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[SANET-MG] gm crops fpr health? part 4

part 4
gm crops for health
Peptides both natural and synthetic
Glutathione (GSH) is an antioxidant consisting of three amino acids that protects cells from free radicals and participates in metabolic reaction. GSH is the most abundant low molecular weight thiol (compounds with -SH group) in plants. It accumulates to high concentrations particularly in response to stress. A bacterial enzyme catalyzing glutathione synthesis and lacking feed back inhibition was used to enhance glutathione production in plants [83]. Glutathione is important in maintaining nutritional homeostasis and great caution must be exercised in manipulating glutathione levels in plants. Increasing glutathione levels in tobacco unexpectedly resulted in continuous oxidative stress in the plants [84]. Anti-microbial peptides provide the first line of defence against invading bacteria, fungi and viruses in both plants and animals and are part of the host’s innate immunity, acting mainly at the cell membrane. They are 15 to 40 amino acids in length, most of them hydrophobic (water-hating) and cationic (positively charged), and are beginning to find applications in medicine and in crop protection. A synthetic peptide D4E1 based on the cecropin B peptide toxin (obtained from the moth, Cecropia), consists of a linear sequence of 17 amino acids: FKLRAKIKVRLRAKIKL (F for phenylalanine, K for lysine, L for leucine, R for arginine, A for alanine, I for isoleucine, V for valine). The peptide protected against Aspergillus and Fusarium fungi. It acts by binding to ergosterol, a sterol present in fungal cell walls [85]. On further tests, D4E1 was found to have broad-spectrum anti-microbial action, and was active against fungi belonging to the orders Ascomycete, Basidiomycete, Deuteromycete and Oomycetes, as well as bacterial pathogens Psuedomonas and Xanthomonas [86]. The D4E1 toxin also proved effective in treating Chlamydia infection in humans [87]. Synthetic peptides of 11 amino acids proved effective against bacterial plant pathogens, with minimal cytotoxicity and protease degradation, offering improved crop protection as an external pesticide or incorporated into transgenic crops [88]. A DNA sequence encoding synthetic peptide 10 amino acids long replaced the active region of the tomato prostemin gene in order to enhance production and processing of the peptide. The hybrid gene facilitated the transfer and insertion of the peptide into tobacco plants where it proved active against microbial pests [89]. Researchers at the National Agricultural Research Center, Niigata, Japan, have created transgenic rice with the anti-microbial peptide defensin from Brassica. The transgenic rice plants were resistant to rice blast disease caused by the fungus Magnaporthe grisea. The researchers then systematically altered the genetic code for defensin to produce synthetic peptides that were far more toxic to the fungus than the natural peptides [90]. Rice with the synthetic genes and peptides are being proposed for field-testing prior to commercial release in Japan. A potato virus X expression system was used to produce a killer peptide derived from a single chain anti-idiotype antibody of a broad spectrum microbiocidal yeast killer toxin with a strong activity against human pathogens. The killer peptide was tested against both bacterial and fungal plant pathogens and proved very effective. The killer toxin was fused to the virus X coat protein in a system that allowed its rapid production [91]. The virus production system is capable of spreading the toxin to potato, for better or for worse. There have been criticism and objections to open field-testing of crops modified with the synthetic peptides. The evolution of resistance to anti-microbial peptides will severely compromise both the natural defence of the human immune system against disease and the possibilities of effective therapies emerging in the wake of the disaster of widespread antibiotic resistance [92] (No to Releases of Transgenic Plants with Antimicrobial Peptides). As versions of the peptides also provide defence against pathogens in other animals and plants, the ecological impact of resistant pathogens could be devastating. Another factor adding to the hazards to health and the environment is that the synthetic transgenes code for peptides that are significantly different from the natural versions. This may itself be responsible for toxic or other harmful effects that cannot be known unless thoroughly tested.

Genetically modified microbes in food
Probiotic microbes including Lactobacillus species, Bifidobacterium species and the yeast Saccharomyces boulardii, have been used as food supplement. The health benefits of probiotic microbes include antagonistic effects on gastroenteric pathogens, neutralisation of food mutagens produced in the colon, shifting the immune system to alleviate allergy and lowering serum cholesterol [93] (Health-promoting Germs). Probiotic microbes are being developed as vectors for gene therapy and genetically modified to “improve” the quality of food. The probiotic lactic acid bacteria have been extensively modified to serve the food industry and other purposes. Modifications included modulation of the proteolytic system to enhance cheese ripening, increasing the production of the Kreb’s cycle enzyme alpha keto glutarate, using antisense RNA to silence lytic Lactoccocus phage, introducing a folate gene cluster, re-routing pyruvate to L-alanine, and over-expressing the riboflavin biosynthesis pathway. Further genetic modifications of lactic acid bacteria involved inactivation of glucose fermentation and introduction of lactose fermentation, introduction of alpha-galactosidase, of phytase, of alpha amylase and cellulose. Lactic acid bacteria have also been genetically modified with bacteriocin toxin to prevent dental carries; for increased activity of beta-galactosidase, for lacticin (a bacteriocin) production, for increased nicin production, and increased proteolytic and acidifying activity. The probiotic bacteria have been enhanced for glutathione production, and for oxidative stress tolerance. Lysostaphin, a glycylglycine endopeptidase that specifically cleaves the pentaglycine cross-bridges found in the staphylococcal peptidoglycan was inserted into lactic acid bacteria for use in destroying the pathogen. The safety assessment and regulation of these GM probiotic bacteria were discussed in recent reviews [94, 95]. The production of nutraceuticals (foods or components with health benefits) with genetically modified food grade microbes and their safety assessment were also reviewed [96]. GM probiotic bacteria have not received much public scrutiny mainly because the regulation of such microbes is separate from the regulation of GM crops. There is a strong likelihood that GM probiotic bacteria may be introduced to the market before serious safety concerns are addressed. Using GM probiotic bacteria requires special caution. These bacteria are natural symbionts of the gastrointestinal tract, and have adapted to their human and animal hosts over millions if not billions of years of evolution. Genetically modifying them could easily turn them into pathogens pre-adapted to invade the human and animal gut [97]. Furthermore, the gastroinstestinal tract is an ideal environment for horizontal gene transfer and recombination, the major route to creating pathogens. For these reasons, we have proposed that any genetic modification of probiotic bacteria should be banned [98, 99] (Ban GM Probiotics; GM Probiotic Bacteria in Gene Therapy). Fowls grew faster when fed transgenic yeast Pichia pastoris modified with a pig growth hormone gene with an alcohol oxidase promoter and an alpha-factor signal peptide. The modified yeast mixed with the fowl diet made the fowl grow about 10 percent faster than controls [100]. Growth hormone food microbes may be attractive for chicken farmers, but their use may carry the microbes over into the human population; and not everyone would want to grow like pigs. Microbial bio-control agents have been developed and the impact of such agents on foods requires careful consideration. A modified Trichoderma atroviride with a glucose oxidase gene from Aspergilus niger rapidly overgrew and lysed the plant pathogens Rhizoctonia solani and Pythium ultimum. The transgenic bio-control agent both defeated the pathogens and induced systemic resistance in treated plants [101], but they should be studied extensively for their impact on food safety and quality. The modification of food microbes requires comprehensive public scrutiny especially as numerous modified strains are awaiting release into the commercial markets.

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