Purification and physicochemical characterization of soyatoxin, a novel toxic protein isolated from soybeans (Glycine max)

Plant proteins, peptides, and non-protein amino acids: Toxicity, sources, and analysis

Plants have evolved various mechanisms to synthesize diverse range of substances that contribute to their survival against pests, pathogens, predators, and adverse environmental conditions. Although several plant metabolites possess therapeutic potential, some can be potentially harmful to human and animal health when consumed in large proportion. Proteins, peptides, and non-protein amino acids are products of plant biochemical pathways with proven beneficial and nutritional effects. Despite these benefits, the in vivo toxicities associated with certain plant-derived proteins, peptides, and non-protein amino acids pose a significant risk to humans and animals. Symptoms of poisoning include nausea, vomiting, diarrhea, hair and weight loss, goiter, cataracts, and infertility. Even though plant processing methods such as soaking and drying can reduce the amount of toxin contained in plants, complete riddance is often impossible. As such, food regulatory bodies need to prevent uncontrolled consumption of the listed and many other toxin-containing plant species to keep the public safe. For this purpose, this review collates crucial insights into the sources, and in vivo toxicity associated with certain plant-derived proteins, peptides, and non-protein amino acids that have the clear potential to adversely affect human health. Additionally, this review provides information on analytical methods suitable for the detection of these substances in plants.

Proteomic insights into synthesis of isoflavonoids in soybean seeds

Soybean seeds are the major human dietary source of isoflavonoids, a class of plant natural products almost entirely exclusive to legumes. Isoflavonoids reduce the risk of a number of chronic human illnesses. Biosynthesis and accumulation of this class of compounds is a multigenic and complex trait, with a great deal of variability among soybean cultivars and with respect to the environment. There is a wealth of genomic, transcriptomic, and metabolomics data regarding isoflavonoid biosynthesis, but the connection between multigene families and their cognate proteins is a missing link that could provide us with a great deal of functional information. The changing proteome of the developing seed can shed light on the correlative increase in isoflavonoids, while the maternal seed coat proteome can provide the link with inherited metabolic and signaling machinery. In this effort, 'seed-filling' proteomics has revealed key secondary metabolite enzymes that quantitatively vary throughout seed development. Seed coat proteomics has revealed the existence of metabolic apparatus specific to isoflavonoid biosynthesis (isoflavonoid reductase) that could potentially influence the chemical content of this organ. The future of proteomic analysis of isoflavonoid biosynthesis should be centered on the development of quantitative, tissue-specific proteomes that emphasize low-abundance metabolic proteins to extract the whole suite of factors involved.

A protein extract and a cysteine protease inhibitor enriched fraction from Jatropha curcas seed cake have in vitro anti-Toxoplasma gondii activity

Toxoplasma gondii is a parasite of great medical and veterinary importance that has worldwide distribution and causes toxoplasmosis. There are few treatments available for toxoplasmosis and the search for plant extracts and compounds with anti-Toxoplasma activity is of utmost importance for the discovery of new active drugs. The objective of this study was to investigate the action of a protein extract and a protease inhibitor enriched fraction from J. curcas seed cake on developing tachyzoites of T. gondii-infected Vero cells. The protein extract (JcCE) was obtained after solubilization of the J. curcas seed cake with 100 mM sodium borate buffer, pH 10, centrifugation and dialysis of the resulting supernatant with the extracting buffer. JcCE was used for the in vitro assays of anti-Toxoplasma activity at 0.01, 0.1, 0.5, 1.5, 3.0 and 5.0 mg/ml concentration for 24 h. The results showed that JcCE reduced the percentage of infection and the number of intracellular parasites, but had no effect on the morphology of Vero cells up to 3.0 mg/mL. The cysteine protease inhibitor enriched fraction, which was obtained after chromatography of JcCE on Sephadex G-75 and presented a unique protein band following SDS-PAGE, reduced both the number of T. gondii infected cells and intracellular parasites. These results suggest that both JcCE and the cysteine protease inhibitor enriched fraction interfere with the intracellular growth of T. gondii.

Impact of dehulling on the physico-chemical properties and in vitro protein digestion of common beans (Phaseolus vulgaris L.)

The dehulling process improves bean flour nutritional functionality making beans more likely to be digested by infants.

JcTI-I: a novel trypsin inhibitor from Jatropha curcas seed cake with potential for bacterial infection treatment

Jatropha curcas seed cake is a low-value by-product resulting from biodiesel production. The seed cake is highly toxic, but it has great potential for biotechnology applications as it is a repository of biomolecules that could be important in agriculture, medicine, and industry. To explore this potential, a novel trypsin inhibitor called JcTI-I was purified by fractionation of the crude extract with trichloroacetic acid (2.5%, v/v) followed by affinity chromatography (Trypsin-Sepharose 4B) and molecular exclusion (Sephacryl S-200). Non-reducing sodium dodecyl sulfate-polyacrylamide gel electrophoresis and gel filtration showed that JcTI-I has approximately 20.0~kDa. Mass spectrometry analysis revealed that the intact molecular mass of JcTI-I is 10.252~kDa. Moreover, JcTI-I is a glycoprotein with 6.4% (m/m) carbohydrates, pI of 6.6, N-terminal sequence similarity around 60% to plant albumins and high stability to heat, pH, and salinity. JcTI-I presented antibacterial activity against the human pathogenic bacteria Salmonella enterica subspecies enterica serovar choleraesuis and Staphylococcus aureus, with minimum inhibitory concentration less than 5~μg/mL. Furthermore, JcTI-I did have inhibitory activity against the serine proteases from the tested bacteria. Otherwise, no hemolytic activity of human erythrocytes and signs of acute toxicity to mice were observed for JcTI-I. The results demonstrate the benefits of J. curcas seed cake as a source of trypsin inhibitor with potential for biotechnological application as a new antimicrobial agent against human pathogenic bacteria.

Interaction and digestibility of phaseolin/polyphenol in the common bean

The polyphenols and phaseolin interaction in common bean varieties was studied. Raw beans of three different colours were analysed: black (BRS Supremo), brown (BRS Pontal) and white (WAF-75). Based on the phaseolin digestibility in vitro and phaseolin-polyphenol complexation obtained by SDS-PAGE on a 10% polyacrylamide gel, it was observed that the polyphenols interfere with the digestibility of beans by decreasing the hydrolysis of phaseolin, especially in the darker ones. Furthermore it was possible to verify a difference in the electrophoretic pattern of phaseolin, indicating an interaction between phaseolin and polyphenols.

Plant ureases and related peptides: understanding their entomotoxic properties

Recently, ureases were included in the arsenal of plant defense proteins, alongside many other proteins with biotechnological potential such as insecticides. Isoforms of Canavalia ensiformis urease (canatoxin—CNTX and jack bean urease—JBURE-I) are toxic to insects of different orders. This toxicity is due in part to the release of a 10 kDa peptide from the native protein, by cathepsin-like enzymes present in the insect digestive tract. The entomotoxic peptide, Jaburetox-2Ec, exhibits potent insecticidal activity against several insects, including many resistant to the native ureases. JBURE-I and Jaburetox-2Ec cause major alterations of post-feeding physiological processes in insects, which contribute to, or can be the cause of, their entomotoxic effect. An overview of the current knowledge on plant urease processing and mechanisms of action in insects is presented in this review.

Toxicological studies on plant proteins: a review

Nowadays, toxicological studies are contributing to human health more than ever. Reports on the toxicological studies of plant proteins, which are continuously growing in number in the literature, have been reviewed. Two important aspects are discussed: dietary safety evaluation, including toxicity tests and the maximum daily intake allowance, and the appropriate proportion in our daily diets of proteins from traditional foods and of new proteins from plant sources not traditionally employed as foods. Water hyacinth leaf proteins, sweet lupin proteins and canola proteins have not been shown to be toxic, although they are not traditionally employed as food proteins. These findings are very important for exploiting valuable new protein sources that are suitable for human or animal consumption and applicable to the food industry. Acutely toxic proteins, including lectins, ribosome-inactivating proteins, inhibitors of proteolytic enzymes and glycohydro-lases, have been isolated from plant materials and identified. Their toxicities and molecular characteristics have been described. The toxicity of proteins depends upon their specific native structures. Once they are denatured by appropriate treatment, such as heating, their toxicity can be reduced or even eliminated. These findings indicate that raw materials that contain this kind of toxic protein are not edible. However, after proper processing, they may be suitable for human or animal consumption. Although the toxicities of type 2 ribosome-inactivating proteins reported by different authors vary, the maximum dosages are still trace amounts.

Soybean toxin (SBTX), a protein from soybeans that inhibits the life cycle of plant and human pathogenic fungi

Soybean toxin (SBTX) is a 44 kDa glycoprotein that is lethal to mice (LD(50) = 5.6 mg/kg). This study reports the toxicity of SBTX on pathogenic fungi and yeasts and the mechanism of its action. SBTX inhibited spore germination of Aspergillus niger and Penicillium herguei and was toxic to Candida albicans, Candida parapsilosis, Kluyveromyces marxiannus , Pichia membranifaciens, and Saccharomyces cerevisiae. In addition, SBTX hampered the growth of C. albicans and K. marxiannus and inhibited the glucose-stimulated acidification of the incubation medium by S. cerevisiae, suggesting that SBTX interferes with intracellular proton transport to the external medium. Moreover, SBTX caused cell-wall disruption, condensation/shrinkage of cytosol, pseudohyphae formation, and P. membranifaciens and C. parapsilosis cell death. SBTX is toxic to fungi at concentrations far below the dose lethal to mice and has potential in the design of new antifungal drugs or in the development of transgenic crops resistant to pathogens.

Water extracts of Brazilian leguminous seeds as rich sources of larvicidal compounds against Aedes aegypti L

This study assessed the toxicity of seed water extracts of 15 leguminous species upon Aedes aegypti larvae. A partial chemical and biochemical characterization of water extracts, as well as the assessment of their acute toxicity in mice, were performed. The extracts of Amburana cearensis, Anadenanthera macrocarpa, Dioclea megacarpa, Enterolobium contortisiliquum and Piptadenia moniliformis caused 100% of mortalit y after 1 to 3 h of exposure. They showed LC50 and LC90 values ranging from 0.43 ± 0.01 to 9.06 ± 0.12 mg/mL and from 0.71 ± 0.02 to 13.03 ± 0.15 mg/mL, respectively. Among the secondary metabolite constituents, the seed water extracts showed tannins, phenols, flavones, favonols, xanthones, saponins and alkaloids. The extracts also showed high soluble proteins content (0.98 to 7.71 mg/mL), lectin (32 to 256 HU/mL) and trypsin inhibitory activity (3.64 = 0.43 to 26.19 = 0.05 gIT/kg of flour) The electrophoretic profiles showed a great diversity of protein bands, many of which already described as insecticide proteins. The extracts showed low toxicity to mice (LD50 > 0.15 = 0.01 g/kg body weight), but despite these promising results, further studies are necessary to understand the toxicity of these extracts and their constituentsfrom primary and secondary metabolism upon Ae. aegypti.

Insecticidal action of sodium anacardate from Brazilian cashew nut shell liquid against Aedes aegypti

Aedes aegypti is the major vector of 1 of the most concerning arboviruses of the world, the dengue fever. The only effective way of reducing the incidence of dengue fever is to control the vector mosquito, mainly by application of insecticides to its breeding places. This study was aimed at assessing the insecticidal activity of sodium anacardate, isolated from Brazilian cashew nut shell liquid (CNSL), against the eggs, 3rd instars or pupae of Ae. aegypti. In addition, the acute toxicity of sodium anacardate to mice was also investigated. Sodium anacardate showed toxicity against Ae. aegypti eggs (median effective concentration [EC50] = 162.93 +/- 29.93 microg/ml), larvae (median lethal concentration [LC50] = 55.47 +/- 3.0 microg/ml) and pupae (LC50 = 369.78 - 52.30 microg/ml). On the other hand, even at high dose (0.3 g/kg body weight), this compound did not cause any adverse effects on mice, suggesting that this compound is safe to mammals. Therefore, sodium anacardate may be a viable low-cost alternative to help combat Ae. aegypti.

Combination of chemical analyses and animal feeding trials as reliable procedures to assess the safety of heat processed soybean seeds

This study assessed whether chemical analyses are sufficient to guarantee the safety of heat processing of soybeans (SB) for human/animal consumption. The effects of extrusion and dry-toasting were analyzed upon seed composition and performance of broiler chicks. None of these induced appreciable changes in protein content and amino acid composition. Conversely, toasting reduced all antinutritional proteins by over 85%. Despite that, the animals fed on toasted SB demonstrated a low performance (feed efficiency 57.8 g/100 g). Extrusion gave place to higher contents of antinutrients, particularly of trypsin inhibitors (27.53 g/kg flour), but animal performance was significantly (p < 0.05) better (feed efficiency 63.2 g/100 g). Upon the basis of chemical analyses, dry-toasting represents the treatment of choice. However, considering the results of the feeding trials, extrusion appears to be the safest method. In conclusion, in order to evaluate the reliability of any processing method intended to improve nutritional value, the combination of chemical and animal studies is necessary.

SBTX, a new toxic protein distinct from soyatoxin and other toxic soybean [Glycine max] proteins, and its inhibitory effect on Cercospora sojina growth

SBTX, a novel toxin from soybean, was purified by ammonium sulfate fractionation followed by chromatographic steps DEAE-Cellulose, CM-Sepharose and Superdex 200 HR fast-protein liquid chromatography (FPLC). Lethality of SBTX to mice (LD(50) 5.6 mg/kg) was used as parameter in the purification steps. SBTX is a 44-kDa basic glycoprotein composed of two polypeptide chains (27 and 17 kDa) linked by a disulfide bond. The N-terminal sequences of the 44 and 27kDa chains were identical (ADPTFGFTPLGLSEKANLQIMKAYD), differing from that of 17 kDa (PNPKVFFDMTIGGQSAGRIVMEEYA). SBTX contains high levels of Glx, Ala, Asx, Gly and Lys and showed maximum absorption at 280 nm, epsilon(1cm)(1%) of 6.3, and fluorescence emission in the 290-450 nm range upon excitation at 280nm. The secondary structure content was 35% alpha-helix, 13% beta-strand and beta-sheet, 27% beta-turn, 25% unordered, and 1% aromatic residues. Immunological assays showed that SBTX was related to other toxic proteins, such as soyatoxin and canatoxin, and cross-reacted weekly with soybean trypsin inhibitor and agglutinin, but it was devoid of protease-inhibitory and hemagglutinating activities. The inhibitory effect of SBTX on growth of Cercospora sojina, fungus causing frogeye leaf spot in soybeans, was observed at 50 microg/ml, concentration 112 times lesser than that found to be lethal to mice. This effect on phytopathogenic fungus is a potential attribute for the development of transgenic plants with enhanced resistance to pathogens.

Plant toxic proteins with insecticidal properties. A review on their potentialities as bioinsecticides

To meet the demands for food of the expanding world population, there is need of new ways for protecting plant crops against predators and pathogens while avoiding the use of environmentally aggressive chemicals. A milestone in this field was the introduction into crop plants of genes expressing Bacillus thuringiensis entomotoxic proteins. In spite of the success of this new technology, however, there are difficulties for acceptance of these 'anti-natural' products by the consumers and some concerns about its biosafety in mammals. An alternative could be exploring the plant's own defense mechanisms, by manipulating the expression of their endogenous defense proteins, or introducing an insect control gene derived from another plant. This review deals with the biochemical features and mechanisms of actions of plant proteins supposedly involved in defense mechanisms against insects, including lectins, ribosome-inactivating proteins, enzymes inhibitors, arcelins, chitinases, ureases, and modified storage proteins. The potentialities of genetic engineering of plants with increased resistance to insect predation relying on the repertoire of genes found in plants are also discussed. Several different genes encoding plant entomotoxic proteins have been introduced into crop genomes and many of these insect resistant plants are now being tested in field conditions or awaiting commercialization.

Nutritional study of two Brazilian soybean (Glycine max) cultivars differing in the contents of antinutritional and toxic proteins

The research was conducted with two different recently released Brazilian soybean cultivars (Rio Balsas and Bays) to evaluate whether there is any correlation between the different levels of antinutritional and/or toxic proteins in the cultivars and their nutritive value as sources of protein for monogastric animals (rats). Furthermore, it is discussed, for the first time, the role of the dietary soyatoxin on the performance of rats fed on diets containing soyatoxin-rich (cv. Bays) and soyatoxin-free (cv. Rio Balsas) soybean cultivars. Feeding rats with diets containing raw soybean cultivars showed a lower growth rate, net protein utilization and digestibility, a much higher dry matter and nitrogen excretion and macroscopic alterations in internal organs when compared to rats fed on egg-white protein. The nutritional parameters measured for the diet based on raw Bays cultivar were poorer than those of the diet prepared with Rio Balsas. In the raw soybeans, trypsin inhibitor and lectin, and urease to a lesser extent, significantly affected at different fashion the soybean protein utilization. Heating treatment of the Bays seeds increased the growth rate, NPU, in vivo protein digestibility and practically eliminated or attenuated all the organ alterations observed. This study might be helpful in the choice of safe and nutritious soybean cultivars.

Characterization of two novel defense peptides from pea (Pisum sativum) seeds

A fraction that possesses antifungal activity against Aspergillus niger has been isolated from seeds of the pea (Pisum sativum) by ammonium sulfate fractionation followed by gel filtration on Sephadex G-75. On further purification by reverse-phase high performance liquid chromatography, two small cysteine-rich polypeptides were obtained (Psd1 and Psd2). They are localized primarily in vascular bundles and epidermis tissues of pea pods and exhibit high antifungal activity toward several fungi, displaying IC(50) values ranging from 0.04 to 22 microg/ml. This inhibitory activity decreases when A. niger growth medium is supplemented with cations such as Ca(2+), Mg(2+), Na(+), and K(+). Although the primary sequence of both Psd1 and Psd2 shows homology with other plant defensins, they cannot easily be assigned to any established group.

Pharmacological evaluation of ricinine, a central nervous system stimulant isolated from Ricinus communis

The extract of the pericarp of castor bean (Ricinus communis) showed some typical central nervous system stimulant effects when administered to mice. The animals became exophthalmic, presented tremors and clonic seizures and died a few minutes after receiving larger doses of the extract. At lower doses the extract improved memory consolidation and showed some neuroleptic-like properties, such as a decrease in exploratory behavior and catalepsy. The memory-improving effect and the seizure-eliciting properties of the extract were also observed with the administration of ricinine, a neutral alkaloid isolated from the extract. However, the neuroleptic-like properties of the extract were not observed with ricinine. As the therapeutic index of ricinine is of the order of 200, the compound may be considered as a promising cognition-enhancing drug that may be used for the treatment of human amnesias.

Toxic, antimicrobial and hemagglutinating activities of the purple fluid of the sea hare Aplysia dactylomela Rang, 1828

The antimicrobial, hemagglutinating and toxic activities of the purple fluid of the sea hare Aplysia dactylomela are described. Intact or dialyzed purple fluid inhibited the growth of species of Gram-positive and Gram-negative bacteria and the action was not bactericidal but bacteriostatic. The active factor or factors were heat labile and sensitive to extreme pH values. The fluid preferentially agglutinated rabbit erythrocytes and, to a lesser extent, human blood cells, and this activity was inhibited by the glycoprotein fetuin, a fact suggesting the presence of a lectin. The fluid was also toxic to brine shrimp nauplii (LD50 141.25 micrograms protein/ml) and to mice injected intraperitoneally (LD50 201.8 +/- 8.6 mg protein/kg), in a dose-dependent fashion. These toxic activities were abolished when the fluid was heated. Taken together, the data suggest that the activities of the purple fluid are due primarily to substance(s) of a protein nature which may be involved in the chemical defense mechanism of this sea hare.

Isolation and partial characterisation of highly toxic lectins from Abrus pulchellus seeds

The seeds of Abrus pulchellus, sub-specie tenuiflorus, belonging to the Leguminosae, subfamily Papilionoideae contain highly toxic lectins exhibiting specificity for galactose and galactose-containing structures. The toxins which agglutinate rabbit erythrocytes, present a highly toxic activity in vivo when injected in the peritoneal cavity of mice (LD50=31 microg x kg(-1)) or when tested with the microcrustacean Arthemia salina (LD50=3.5 microg x ml(-1)). The active fraction was purified in a single step, by affinity chromatography on a Sepharose-4B column. The purified toxins migrated as two single bands of Mr 63000 and 61500 Da (SDS-PAGE) and Mr 31500 and 29000 Da (SDS-PAGE with 2-mercaptoethanol), respectively, suggesting the presence of disulphide-bridge interchains as occurs in other plant toxins. The antibodies anti-A. pulchellus toxins did not recognize ricin preparation and only partial identity was observed to A. precatorius toxic lectins prepared in a similar way to ricin and A. pulchellus toxins.