l-Canavanine Metabolism in Jack Bean, Canavalia ensiformis (L.) DC. (Leguminosae)

Canavanine-Induced Decrease in Nitric Oxide Synthesis Alters Activity of Antioxidant System but Does Not Impact S-Nitrosoglutathione Catabolism in Tomato Roots

Canavanine (CAN) is a nonproteinogenic amino acid synthesized in legumes. In mammalians, as arginine analogue, it is an inhibitor of nitric oxide synthase (NOS) activity. The aim of this study was to investigate the impact of CAN-induced nitric oxide level limitation on the antioxidant system and S-nitrosoglutathione (GSNO) metabolism in roots of tomato seedlings. Treatment with CAN (10 or 50 µM) for 24-72 h led to restriction in root growth. Arginine-dependent NOS-like activity was almost completely inhibited, demonstrating direct effect of CAN action. CAN increased total antioxidant capacity and the level of sulphydryl groups. Catalase (CAT) and superoxide dismutase (SOD) activity decreased in CAN exposed roots. CAN supplementation resulted in the decrease of transcript levels of genes coding CAT (with the exception of CAT1). Genes coding SOD (except MnSOD and CuSOD) were upregulated by CAN short treatment; prolonged exposition to 50-µM CAN resulted in downregulation of FeSOD, CuSOD, and SODP-2. Activity of glutathione reductase dropped down after short-term (10-µM CAN) supplementation, while glutathione peroxidase activity was not affected. Transcript levels of glutathione reductase genes declined in response to CAN. Genes coding glutathione peroxidase were upregulated by 50-µM CAN, while 10-µM CAN downregulated GSHPx1. Inhibition of NOS-like activity by CAN resulted in lower GSNO accumulation in root tips. Activity of GSNO reductase was decreased by short-term supplementation with CAN. In contrast, GSNO reductase protein abundance was higher, while transcript levels were slightly altered in roots exposed to CAN. This is the first report on identification of differentially nitrated proteins in response to supplementation with nonproteinogenic amino acid. Among nitrated proteins differentially modified by CAN, seed storage proteins (after short-term CAN treatment) and components of the cellular redox system (after prolonged CAN supplementation) were identified. The findings demonstrate that due to inhibition of NOS-like activity, CAN leads to modification in antioxidant system. Limitation in GSNO level is due to lower nitric oxide formation, while GSNO catabolism is less affected. We demonstrated that monodehydroascorbate reductase, activity of which is inhibited in roots of CAN-treated plants, is the protein preferentially modified by tyrosine nitration.

Heteroatom-Heteroatom Bond Formation in Natural Product Biosynthesis

Natural products that contain functional groups with heteroatom-heteroatom linkages (X-X, where X = N, O, S, and P) are a small yet intriguing group of metabolites. The reactivity and diversity of these structural motifs has captured the interest of synthetic and biological chemists alike. Functional groups containing X-X bonds are found in all major classes of natural products and often impart significant biological activity. This review presents our current understanding of the biosynthetic logic and enzymatic chemistry involved in the construction of X-X bond containing functional groups within natural products. Elucidating and characterizing biosynthetic pathways that generate X-X bonds could both provide tools for biocatalysis and synthetic biology, as well as guide efforts to uncover new natural products containing these structural features.

Modification of the endogenous NO level influences apple embryos dormancy by alterations of nitrated and biotinylated protein patterns

NO donors and Arg remove dormancy of apple embryos and stimulate germination. Compounds lowering NO level (cPTIO, L -NAME, CAN) strengthen dormancy. Embryo transition from dormancy state to germination is linked to increased nitric oxide synthase (NOS)-like activity. Germination of embryos is associated with declined level of biotin containing proteins and nitrated proteins in soluble protein fraction of root axis. Pattern of nitrated proteins suggest that storage proteins are putative targets of nitration. Nitric oxide (NO) acts as a key regulatory factor in removal of seed dormancy and is a signal necessary for seed transition from dormant state into germination. Modulation of NO concentration in apple (Malus domestica Borkh.) embryos by NO fumigation, treatment with NO donor (S-nitroso-N-acetyl-D,L-penicillamine, SNAP), application of 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (cPTIO), N ω-nitro-L-arginine methyl ester (L-NAME), canavanine (CAN) or arginine (Arg) allowed us to investigate the NO impact on seed dormancy status. Arg analogs and NO scavenger strengthened embryo dormancy by lowering reactive nitrogen species level in embryonic axes. This effect was accompanied by strong inhibition of NOS-like activity, without significant influence on tissue NO2 (-) concentration. Germination sensu stricto of apple embryos initiated by dormancy breakage via short term NO treatment or Arg supplementation were linked to a reduced level of biotinylated proteins in root axis. Decrease of total soluble nitrated proteins was observed at the termination of germination sensu stricto. Also modulation of NO tissue status leads to modification in nitrated protein pattern. Among protein bands that correspond to molecular mass of approximately 95 kDa, storage proteins (legumin A-like and seed biotin-containing protein) were identified, and can be considered as good markers for seed dormancy status. Moreover, pattern of nitrated proteins suggest that biotin containing proteins are also targets of nitration.

Cyanamide is biosynthesized from L-canavanine in plants

Cyanamide had long been recognized as a synthetic compound but more recently has been found as a natural product from several leguminous plants. This compound’s biosynthetic pathway, as yet unelaborated, has attracted attention because of its utility in many domains, such as agriculture, chemistry, and medicine. We noticed that the distribution of L-canavanine in the plant kingdom appeared to include that of cyanamide and that the guanidino group structure in L-canavanine contained the cyanamide skeleton. Here, quantification of these compounds in Vicia species suggested that cyanamide was biosynthesized from L-canavanine. Subsequent experiments involving L-[guanidineimino-¹⁵N₂]canavanine addition to young Vicia villosa seedlings resulted in significant incorporation of ¹⁵N-label into cyanamide, verifying its presumed biosynthetic pathway.

Avoidance of nonprotein amino acid incorporation into protein by the seed predator,Caryedes brasiliensis (Bruchidae)

Larvae of the bruchid beetle,Caryedes brasiliensis (Bruchidae) have the ability to avoid significant incorporation ofL-canavanine, the guanidinooxy structural analog ofL-arginine, into de novo synthesized proteins. This ability is related to a highly discriminatory protein-synthesizing system which exhibits marked ability to avoid processing an array of nonprotein amino acids structurally related to arginine.

Antidiabetic II drug metformin in plants: uptake and translocation to edible parts of cereals, oily seeds, beans, tomato, squash, carrots, and potatoes

Residues of pharmaceuticals present in wastewater and sewage sludge are of concern due to their transfer to aquatic and terrestrial food chains and possible adverse effects on nontargeted organisms. In the present work, uptake and translocation of metformin, an antidiabetic II medicine, by edible plant species cultivated in agricultural soil have been investigated in greenhouse experiment. Metformin demonstrated a high uptake and translocation to oily seeds of rape ( Brassica napus cv. Sheik and Brassica rapa cv. Valo); expressed as an average bioconcentration factor (BCF, plant concentration over initial concentration in soil, both in dry weight), BCF values as high as 21.72 were measured. In comparison, BCFs for grains of the cereals wheat, barley, and oat were in the range of 0.29-1.35. Uptake and translocation to fruits and vegetables of tomato (BCFs 0.02-0.06), squash (BCFs 0.12-0.18), and bean (BCF 0.88) were also low compared to rape. BCFs for carrot, potato, and leaf forage B. napus cv. Sola were similar (BCF 1-4). Guanylurea, a known degradation product of metformin by microorganisms in activated sludge, was found in barley grains, bean pods, potato peel, and small potatoes. The mechanisms for transport of metformin and guanidine in plants are still unknown, whereas organic cation transporters (OCTs) in mammals are known to actively transport such compounds and may guide the way for further understanding of mechanisms also in plants.

Identification of the biosynthetic gene cluster for 3-methylarginine, a toxin produced by Pseudomonas syringae pv. syringae 22d/93

The epiphyte Pseudomonas syringae pv. syringae 22d/93 (Pss22d) produces the rare amino acid 3-methylarginine (MeArg), which is highly active against the closely related soybean pathogen Pseudomonas syringae pv. glycinea. Since these pathogens compete for the same habitat, Pss22d is a promising candidate for biocontrol of P. syringae pv. glycinea. The MeArg biosynthesis gene cluster codes for the S-adenosylmethionine (SAM)-dependent methyltransferase MrsA, the putative aminotransferase MrsB, and the amino acid exporter MrsC. Transfer of the whole gene cluster into Escherichia coli resulted in heterologous production of MeArg. The methyltransferase MrsA was overexpressed in E. coli as a His-tagged protein and functionally characterized (K(m), 7 mM; k(cat), 85 min(-1)). The highly selective methyltransferase MrsA transfers the methyl group from SAM into 5-guanidino-2-oxo-pentanoic acid to yield 5-guanidino-3-methyl-2-oxo-pentanoic acid, which then only needs to be transaminated to result in the antibiotic MeArg.

The supernumerary chromosome of Nectria haematococca that carries pea-pathogenicity-related genes also carries a trait for pea rhizosphere competitiveness

Fungi are found in a wide range of environments, and the ecological and host diversity of the fungus Nectria haematococca has been shown to be due in part to unique genes on different supernumerary chromosomes. These chromosomes have been called "conditionally dispensable" (CD) since they are not needed for axenic growth but are important for expanding the host range of individual isolates. From a biological perspective, the CD chromosomes can be compared to bacterial plasmids that carry unique genes that can define the habits of these microorganisms. The current study establishes that the N. haematococca PDA1-CD chromosome, which contains the genes for pea pathogenicity (PEP cluster) on pea roots, also carries a gene(s) for the utilization of homoserine, a compound found in large amounts in pea root exudates. Competition studies demonstrate that an isolate that lacks the PEP cluster but carries a portion of the CD chromosome which includes the homoserine utilization (HUT) gene(s) is more competitive in the pea rhizosphere than an isolate without the CD chromosome.

Genes, enzymes and regulation of arginine biosynthesis in plants

Arabidopsis genes encoding enzymes for each of the eight steps in L-arginine (Arg) synthesis were identified, based upon sequence homologies with orthologs from other organisms. Except for N-acetylglutamate synthase (NAGS; EC 2.3.1.1), which is encoded by two genes, all remaining enzymes are encoded by single genes. Targeting predictions for these enzymes, based upon their deduced sequences, and subcellular fractionation studies, suggest that most enzymes of Arg synthesis reside within the plastid. Synthesis of the L-ornthine (Orn) intermediate in this pathway from L-glutamate occurs as a series of acetylated intermediates, as in most other organisms. An N-acetylornithine:glutamate acetyltransferase (NAOGAcT; EC 2.3.1.35) facilitates recycling of the acetyl moiety during Orn formation (cyclic pathway). A putative N-acetylornithine deacetylase (NAOD; EC 3.5.1.16), which participates in the "linear" pathway for Orn synthesis in some organisms, was also identified. Previous biochemical studies have indicated that allosteric regulation of the first and, especially, the second steps in Orn synthesis (NAGS; N-acetylglutamate kinase (NAGK), EC 2.7.2.8) by the Arg end-product are the major sites of metabolic control of the pathway in organisms using the cyclic pathway. Gene expression profiling for pathway enzymes further suggests that NAGS, NAGK, NAOGAcT and NAOD are coordinately regulated in response to changes in Arg demand during plant growth and development. Synthesis of Arg from Orn is further coordinated with pyrimidine nucleotide synthesis, at the level of allocation of the common carbamoyl-P intermediate.

Identification of an isoform of ornithine carbamoyltransferase that can effectively utilize canaline as a substrate from the leaves of Canavalia lineata

An isoform of ornithine carbamoyltransferase that can utilize effectively canaline as a substrate (C-OCT) was identified from the leaves of Canavalia lineata (Thunb.) DC. The molecular mass of native C-OCT was 109 kDa by Sephacryl S-200 gel filtration and that of the subunit was 36 kDa by SDS-PAGE and immunoblotting using the antibody against kidney bean ornithine carbamoyltransferase (OCT). C-OCT has pH optimum at 8.0 for canaline-dependent OCT activity and Michaelis constants of 9.6 mM for canaline and 0.24 mM for carbamoyl phosphate. To some extent, the C-OCT also showed ornithine-dependent OCT activity and a pH optimum of 8.5; Michaelis constants for ornithine and carbamoyl phosphate were 0.21 and 0.086 mM, respectively. The enzyme exhibit V(max) for canaline-dependent activity 14-fold higher than that for ornithine-dependent activity and the ratio of canaline-dependent activity to ornithine-dependent activity was 66-fold higher than that of OCT of the same plant. It is likely that this enzyme plays an important role in the canavanine synthesis from the canavanine-containing plants.

Metabolism of l-Canavanine and l-Canaline in Leguminous Plants

Massive accumulation of l-canavanine, the 2-amino-4-(guanidinooxy)butyric acid structural analog of l-arginine, occurs in many legumes. Accumulation of large amounts of this nonprotein amino acid results in large part from canavanine's protective efficacy; it forms an effective chemical barrier to predation, disease, and even competition with other plants. Diversion of metabolic resources for the synthesis and storage of appreciable canavanine does not place an inordinate burden on the plant. Catabolism of this nonprotein amino acid provides respiratory carbon, generates essential primary metabolites, and ammoniacal nitrogen for the developing plant.

A higher plant enzyme exhibiting broad acceptance of stereoisomers

An arginase, purified from the leaf of the jack bean, Canavalia ensiformis, can effectively hydrolyze both l- and d-arginine. Arginases, examined from a number of other plant and animal sources, exhibit marked substrate stereospecificity and fail to catabolize d-arginine. In order to provide essential nitrogen, jack bean leaf arginase also catabolizes l-canavanine, an arginine analog that is a predominant nitrogen-storing metabolite of this legume. The ability of arginase to metabolize both stereoisomers of arginine may result from the requirement for this enzyme to exhibit limited substrate specificity in order to hydrolyze both arginine and canavanine.

Aberrant, canavanyl protein formation and the ability to tolerate or utilize L-canavanine

L-Canavanine, 2-amino-4-(guanidinooxy)butyric acid, and L-arginine incorporation into de novo synthesized proteins was compared in six organisms. Utilizing L-[guanidinooxy14C]canavanine and L-[guanidino14C]arginine at substrate saturation, the canavanine to arginine incorporation ratio was determined in de novo synthesized proteins. Caryedes brasiliensis and Sternechus tuberculatus, canavanine utilizing insects; Canavalia ensiformis, a canavanine storing plant; and to a lesser extent Heliothis virescens, a canavanine resistant insect, failed to accumulate significant canavanyl proteins. By contrast, Manduca sexta, a canavanine-sensitive insect, and Glycine max, a canavanine free plant, readily incorporated canavanine into newly synthesized proteins. This study supports the contention that the incorporation of canavanine into proteins in place of arginine contributes significantly to canavanine's antimetabolic properties.

L-Canavanine and protein synthesis in the tobacco hornworm Manduca sexta

L-Canavanine, a nonprotein amino acid of certain leguminous plants, manifests potent insecticidal properties in a canavanine-sensitive insect such as the tobacco hornworm Manduca sexta (L.) (Sphingidae). This arginine analog is activated and aminoacylated by arginyl-tRNA synthetase and incorporated into nascent polypeptide chains to create structurally aberrant, canavanine-containing proteins. Analysis of incorporation of [3H]leucine into protein in M. sexta larvae that had been injected with canavanine revealed that this arginine analog stimulates protein synthesis. During the first 3 hr after injection of canavanine, canavanine-mediated net stimulation of protein formation was readily discerned. Thereafter, the stimulation of protein synthesis appeared to be offset by the preferential degradation of anomalous proteins. Double-label protein-turnover experiments with larvae injected with [14C]canavanine- and [3H]arginine-containing hemolymph proteins showed that canavanine-containing proteins were degraded preferentially.

A radiometric assay for determining the incorporation of L-canavanine or L-arginine into protein

Procedures for a radiometric assay of L-[guanidinooxy-14C]canavanine were developed which provide a convenient and accurate measure of the incorporation of [14C]canavanine into de novo-synthesized proteins. These methods are also applicable to determining [14C]arginine incorporation into protein. These procedures have been employed to study the synthesis of L-[guanidinooxy-14C]canavanine- and L-[guanidino-14C]arginine-containing proteins from the hemolymph of Manduca sexta and Heliothis virescens, two highly destructive insect pests.

l-Canavanine Transport and Utilization in Developing Jack Bean, Canavalia ensiformis (L.) DC. [Leguminosae]

l-Canavanine, the guanidinooxy structural analog of l-arginine, is an important nonprotein amino acid of many leguminous plants with nitrogen storage a major proported role. l-[Guanidinooxy-(14)C]canavanine, [(14)C] urea, and [(15)N]urea were injected separately into the fleshy, green cotyledons of 9-day old jack bean plants, Canavalia ensiformis (L.) DC. [Leguminosae]. There was significant transport of canavanine from the cotyledons to the aboveground portions of the plant, but not to the roots. Within 1.5 hours of isotope administration, the remaining labeled canavanine was divided equally between the cotyledons and the aboveground portions of the plant. During the 48-hour postinjection period, the contribution of l-[guanidinooxy-(14)C]canavanine to the total (14)carbon of the cotyledons decreased rapidly while it increased in the aboveground portions of the plant.[(14)C]Urea is degraded very rapidly; only 4.4% of the initial dose remained after 1.5 hours. Urea is catabolized so effectively within the cotyledons that not even 2% of the administered urea can be detected in tissues outside of these storage organs. [(15)N]Urea supplied to the developing coytledons leads to rapid (15)N incorporation into the amino nitrogen of glutamic acid and/or glutamine (28% (15)N abundance after 3 hours). Other amino acids are labeled but less heavily. The data are consistent with the proported role for l-canavanine of nitrogen storage within the developing cotyledons and cotyledonary canavanine is transported very effectively to the aboveground portions of the plant. It is not yet clear how efficiently this transported canavanine supports the nitrogen metabolism of the developing plant.