Proteins
Protease inhibitors: Protease inhibitors are protein-based substances widely distributed within the plant kingdom, including the seeds of most cultivated legumes which have the ability to inhibit the activity of proteolytic enzymes within the gastro- intestinal tract of animals. The commercial soybean processing methods have generally been targeted almost entirely on the destruction of their activity (through denaturation by heat processing techniques) rather than the removal or destruction of any other of the numerous anti-nutrients present within raw soybeans. Although relatively few feeding trials have been conducted as yet concerning the effect of purified protease inhibitors, a negative effect of soybean protease inhibitor has been reported on protein and amino acid digestion by inhibiting intestinal protease activity.
Heamagglutinins: Lectins are found in most types of beans, including soybeans. Different lectins have different levels of toxicity, though not all lectins are toxic. Lectins in soybean plant are different from those in seeds. Concanavalin A is one of such lectins from Concanavalia ensiformis (jack bean) which is most widely studied. Although lectins have been found to occur naturally in the blood serum of some fish species there is no published information concerning the dietary effect of purified lectins from legumes or oilseeds on fish and shrimp. By contrast, studies with humans and laboratory animals have reported toxicity symptoms ranging from reduced growth, alterations in gut microflora, to death. Some of phytohaemgglutinins are extremely toxic, such as ricin from castor bean and others, such as kidney beans. The seeds from the castor bean plant, Ricinus communis, are poisonous to people, animals and insects. However, now it is known that the agglutination is due to another toxin that was also
present, called RCA (Ricinus communis agglutinin). Many cytotoxic proteins from a variety of plants have been identified, and they are related to ricin both in structure and function. For both ricin and kidney bean agglutinin, no agglutination of erythrocytes occurs at low pH, and adsorbed lectins dissociate from the erythrocytes. Although lectins are usually heat-labile, their stability varies between plant species; many lectins being resistant to inactivation by dry heat and requiring the presence of moisture for more complete destruction. Moreover, although it has also been reported that lectins are readily inactivated by proteolytic enzymes (in vitro) during the fact that raw legumes or partially processed meals may still confer residual lectin activity and therefore toxicity.
Toxic amino acids: A variety of naturally occurring toxic amino acids have been identified with plant feedstuffs. There are hundreds of amino acids produced by plants that do not occur in proteins. Some of these are toxic to both livestock and man and occur in agronomically important legumes. One such most studied toxic amino acid is mimosine which is present in Leucaena Leucocephala and other members of the l e g u mi n o s e a e . O t he r t o xi c a min o a c i d s i nc l ud e c a na v a n i ne in Se s b a n ia ; dihydroxyphenylalanine in faba beans and indospicine in Indigofera spp. Selenoamino acids like methylselenocysteine and selenocystathionine present in selenium accumulating plants of the genera Astragalus and Machaeranthera. A few of the less studied toxic amino acid includes N-oxalyl-L-diaminopropionic acid in Lathyrus and 4-N-acetyl-p-2,4-diamino butanoic acid (ADAB). Thiaminases are enzymes that cleave the thiamin molecule and render it biologically inactive.
Although heat stable, mimosine can be readily extracted (by removal or by endogenous enzymatic conversion to a less toxic form) from ipil-ipil leaf meal by soaking in water with a marked improvement in nutritive value. Since mimosine is a water soluble amino acid, water soaking and washing the leucaena leaves for one day before sun drying and grinding was reported as an economical way to improve their nutritive value. Interestingly, however, the best performance was obtained by simply sun-drying the leaves prior to use. The drying and water soaking procedures gave the best combination of mimosine reduction and least loss of CP. The degradation of mimosine to 3-hydroxy-4(1H)-pyridone during ensiling of Leucaena leaves in small- scale laboratory preparations was caused almost entirely by endogenous enzymes in the leaf and not by fermentation during ensiling. The leucaena-nodulating Rhizobium sp. strain TAL1145 degrades mimosine (Mid+) and utilizes it as a source of carbon and nitrogen.
Canavanine is found in the seeds of the legume Sesbania spp and jack bean Canavalia spp and acts as an arginine antagonist. The toxicity of the seed proteins increases with increasing dietary inclusion level, and being reduced with water extraction (canavanine, being soluble in water). The results showed that all processing methods were efficient to eliminate canavanine from seeds. The treatments such as: soaked in water for 12 h, autoclaved, then dried at room temperature and coarsely ground, soaked in water for 24 h, autoclaved and dried treatments were more effective to detoxification of the bitter vetch for broiler chicken.
Thiaminases are enzymes that cleave the thiamin molecule and render it biologically inactive. The ingestion of significant amounts of thiaminases can induce thiamin deficiency even though there may be a sufficient amount of thiamin in the diet. Thiaminases are denatured by heat, therefore subjecting any of the sources of thiaminases to cooking or other heat treatment renders the thiaminases inactive. Indospicine occurs in Indigofera spicata and other Indigofera species. Toxicity problems include weight loss, liver and kidney damage, and abortions.
Food allergens: Allergens are heat-stable proteins with the ability to stimulate antigenic challenge or immunological activity on ingestion and consequently causing gastrointestinal hypersensitivity within the animal. For example, soybean is reported to contain specific allergens (globular proteins) which have been found to orally sensitize pre-ruminant calves, baby pigs, guinea pigs and rabbits; the sensitivity being manifested in gastrointestinal hypersensitivity reactions and digestive disturbances, including mucosal inflammation and luminal osmolality. Considerable further work is required concerning the sensitivity of individual fish and shrimp species to different potential food allergens.