depletion of blood asparagine in combination with standard chemotherapeutic agents. Significant side effects can arise in these protocols and, in many cases, patients develop drug-resistant forms of the disease that may be correlated with up-regulation of the enzyme glutamine-dependent asparagine synthetase (ASNS). Though the precise molecular mechanisms that result in the appearance of drug resistance are the subject of active study, potent ASNS inhibitors may have clinical utility in treating asparaginase-resistant forms of childhood ALL. This review provides an overview of recent developments in our understanding of (a) the structure and catalytic mechanism of ASNS, and (b) the role that ASNS may play in the onset of drug-resistant childhood ALL. In addition, the first successful, mechanism-based efforts to prepare and characterize nanomolar ASNS inhibitors are discussed, together with the implications of these studies for future efforts to develop useful drugs.

Automation of gene assignments to metabolic pathways using high-throughput expression data

BACKGROUND

Accurate assignment of genes to pathways is essential in order to understand the functional role of genes and to map the existing pathways in a given genome. Existing algorithms predict pathways by extrapolating experimental data in one organism to other organisms for which this data is not available. However, current systems classify all genes that belong to a specific EC family to all the pathways that contain the corresponding enzymatic reaction, and thus introduce ambiguity.

RESULTS

Here we describe an algorithm for assignment of genes to cellular pathways that addresses this problem by selectively assigning specific genes to pathways. Our algorithm uses the set of experimentally elucidated metabolic pathways from MetaCyc, together with statistical models of enzyme families and expression data to assign genes to enzyme families and pathways by optimizing correlated co-expression, while minimizing conflicts due to shared assignments among pathways. Our algorithm also identifies alternative ("backup") genes and addresses the multi-domain nature of proteins. We apply our model to assign genes to pathways in the Yeast genome and compare the results for genes that were assigned experimentally. Our assignments are consistent with the experimentally verified assignments and reflect characteristic properties of cellular pathways.

CONCLUSION

We present an algorithm for automatic assignment of genes to metabolic pathways. The algorithm utilizes expression data and reduces the ambiguity that characterizes assignments that are based only on EC numbers.

Up-regulation and localization of asparagine synthetase in tomato leaves infected by the bacterial pathogen Pseudomonas syringae

Nitrogen metabolism is one aspect of basic metabolism, which is still quite unknown in the field of plant-pathogen interactions. Evidence derived from previous studies conducted in our laboratory strongly suggests that during microbial pathogenesis an important nitrogen mobilization process takes place in diseased tissues. Here we describe the expression pattern of asparagine synthetase (AS; EC 6.3.5.4) in tomato leaves infected by the bacterial pathogen Pseudomonas syringae pv. tomato. Using an homologous AS cDNA probe isolated by RT-PCR from infected leaves, we have observed a high level induction of AS expression during the course of infection. Concomitantly, a single AS polypeptide also accumulated in response to bacterial infection. Furthermore, immunohistochemical analysis of AS in infected leaves revealed a strong immunostaining in phloem cells of the main vascular bundles and in secondary veins of the leaf blade. These data correlate with those previously reported for expression of a cytosolic isoform of glutamine synthetase (GS1) also induced during development of the infectious process. Taken together, our results suggest the existence of a GS1/AS pathway representing a metabolic route for transferring ammonium released from protein catabolism into asparagine, an amino acid that may have a major role in nitrogen mobilization from diseased tissues.

Removal of cyanide by woody plants

Hydrogen cyanide is a high volume production chemical that causes severe environmental problems. The toxicity of potassium cyanide (KCN) to basket willow trees (Salix viminalis) was tested. In aqueous solution, 2 mg CN l(-1) as KCN depressed the transpiration after 72 h about 50%. Trees exposed to 0.4 mg CN l(-1) in aqueous solution showed initially a depression of transpiration, but recovered. Doses of 8 and 20 mg CN l(-1) in aqueous solution were quickly mortal to the trees. At the end of the test, almost all cyanide had disappeared from the solutions. Levels of cyanide in plants were related to the toxicity, with no elevated levels of cyanide in plants exposed to 0.4 mg CN l(-1). Willows grown in sand survived 423.5 h irrigation with 20 mg CN l(-1). Willows grown in sand irrigated with 50 mg CN l(-1) died within a few days. The roots of the surviving willows were able to consume about 10 mg CN kg fresh weight(-1)h(-1). Vascular plants possess the enzymes beta-cyanoalanine synthase and beta-cyanoalanine hydrolase, which convert free cyanide to the amino acid asparagine. The in vivo capacity of woody plants (willow, poplar, elder, rose, birch) to remove cyanide was evaluated. Tests were performed with detached leaves and roots in KCN solutions of different concentrations. The highest removal capacity was obtained for basket willow hybrids (Salix viminalis x schwerinii). The Michaelis-Menten kinetics was determined. Realistic values of the half-saturation constant, K(M), were between 0.6 and 1.7 mg CN l(-1); the maximum metabolic capacity, v(max), was around 9.3 mg CN kg fresh weight(-1)h(-1). The removal of cyanide by plants might be useful in phytoremediation and treatment of wastewater from gold mining.

Revisiting the steady state kinetic mechanism of glutamine-dependent asparagine synthetase from Escherichia coli

Escherichia coli asparagine synthetase B (AS-B) catalyzes the formation of asparagine from aspartate in an ATP-dependent reaction for which glutamine is the in vivo nitrogen source. In an effort to reconcile several different kinetic models that have been proposed for glutamine-dependent asparagine synthetases, we have used numerical methods to investigate the kinetic mechanism of AS-B. Our simulations demonstrate that literature proposals cannot reproduce the glutamine dependence of the glutamate/asparagine stoichiometry observed for AS-B, and we have therefore developed a new kinetic model that describes the behavior of AS-B more completely. The key difference between this new model and the literature proposals is the inclusion of an E.ATP.Asp.Gln quaternary complex that can either proceed to form asparagine or release ammonia through nonproductive glutamine hydrolysis. The implication of this model is that the two active sites in AS-B become coordinated only after formation of a beta-aspartyl-AMP intermediate in the synthetase site of the enzyme. The coupling of glutaminase and synthetase activities in AS is therefore different from that observed in all other well-characterized glutamine-dependent amidotransferases.

Amino-acid cycling drives nitrogen fixation in the legume-Rhizobium symbiosis

The biological reduction of atmospheric N2 to ammonium (nitrogen fixation) provides about 65% of the biosphere's available nitrogen. Most of this ammonium is contributed by legume-rhizobia symbioses, which are initiated by the infection of legume hosts by bacteria (rhizobia), resulting in formation of root nodules. Within the nodules, rhizobia are found as bacteroids, which perform the nitrogen fixation: to do this, they obtain sources of carbon and energy from the plant, in the form of dicarboxylic acids. It has been thought that, in return, bacteroids simply provide the plant with ammonium. But here we show that a more complex amino-acid cycle is essential for symbiotic nitrogen fixation by Rhizobium in pea nodules. The plant provides amino acids to the bacteroids, enabling them to shut down their ammonium assimilation. In return, bacteroids act like plant organelles to cycle amino acids back to the plant for asparagine synthesis. The mutual dependence of this exchange prevents the symbiosis being dominated by the plant, and provides a selective pressure for the evolution of mutualism.

Amino-acid cycling drives nitrogen fixation in the legume-Rhizobium symbiosis

The biological reduction of atmospheric N2 to ammonium (nitrogen fixation) provides about 65% of the biosphere's available nitrogen. Most of this ammonium is contributed by legume-rhizobia symbioses, which are initiated by the infection of legume hosts by bacteria (rhizobia), resulting in formation of root nodules. Within the nodules, rhizobia are found as bacteroids, which perform the nitrogen fixation: to do this, they obtain sources of carbon and energy from the plant, in the form of dicarboxylic acids. It has been thought that, in return, bacteroids simply provide the plant with ammonium. But here we show that a more complex amino-acid cycle is essential for symbiotic nitrogen fixation by Rhizobium in pea nodules. The plant provides amino acids to the bacteroids, enabling them to shut down their ammonium assimilation. In return, bacteroids act like plant organelles to cycle amino acids back to the plant for asparagine synthesis. The mutual dependence of this exchange prevents the symbiosis being dominated by the plant, and provides a selective pressure for the evolution of mutualism.

Effect of lignins and their precursors on nitric oxide, citrulline and asparagine production by mouse macrophage-like Raw 264.7 cells

Lignins, tannins and flavonoids are commonly found polyphenols. Among these polyphenols, lignins, polymers of phenylpropenoids complexed with polysaccharides, were the least cytotoxic and most potently stimulated the production of nitric oxide (NO), citrulline and asparagine by mouse macrophage-like Raw 264.7 cells. The maximum production of these substances reached the level attained by lipopolysaccharide (LPS). However, epigallocatechin gallate, phenylpropenoid monomers (ferulic acid, caffeic acid) and gallic acid (component unit of tannin) were inactive. These data suggest that the macrophage-stimulation activity of polyphenols depends, at least in part, on their molecular weight or structural configuration. There was a positive relationship between the extent of asparagine production and that of NO or citrulline. Western blot analysis demonstrated that both lignins and LPS elevated the cellular level of asparagine synthetase. The present study suggests the possible link between the stimulated asparagine production and macrophage activation.

Transfer RNA-dependent amino acid biosynthesis: an essential route to asparagine formation

Biochemical experiments and genomic sequence analysis showed that Deinococcus radiodurans and Thermus thermophilus do not possess asparagine synthetase (encoded by asnA or asnB), the enzyme forming asparagine from aspartate. Instead these organisms derive asparagine from asparaginyl-tRNA, which is made from aspartate in the tRNA-dependent transamidation pathway [Becker, H. D. & Kern, D. (1998) Proc. Natl. Acad. Sci. USA 95, 12832-12837; and Curnow, A. W., Tumbula, D. L., Pelaschier, J. T., Min, B. & Söll, D. (1998) Proc. Natl. Acad. Sci. USA 95, 12838-12843]. A genetic knockout disrupting this pathway deprives D. radiodurans of the ability to synthesize asparagine and confers asparagine auxotrophy. The organism's capacity to make asparagine could be restored by transformation with Escherichia coli asnB. This result demonstrates that in Deinococcus, the only route to asparagine is via asparaginyl-tRNA. Analysis of the completed genomes of many bacteria reveal that, barring the existence of an unknown pathway of asparagine biosynthesis, a wide spectrum of bacteria rely on the tRNA-dependent transamidation pathway as the sole route to asparagine.

Stimulation of arginine consumption and asparagine production in LPS-activated macrophages

Changes in amino acid utilization during lipopolysaccharide (LPS)-induced activation of mouse macrophage-like cells Raw264.7 were investigated. Amino acids in the medium and cell fractions were extracted by 5% trichloroacetic acid and quantitated by amino acid analyzer. Glutamine was utilized by cells at the highest rate, followed by serine and arginine, a precursor of nitric oxide (NO). When Raw264.7 cells were incubated with 10 or 100 ng/mL LPS, the consumption of arginine and the production of citrulline, nitric oxide (NO) and asparagine were significantly increased. The intracellular amino acid concentration was not significantly changed. These data suggest that arginine consumption and asparagine production might be possible markers of macrophage activation.

The role of the hydrophilic Asn230 residue of the mu-opioid receptor in the potency of various opioid agonists

1. To investigate the effect of the hydrophilic Asn amino acid at position 230 of the human mu-opioid receptor (hMOR230) on the potency of various agonists, we mutated this residue to Thr and Leu (hMORN230T and hMORN230L respectively). 2. Taking advantage of the functional coupling of the opioid receptor with the heteromultimeric G-protein-coupled inwardly rectifying K(+) (GIRK1/GIRK2) channel, either the wild type hMOR or one of the mutated receptors (hMORN230L or hMORN230T) were functionally coexpressed with GIRK1/GIRK2 channels and a regulator of G-protein signalling (RGS4) in Xenopus laevis oocytes. 3. The two-microelectrode voltage clamp technique was used to measure the opioid receptor-activated GIRK1/GIRK2 channel responses. The potency of [D-Ala(2),N-MePhe(4),Gly(5)-ol]-enkephalin (DAMGO), remained unaffected as measured via hMORN230T and hMORN230L, while the potency of fentanyl and morphine significantly increased via these mutated receptors. 4. Our results are indicative for the existence of hydrophobic interactions between a methyl-group of the side chain of Thr or Leu on the one hand and the piperidine-ring of fentanyl and the hexene-ring of morphine on the other. The mutations also had no influence on the potency of morphine-6-glucuronide (M6G) and morphine-3-glucuronide (M3G). 5. We conclude that the hydrophilic side chain of Asn in position 230 is not involved in the formation of a H-bond with the aliphatic alcohol of morphine and that an enhancement of the potency of morphine and fentanyl can be explained by mutating this residue towards more hydrophobic amino acids.

Characterization of inhibitors acting at the synthetase site of Escherichia coli asparagine synthetase B

Asparagine synthetase catalyzes the ATP-dependent formation of L-asparagine from L-aspartate and L-glutamine, via a beta-aspartyl-AMP intermediate. Since interfering with this enzyme activity might be useful for treating leukemia and solid tumors, we have sought small-molecule inhibitors of Escherichia coli asparagine synthetase B (AS-B) as a model system for the human enzyme. Prior work showed that L-cysteine sulfinic acid competitively inhibits this enzyme by interfering with L-aspartate binding. Here, we demonstrate that cysteine sulfinic acid is also a partial substrate for E. coli asparagine synthetase, acting as a nucleophile to form the sulfur analogue of beta-aspartyl-AMP, which is subsequently hydrolyzed back to cysteine sulfinic acid and AMP in a futile cycle. While cysteine sulfinic acid did not itself constitute a clinically useful inhibitor of asparagine synthetase B, these results suggested that replacing this linkage by a more stable analogue might lead to a more potent inhibitor. A sulfoximine reported recently by Koizumi et al. as a competitive inhibitor of the ammonia-dependent E. coli asparagine synthetase A (AS-A) [Koizumi, M., Hiratake, J., Nakatsu, T., Kato, H., and Oda, J. (1999) J. Am. Chem. Soc. 121, 5799-5800] can be regarded as such a species. We found that this sulfoximine also inhibited AS-B, effectively irreversibly. Unlike either the cysteine sulfinic acid interaction with AS-B or the sulfoximine interaction with AS-A, only AS-B productively engaged in asparagine synthesis could be inactivated by the sulfoximine; free enzyme was unaffected even after extended incubation with the sulfoximine. Taken together, these results support the notion that sulfur-containing analogues of aspartate can serve as platforms for developing useful inhibitors of AS-B.

Glutamyl-tRNA(Gln) amidotransferase in Deinococcus radiodurans may be confined to asparagine biosynthesis

Asparaginyl-tRNA (Asn-tRNA) and glutaminyl-tRNA (Gln-tRNA) are essential components of protein synthesis. They can be formed by direct acylation by asparaginyl-tRNA synthetase (AsnRS) or glutaminyl-tRNA synthetase (GlnRS). The alternative route involves transamidation of incorrectly charged tRNA. Examination of the preliminary genomic sequence of the radiation-resistant bacterium Deinococcus radiodurans suggests the presence of both direct and indirect routes of Asn-tRNA and Gln-tRNA formation. Biochemical experiments demonstrate the presence of AsnRS and GlnRS, as well as glutamyl-tRNA synthetase (GluRS), a discriminating and a nondiscriminating aspartyl-tRNA synthetase (AspRS). Moreover, both Gln-tRNA and Asn-tRNA transamidation activities are present. Surprisingly, they are catalyzed by a single enzyme encoded by three ORFs orthologous to Bacillus subtilis gatCAB. However, the transamidation route to Gln-tRNA formation is idled by the inability of the discriminating D. radiodurans GluRS to produce the required mischarged Glu-tRNAGln substrate. The presence of apparently redundant complete routes to Asn-tRNA formation, combined with the absence from the D. radiodurans genome of genes encoding tRNA-independent asparagine synthetase and the lack of this enzyme in D. radiodurans extracts, suggests that the gatCAB genes may be responsible for biosynthesis of asparagine in this asparagine prototroph.

Nitrogen assimilation in alfalfa: isolation and characterization of an asparagine synthetase gene showing enhanced expression in root nodules and dark-adapted leaves

Asparagine, the primary assimilation product from N2 fixation in temperate legumes and the predominant nitrogen transport product in many plant species, is synthesized via asparagine synthetase (AS; EC 6.3.5.4). Here, we report the isolation and characterization of a cDNA and a gene encoding the nodule-enhanced form of AS from alfalfa. The AS gene is comprised of 13 exons separated by 12 introns. The 5' flanking region of the AS gene confers nodule-enhanced reporter gene activity in transformed alfalfa. This region also confers enhanced reporter gene activity in dark-treated leaves. These results indicate that the 5' upstream region of the AS gene contains elements that affect expression in root nodules and leaves. Both AS mRNA and enzyme activity increased approximately 10- to 20-fold during the development of effective nodules. Ineffective nodules have strikingly reduced amounts of AS transcript. Alfalfa leaves have quite low levels of AS mRNA and protein; however, exposure to darkness resulted in a considerable increase in both. In situ hybridization with effective nodules and beta-glucuronidase staining of nodules from transgenic plants showed that AS is expressed in both infected and uninfected cells of the nodule symbiotic zone and in the nodule parenchyma. RNA gel blot analysis and in situ hybridization results are consistent with the hypothesis that initial AS expression in nodules is independent of nitrogenase activity.

Mutagenesis and chemical rescue indicate residues involved in beta-aspartyl-AMP formation by Escherichia coli asparagine synthetase B

Site-directed mutagenesis and kinetic studies have been employed to identify amino acid residues involved in aspartate binding and transition state stabilization during the formation of beta-aspartyl-AMP in the reaction mechanism of Escherichia coli asparagine synthetase B (AS-B). Three conserved amino acids in the segment defined by residues 317-330 appear particularly crucial for enzymatic activity. For example, when Arg-325 is replaced by alanine or lysine, the resulting mutant enzymes possess no detectable asparagine synthetase activity. The catalytic activity of the R325A AS-B mutant can, however, be restored to about 1/6 of that of wild-type AS-B by the addition of guanidinium HCl (GdmHCl). Detailed kinetic analysis of the rescued activity suggests that Arg-325 is involved in stabilization of a pentacovalent intermediate leading to the formation beta-aspartyl-AMP. This rescue experiment is the second example in which the function of a critical arginine residue that has been substituted by mutagenesis is restored by GdmHCl. Mutation of Thr-322 and Thr-323 also produces enzymes with altered kinetic properties, suggesting that these threonines are involved in aspartate binding and/or stabilization of intermediates en route to beta-aspartyl-AMP. These experiments are the first to identify residues outside of the N-terminal glutamine amide transfer domain that have any functional role in asparagine synthesis.

Mapping the aspartic acid binding site of Escherichia coli asparagine synthetase B using substrate analogs

Novel inhibitors of asparagine synthetase, that will lower circulating levels of blood asparagine, have considerable potential in developing new protocols for the treatment of acute lymphoblastic leukemia. We now report the indirect characterization of the aspartate binding site of Escherichia coli asparagine synthetase B (AS-B) using a number of stereochemically, and conformationally, defined aspartic acid analogs. Two compounds, prepared using novel reaction conditions for the stereospecific beta-functionalization of aspartic acid diesters, have been found to be competitive inhibitors with respect to aspartate in kinetic studies on AS-B. Chemical modification experiments employing [(fluorosulfonyl)benzoyl]adenosine (FSBA), an ATP analog, demonstrate that both inhibitors bind to the aspartate binding site of AS-B. Our results reveal that large steric alterations in the substrate are not tolerated by the enzyme, consistent with the failure of previous efforts to develop AS inhibitors using random screening approaches, and that all of the ionizable groups are placed in close proximity in the bound conformation of aspartate.

Molecular cloning and characterisation of asparagine synthetase from Lotus japonicus: dynamics of asparagine synthesis in N-sufficient conditions

Two cDNA clones, LJAS1 and LJAS2, encoding different asparagine synthetases (AS) have been identified and sequenced and their expression in Lotus japonicus characterised. Analysis of predicted amino acid sequences indicted a high level of identity with other plant AS sequences. No other AS genes were detected in the L. japonicus genome. LJAS1 gene expression was found to be root-enhanced and lower levels of transcript were also identified in photosynthetic tissues. In contrast, LJAS2 gene expression was root-specific. These patterns of AS gene expression are different from those seen in pea. AS gene expression was monitored throughout a 16 h light/8 h dark day, under nitrate-sufficient conditions. Neither transcript showed the dark-enhanced accumulation patterns previously reported for other plant AS genes. To evaluate AS activity, the molecular dynamics of asparagine synthesis were examined in vivo using 15N-ammonium labelling. A constant rate of asparagine synthesis in the roots was observed. Asparagine was the most predominant amino-component of the xylem sap and became labelled at a slightly slower rate than the asparagine in the roots, indicating that most root asparagine was located in a cytoplasmic 'transport' pool rather than in a vacuolar 'storage' pool. The steady-state mRNA levels and the 15N-labelling data suggest that light regulation of AS gene expression is not a factor controlling N-assimilation in L. japonicus roots during stable growth in N-sufficient conditions.

Post-transcriptional regulation of nitrate reductase by light is abolished by an N-terminal deletion

Higher plant nitrate reductases (NRs) carry an N-terminal domain whose sequence is not conserved in NRs from other organisms. A gene composed of a full-length tobacco NR cDNA with an internal deletion of 168 bp in the 5' end fused to the cauliflower mosaic virus 35S promoter and appropriate termination signals was constructed and designated as delta NR. An NR-deficient mutant of Nicotiana plumbaginifolia was transformed with this delta NR gene. In transgenic plants expressing this construct, NR activity was restored and normal growth resulted. Apart from a higher thermosensitivity, no appreciable modification of the kinetic parameters of the enzyme was detectable. The post-transcriptional regulation of NR by light was abolished in delta NR transformants. Consequently, deregulated production of glutamine and asparagine was detected in delta NR transformants. The absence of in vitro delta NR activity modulation by ATP suggests the impairment of delta NR phosphorylation and thereby suppression of delta NR post-translational regulation. These data imply that post-transcriptional control of NR expression is important for the flow of the nitrate assimilatory pathway.

Arginine 30 and asparagine 74 have functional roles in the glutamine dependent activities of Escherichia coli asparagine synthetase B

Although Arg-30, Asn-74, and Asn-79 appear totally conserved throughout the purF glutamine-dependent amidotransferases, their potential roles in catalysis and binding remain unexplored for any member of the enzyme family. Here we report the overexpression, purification, and kinetic characterization of a series of AS-B mutants which allow an examination of the functional roles of these 3 residues in glutamine-dependent nitrogen transfer. While Asn-79 appears to possess no catalytic function in AS-B, site-directed mutagenesis of Asn-74 has implicated this residue as playing a role in catalysis of nitrogen transfer from glutamine. The kinetic properties of the Asn-74 AS-B mutant enzymes appear consistent with both ammonia-mediated nitrogen transfer and two apparently novel mechanistic suggestions for this reaction involving either an oxyanion or imide intermediate (Richards, N. G. J., and Schuster, S. M. (1992) FEBS Lett. 313, 98-102). We also demonstrate that replacement of Arg-30 by an alanine residue yields an AS-B mutant for which the apparent Km for glutamine is increased in the glutamine-dependent synthesis of asparagine. In addition, ATP-dependent stimulation of the glutaminase activity of AS-B is modified or completely eliminated when Arg-30 is replaced by other amino acids. The latter observation may indicate the existence of a molecular switch involving Arg-30 which coordinates the two half-reactions catalyzed by the glutamine-dependent amidotransferases and synthetase domains of cellular AS-B.

[The effect of amino acids in the asparagine family on the aspartate kinase and homoserine dehydrogenase of ethionine-resistant mutants of Pseudomonas putida]

The activity of aspartate kinase and homoserin dehydrogenase from ethionine resistant mutants Pseudomonas putida 25 and 6 have been studied as affected by amino acids from the family of asparagine. They are characterized by a capacity to the surplus synthesis of methionine. It is shown that mutants have negative regulation of the level of activity of the studied enzymes. It is supposed that the mutations (or mutation) could take place which affected properties of enzymes, which participated directly in the biosynthesis of methionine, in the analogue resistant clones 25 and 6.

Glutamine and asparagine synthesis in the nematodes Heligmosomoides polygyrus and Panagrellus redivivus

Low levels of glutamine synthetase were demonstrated in Panagrellus redivivus and Heligmosomoides polygyrus. Asparagine synthetase was detected in P. redivivus but activity was too low to be detected in H. polygyrus.

gamma-N-methylasparagine in phycobiliproteins. Occurrence, location, and biosynthesis

The novel post-translationally modified residue gamma-N-methylasparagine, previously detected in the beta subunit of allophycocyanin (Klotz, A. V., Leary, J. A., and Glazer, A. N. (1986) J. Biol. Chem. 261, 15891-15894), has been found in the beta subunits of a variety of other phycobiliproteins. Representatives of C- and R-phycocyanins and B-, C-, and R-phycoerythrins all contain 1 eq of gamma-N-methylasparagine on their beta subunits as judged by the presence of methylamine in acid hydrolysates. Radiotracer experiments show that the methyl group is derived from the S-methyl of methionine, implicating S-adenosylmethionine as an intermediate methyl transfer agent. Isolation of peptides from C-phycocyanins, prepared from cells labeled by L-[methyl-14C]methionine, showed that the gamma-N-methylasparagine residue is at position beta-72, within a highly conserved region in phycobiliproteins. This location corresponds to that reported earlier for the position of gamma-N-methylasparagine in allophycocyanin and R-phycoerythrin. Phycobiliprotein alpha subunits contain insignificant amounts of the adduct. Methylamine is absent from the hydrolysates of the beta subunits or alpha beta monomers of phycobiliproteins from certain organisms. These latter data indicate that the gamma-N-methylasparagine residue is dispensable in some circumstances. The function of this modification remains to be established. gamma-N-methylasparagine was also absent from several other proteins including bovine histones, porcine myelin basic peptide, and the Salmonella typhimurium aspartate chemoreceptor, all known to undergo post-translational methylations.

Characterization of a mannosidase acting on alpha 1----3- and alpha 1----6-linked mannose residues of oligomannosidic intermediates of glycoprotein processing

Baby hamster kidney (BHK) cell extracts catalyze the conversion of [3H]mannose-labelled (Man)5GlcNAc and (Man)6GlcNAc oligosaccharides to a (Man)3GlcNAc species that retains affinity for concanavalin-A-Sepharose and appears to be Man alpha 1----3[Man alpha 1----6]Man beta 1----4GlcNAc. The properties of the (Man)5GlcNAc-hydrolase activity differ from lysosomal alpha-mannosidases as well as previously described processing mannosidases acting on oligosaccharide intermediates of N-glycan assembly. Mosquito cell extracts catalyze hydrolysis of (Man)6GlcNAc but lack the (Man)5GlcNAc hydrolase activity detected in BHK cell extracts. Glycopeptide analysis has been carried out on a ricin-resistant BHK mutant RicR14 that lacks N-acetylglucosaminyl transferase I and fails to convert oligomannosidic N-glycans to complex-type chains, and mosquito cells that constitutively lack N-acetylglucosaminyl transferase I. In both cell lines, the cellular glycoproteins contain (Man)5GlcNAc oligosaccharide as the major stable component equivalent to a 15-20-fold increase compared with normal BHK cells. Although containing very high amounts of asparagine-linked (Man)5(GlcNAc)2, RicR14 cells exhibit (Man)5GlcNAc hydrolase activity at levels similar to wild-type BHK cells. This result, together with previous work [Foddy, L., Feeney, J. & Hughes, R.C. (1986) Biochem. J. 233, 697-706] showing the complete inhibition of conversion of oligomannosidic intermediates to complex-type N-glycans in BHK cells treated with swainsonine, an inhibitor of mannosidase II but not the (Man)5GlcNAc hydrolase activity, argues against a major role for the (Man)5GlcNAc hydrolase activity in N-glycan assembly and suggesting other functions for the mannosidase activity in BHK cells.

Metabolism of a transformation-sensitive amino acid fucoside in normal and simian virus 40-transformed human embryonic lung cells

We have demonstrated previously that there is a marked decrease in the level of labeled fucose incorporated into an amino acid fucoside, i.e., fucosyl-N-acetylglucosaminylasparagine (FL4c) in SV40-transformed human embryonic lung cells (SV40-WI-38) as compared to WI-38 cells (Morton, P.A., Klinger, M. M., and Steiner, S. Cancer Res., 42: 3022-3027, 1982). In the current study, we have observed that the reduction in labeled fucose incorporated into FL4c in the SV40-WI-38 cells is paralleled by reduced chemical quantity of that component. [3H]-Fucose pulse/chase and long-term fucose labeling/chase studies, in some instances in the presence of tunicamycin, have revealed that FL4c is a relatively stable end produce of N-linked-type glycoprotein metabolism. The relative metabolic stability argues against breakdown of FL4c as the basis for the markedly reduced level in the SV40-WI-38 cells. However, the transformed cells manifested an almost 3-fold higher level of alpha-L-fucosidase activity when p-nitrophenyl-alpha-fucoside was used as substrate. These results raise the possibility that the parent glycoprotein of FL4c might be more rapidly catabolized in the SV40-WI-38 cells than in the WI-38 cells. Whatever the biochemical basis for the decrease in level of FL4c in transformed human cells, it would seem to underlie a difference between normal and transformed cells in membrane glycoprotein catabolism.

[Role of beta-cyanoalanine hydratase in the synthesis of asparagine in white lupine]

A highly active beta-cyanoalanine hydratase catalyzing the asparagine synthesis from beta-cyanoalanine was isolated from 11-day-old etyolated seedlings of white lupine (Lupinus albus) and subjected to purification. In white lupine seedlings the enzyme activity is 300 times as high as that in extracts of blue lupine (Lupinus angustifolius). The experimental data suggest that in white lupine seedlings asparagine synthesis from cysteine and cyanide occurs via beta-cyanoalanine production with its subsequent hydratation. The physiological role of beta-cyanoalanine hydratase from white lupine consists in an extensive synthesis of asparagine, whose concentration reaches 0.1 M.

Asparagine synthetases of Klebsiella aerogenes: properties and regulation of synthesis

We isolated pleiotropic mutants of Klebsiella aerogenes with the transposon Tn5 which were unable to utilize a variety of poor sources of nitrogen. The mutation responsible was shown to be in the asnB gene, one of two genes coding for an asparagine synthetase. Mutations in both asnA and asnB were necessary to produce an asparagine requirement. Assays which could distinguish the two asparagine synthetase activities were developed in strains missing a high-affinity asparaginase. The asnA and asnB genes coded for ammonia-dependent and glutamine-dependent asparagine synthetases, respectively. Asparagine repressed both enzymes. When growth was nitrogen limited, the level of the ammonia-dependent enzyme was low and that of the glutamine-dependent enzyme was high. The reverse was true in a nitrogen-rich (ammonia-containing) medium. Furthermore, mutations in the glnG protein, a regulatory component of the nitrogen assimilatory system, increased the level of the ammonia-dependent enzyme. The glutamine-dependent asparagine synthetase was purified to 95%. It was a tetramer with four equal 57,000-dalton subunits and catalyzed the stoichiometric generation of asparagine, AMP, and inorganic pyrophosphate from aspartate, ATP, and glutamine. High levels of ammonium chloride (50 mM) could replace glutamine. The purified enzyme exhibited a substrate-independent glutaminase activity which was probably an artifact of purification. The tetramer could be dissociated; the monomer possessed the high ammonia-dependent activity and the glutaminase activity, but not the glutamine-dependent activity. In contrast, the purified ammonia-dependent asparagine synthetase, about 40% pure, had a molecular weight of 80,000 and is probably a dimer of identical subunits. Asparagine inhibited both enzymes. Kinetic constants and the effect of pH, substrate, and product analogs were determined. The regulation and biochemistry of the asparagine synthetases prove the hypothesis strongly suggested by the genetic and physiological evidence that a glutamine-dependent enzyme is essential for asparagine synthesis when the nitrogen source is growth rate limiting.

Potential inhibitors of L-asparagine biosynthesis. 5. Electrophilic amide analogues of (S)-2,3-diaminopropionic acid

Three electrophilic amide analogues of (S)-2,3-diaminopropionic acid (1, DAP) have been prepared as potential inhibitors of L-asparagine synthetase (ASase, from Novikoff hepatoma, EC 6.3.5.4). DAP was selectively blocked by the carbobenzoxy (Cbz) group to give 3-N-Cbz-DAP (2a). Esterification of 2a with isobutylene afforded tert-butyl 3-N-carbobenzoxy-(S)-2,3-diaminopropionate (3a), which was then blocked at the 2 position with the tert-butoxycarbonyl (Boc) group to give tert-butyl 2-[(S)-(tert-butoxycarbonyl)amino]-3-[(carbobenzoxy)amino]propionate (4). Selective cleavage of the Cbz group by H2/Pd gave the key intermediate tert-butyl 2-N-(tert-butoxycarbonyl)-(S)-2,3-diaminopropionate (5), which was acylated, via the N-hydroxysuccinimide esters, with bromoacetic acid, dichloroacetic acid, and fumaric acid monoethyl ester to give tert-butyl 2-[(S)-(tert-butoxycarbonyl)-amino]-3-(2-bromoacetamido)propionate (6a), tert-butyl 2-[(S)-(tert-butoxycarbonyl)amino]-3-(2,2-dichloroacetamido)propionate (6b), and tert-butyl 2-[(S)-(tert-butoxycarbonyl)amino]-3-(ethoxycarbonyl)acrylamido]-propionate (6c), respectively. Deblocking of 6a-c gave the corresponding amino acids (S)-2-amino-3-(2-bromoacetamido)propionic acid hydrobromide (7a), (S)-2-amino-3-(2,2-dichloroacetamido)propionic acid (7b), and ethyl N-[(S)-2-amino-2-carboxyethyl]fumarate (7c). By a slightly different procedure, 5 was converted in two steps to (S)-2-amino-3-acetamidopropionic acid hydrobromide (7d). The inhibition of ASase by 7a-c at 1 mM was 93, 19, and 37%, respectively, while 7d was without inhibition at 2 mM. Compounds 7a-c failed to increase the life span of mice infected with B16 melanoma.

Genetic and biomedical studies demonstrating a second gene coding for asparagine synthetase in Escherichia coli

Genetic experiments have indicated that asparagine auxotrophs of Escherichias coli K-12 can be made asparagine prototrophs at either of two sites on the chromosome and that wild-type strains require both sites to be mutated to produce asparagine auxotrophy. The former asn locus is now called asnA, and the new gene is designated asnB. The asnB gene is located near gal.AsnA+ asnB and asnA asnB+ strains were constructed, and the asparagine synthetic reaction was characterized in extracts. These studies revealed that the asnA gene codes for the enzyme previously described (H. Cedar and J.H. Schwartz, J. Biol. Chem. 244: 4112-4121, 1969), whereas the asnB gene is involved in the production of an enzyme which differs from the one previously described in its specific activity in extracts, its stability at low and high temperatures, and its apparent ability to use either glutamine or ammonia as amide nitrogen donor. Physiological studies showed that either enzyme alone is sufficient to allow a maximal growth rate under conditions of asparagine limitation.

L-asparaginyl-tRNA synthetase and L-asparagine synthetase activities of L-asparaginase-sensitive and -resistant forms of the mouse Gardner lymphoma 6C3HED

1. The mouse Gardner lymphoma 6C3HED was grown in ascites fluid in a form sensitive to the action of L-asparaginase (line 1), in another form which was resistant to L-asparaginase (line 2) and in a third form with partial sensitivity to L-asparaginase (line 3). 2. The L-asparaginyl-tRNA synthetase activities of extracts of the tumour cells, cultured both in the mouse and in vitro, were determined. Two of the lines, 1 and 3, in early passage numbers, showed a derepression mechanism involving L-asparagine. Mutation occurred with these lines resulting in the L-asparaginyl-tRNA synthetase activity of all the tumour cell lines being the same. 3. Cells of line 1 had low L-asparagine synthetase activity, which was unchanged by altering the supply of L-asparagine in vitro. Cells of lines 2 and 3 exhibited L-asparagine synthetase activities, which changed with the supply of L-asparagine. 4. It is not certain that L-asparagine synthetase activity of L-asparaginase-sensitive cells is controlled by L-asparaginyl-tRNA acting as a corepressor.

L-Asparagine auxotrophs of Saccharomyces cerevisiae: genetic and phenotypic characterization

L-Asparagine auxotrophy in Saccharomyces cerevisiae is the result of mutation in each of two unlinked cistrons, ASN1 and ASN2. Mutation in only one of these cistrons yields growth indistinguishable from that of wild-type cells under a variety of nutritional stresses. Relatively high concentrations of L-asparagine are required to permit maximal growth of the auxotrophs, and the amino acid requirement cannot be satisfied by a variety of other amino acids that serve as nitrogen sources for cell growth. Although reversion of the mutations can occur, haploid populations of cells containing only low frequencies of prototrophs can be maintained easily. In diploid cells heteroallelic for certain combinations of alleles of the two genes, mitotic recombination gives rise to prototrophic cells that accumulate to high frequency in populations of the cells.

Evidence for cycloheximide acting as a glutamine analogue in plant tissue

In growing maize root tissue [14C]asparagine formation in inhibited and [14C]glutamine accumulation stimulated by treatment with cycloheximide or glutamine analogs such as azaserine. In contrast, puromycin enhances the accumulation of [14C]asparagine but not [14C]glutamine. Cycloheximide and puromycin alone inhibit protein synthesis. This is interpreted to mean that the alteration in amide metabolism following cycloheximide treatment is a direct result of the antibiotic acting as a glutamine analog. While cycloheximide is often the cytoplasmic protein synthesis inhibitor of choice due to its potency and rapid action, its assumed specificity of action of eukaryotes is doubtful.