Diverse effects of phytohaemagglutinin on gastrointestinal secretions in rats

Structure-function and application of plant lectins in disease biology and immunity

Lectins are proteins with a high degree of stereospecificity to recognize various sugar structures and form reversible linkages upon interaction with glyco-conjugate complexes. These are abundantly found in plants, animals and many other species and are known to agglutinate various blood groups of erythrocytes. Further, due to the unique carbohydrate recognition property, lectins have been extensively used in many biological functions that make use of protein-carbohydrate recognition like detection, isolation and characterization of glycoconjugates, histochemistry of cells and tissues, tumor cell recognition and many more. In this review, we have summarized the immunomodulatory effects of plant lectins and their effects against diseases, including antimicrobial action. We found that many plant lectins mediate its microbicidal activity by triggering host immune responses that result in the release of several cytokines followed by activation of effector mechanism. Moreover, certain lectins also enhance the phagocytic activity of macrophages during microbial infections. Lectins along with heat killed microbes can act as vaccine to provide long term protection from deadly microbes. Hence, lectin based therapy can be used as a better substitute to fight microbial diseases efficiently in future.

Dietary Lectin exclusion: The next big food trend?

Until recently, with the exception of coeliac disease, gastroenterologists have not been particularly interested in the role of diet in the management of gastrointestinal disorders. However, patients have always felt that diet must play a part in their symptoms and, in the absence of any medical interest, have turned to alternative dietary practitioners for help, which can often have no evidence base. Fortunately, with the advent of the FODMAP diet (fermentable oligosaccharides, disaccharides, monosaccharides, and polyols) and the realisation that diet can have a profound effect on the microbiome, medical opinion is now changing. Nevertheless, research on the various diets that are now available is often completely lacking. Lectins are carbohydrate binding proteins which are widely distributed in nature and are found in a whole variety of commonly consumed foods. It seems likely that the exclusion of lectins from the diet could become the next “food fashion” for alternative practitioners to promote, especially as there is some evidence to suggest that certain lectins may be harmful to health. It is, therefore, the purpose of this viewpoint to try and stimulate research on the dietary effects of lectins, which is currently minimal, so that we can pre-empt a situation where we are unable to give patients or the public evidence based advice on this topic.

Early effects on the intestinal barrier and pancreatic function after enteral stimulation with protease or kidney bean lectin in neonatal rats

Gut maturation naturally accelerates at weaning in altricial mammalian species, such as the rat. Mimicking this, gut development can also be induced precociously, 3-4 d earlier than it would occur naturally, by enteral exposure to phytohaemagglutinin (PHA), or various proteases. We investigated the early effects of gut provocation on intestinal barrier and pancreatic functions, to get a better understanding of the mechanisms that initiate gut maturation. The effects of oral administration of protease (trypsin) or PHA to 14-d-old suckling rats were studied during 24 h in comparison with water-fed controls. Intestinal in vivo permeability was assessed by oral administration of different-sized marker molecules and measuring their passage into the blood or urine 3 h later. A period of 24 h following oral administration, both PHA and protease provocation stimulated small intestinal (SI) growth and pancreatic secretion, as indicated by decreased pancreatic trypsin and increased luminal enzyme content. Within 1 h of oral administration, both treatments prevented the absorption of macromolecules to blood that was observed in controls. PHA treatment hindered the passage of fluorescein isothiocyanate-dextran (FD) 4 to blood, whereas protease treatment temporarily increased plasma levels of FD4, and the urine lactulose:mannitol ratio, indicating increased intestinal leakiness. Following protease treatment, fluorescence microscopy showed decreased vesicular uptake of FD70 in the proximal SI and increased epithelial fluorescence in the distal SI. In conclusion, PHA and protease differed in their early effects on the intestinal barrier; both exerted a blocking effect on epithelial endocytosis, whereas protease treatment alone temporarily increased epithelial leakiness, which seemed to be confined to the distal SI.

Effect of White Kidney Beans (Phaseolus vulgaris L. var. Beldia) on Small Intestine Morphology and Function in Wistar Rats

The chronic ingestion of raw or undercooked kidney beans (Phaseolus vulgaris L.) causes functional and morphological derangement in various tissues. The major objectives of this study were to investigate the gavage effects of a raw Beldia bean variety that is widely consumed in Tunisia, on the small intestine morphology and jejunal absorption of water, electrolytes, and glucose in Wistar rats. Twenty young male rats were randomly divided into two groups of 10 rats. The first group served as the control and was gavaged with 300 mg of a rodent pellet flour suspension (RPFS), whereas the second experimental group was challenged with 300 mg of a Beldia bean flour suspension (BBFS) for 10 days. Histological studies were performed using light and electron microcopy. The intestinal transport of water, sodium, potassium, and glucose was studied by perfusing the jejunal loops of the small bowels in vivo. The feeding experiments indicated that BBFS did not affect weight gain. Histomorphometric analyses showed that the villus heights, crypt depths, and crypt/villus ratios in the jejunum and ileum were greater in the BBFS-fed rats than controls. Electron microscopy studies demonstrated that the rats exposed to RPFS exhibited intact intestinal tracts; however, the BBFS-treated rats demonstrated intestinal alterations characterized by abnormal microvillus architectures, with short and dense or long and slender features, in addition to the sparse presence of vesicles near the brush border membrane. BBFS administration did not significantly affect glucose absorption. However, significant decreases were observed in water and electrolyte absorption compared with the uptake of the controls. In conclusion, raw Beldia beans distorted jejunum morphology and disturbed hydroelectrolytic flux.

Allergenicity assessment of Allium sativum leaf agglutinin, a potential candidate protein for developing sap sucking insect resistant food crops

Background

Mannose-binding Allium sativum leaf agglutinin (ASAL) is highly antinutritional and toxic to various phloem-feeding hemipteran insects. ASAL has been expressed in a number of agriculturally important crops to develop resistance against those insects. Awareness of the safety aspect of ASAL is absolutely essential for developing ASAL transgenic plants.

Methodology/Principal Findings

Following the guidelines framed by the Food and Agriculture Organization/World Health Organization, the source of the gene, its sequence homology with potent allergens, clinical tests on mammalian systems, and the pepsin resistance and thermostability of the protein were considered to address the issue. No significant homology to the ASAL sequence was detected when compared to known allergenic proteins. The ELISA of blood sera collected from known allergy patients also failed to show significant evidence of cross-reactivity. In vitro and in vivo assays both indicated the digestibility of ASAL in the presence of pepsin in a minimum time period.

Conclusions/Significance

With these experiments, we concluded that ASAL does not possess any apparent features of an allergen. This is the first report regarding the monitoring of the allergenicity of any mannose-binding monocot lectin having insecticidal efficacy against hemipteran insects.

Potential efficacy of preparations derived from Phaseolus vulgaris in the control of appetite, energy intake, and carbohydrate metabolism

Preclinical data on extracts of and preparations derived from beans of Phaseolus vulgaris are reviewed as potential remedies for use in controlling food consumption, body weight, lipid accumulation, and glycemia. A growing body of evidence suggests that acute and chronic administration of P. vulgaris derivatives reduces food intake (including highly palatable foods), body weight, lipid deposit, and glycemia in rats exposed to multiple experimental procedures. Two possible lectin-mediated mechanisms of action have been proposed: (a) inhibition of α-amylase, resulting in a reduced carbohydrate metabolism and absorption; (b) phytohemoagglutinin-induced modulation of the activity of cholecystokinin and glucagon-like peptides, resulting in a reduced appetite. Preliminary clinical data, as well as reports focusing on the use of several traditional medicines, apparently extend these findings to humans. Should these initial clinical data be confirmed by future surveys, P. vulgaris derivatives might constitute novel remedies for the treatment of obesity and metabolic syndrome. Future studies are also expected to identify active structures leading to the development of new pharmaceutical agents.

Origin and mediation of secretion induced by oral phytohaemagglutinin (PHA) in rats

After oral administration several gut-binding lectins induce accumulation of liquor and amylase in the proximal small intestine. Orally administered Phaseolus vulgaris phytohaemagglutinin (PHA) was used to study the mediation of these effects in rats. The regulation of amylase secretion clearly differed from that of the liquor. The amylase activity was of pancreatic origin, in agreement with the known cholecystokinin-releasing effect of PHA. It appears that CCK exerts its effect both directly and by facilitating neural stimulatory pathways. Intestinal secretion was identified as the source of the liquor, without a contribution by other secretions. It was mediated by a local cholinergic reflex with the involvement of both muscarinic and nicotinic acetylcholine receptors. It is speculated that the observed enteric reflex may enable the gut to transport secreted antibacterial peptides or secretory antibodies from the crypts to adherent bacteria on adjacent villi.

Enteral crude red kidney bean (Phaseolus vulgaris) lectin--phytohemagglutinin--induces maturational changes in the enterocyte membrane proteins of suckling rats

This study aimed to investigate the effect of enterally administered crude red kidney bean (Phaseolus vulgaris) lectin, PHA, on the expression of brush-border membrane vesicle (BBMV) proteins, in particular Na(+)/H(+) exchangers (NHEs), in the small intestine of suckling rats. Gavage of PHA to 14-day-old rats for 3 days resulted in altered protein/glycoprotein patterns as analyzed by SDS-PAGE. Immunoblots demonstrated the appearance of two 71- and 27-kD protein bands indicative for NHE3--one of the NHE isoforms--and PHA, respectively. PHA treatment also resulted in an augmented uptake of (22)Na(+) by the BBMV indicating an increase in NHE activity. Overall, the data suggests that enteral PHA exposure may induce maturational changes in enterocyte membrane proteins in young rats. In view of these findings, an investigation into the addition of PHA to infant formulas and weaning diets is warranted.

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.

Phytohaemagglutinin inhibits gastric acid but not pepsin secretion in conscious rats

Phytohaemagglutinin (PHA), a kidney bean lectin, is known for its binding capability to the small intestinal surface. There has been no data available, however, on the biological activity of PHA in the stomach. Recent observations indicate that PHA is able to attach to gastric mucosal and parietal cells. Therefore, we examined whether PHA affects gastric acid and pepsin secretion in rats. Rats were surgically prepared with chronic stainless steel gastric cannula and with indwelling polyethylene jugular vein catheter. During experiments, animals were slightly restrained. Gastric acid secretion was collected in 30 min periods. Acid secretion was determined by titration of the collected gastric juice with 0.02 N NaOH to pH 7.0. Pepsin activity was estimated by measuring enzymatic activity. Saline, pentagastrin and histamine were infused intravenously. PHA or bovine serum albumin (BSA) were dissolved in saline and given intragastrically through the gastric cannula. PHA significantly inhibited basal acid secretion. Inhibition of acid output reached 72% during the first collection period following PHA administration when compared, then gradually disappeared. Pentagastrin-stimulated acid secretion was repressed dose-dependently by PHA as well. Maximal inhibition was observed during the first 30 min following application of PHA. Histamine-stimulated acid secretion was inhibited by PHA in a similar manner. Pepsin secretion was not affected by PHA under either basal or stimulated conditions. These results provide evidence that PHA is a potent inhibitor of gastric acid secretion in conscious rats, but it does not affect pepsin output from the stomach.