Toxic Amino Acids
Toxic Amino Acids: A wide range of toxic non-protein amino acids occur in the foliage and seeds of plants. These toxic non-protein amino acids appear to play a major role in determining the nutritional value of a number of tropical legumes (D’Mello, 1982). It has been proposed that these amino acids act antagonistically towards certain nutritionally important amino acids (Liener, 1980). Fowden (1971) suggested that the metabolic pathways culminating in the synthesis of certain non-protein amino acids might reflect subtle alteration in the genome responsible for directing the formation of crucial amino acids. Bell (1971) reported that while non-protein amino acids function primarily as storage metabolites, they may also provide an adaptive advantage to the plants, for example to render the plant less susceptible to attack by various animals and lower plants. Some of these toxic amino acids includes; djenkolic acids, mimosine and canavanine. Mimosine, a toxic non-protein amino acid structurally similar to tyrosine, is contained in the legume Leucaena leucocephala (D’Mello and Acamovic 1989; D’Mello, 2000). Mimosine has been proven effective in defleecing sheep and goats (Jacquemet et al.,1990; Luo et al., 2000). Mimosine a pyridoxal antagonist, which inhibits DNA replication and protein synthesis; thus, it may elicit defleecing by arresting cell division in the follicle bulb (Reis, 1979). In monogastric animals, mimosine causes poor growth, alopecia and reproductive problems. Levels of Leucaena meal above 5-10% of the diet for swine, poultry and rabbits generally result in poor animal performance. The major symptoms of toxicity in ruminants are poor growth, loss of hair and wool, lameness, mouth andoesophageal lesions, depressed serum thyroxine level and goitre. Some of these symptoms may be due to mimosine and others to 3, 4-dihydroxypyridine, a metabolite of mimosine in the rumen (Jones and Hegarty, 1984). Djenkol beans (Pithecolobium lubatum) when ingested sometimes lead to kidney failure which is accompanied by the appearance of blood and white needle-like clusters in the urine. The clusters are sulphur-containing amino acids known as djenkolic acids which are present in the bean in the free state, to the extent of 1-4%. This toxic amino acid is structurally similar to cystine, but it is not degraded in the animal body. Due to its insolubility it crystallizes out in the kidney tubules and escapes through urine (Enwere, 1998). The toxic, non-protein amino acid, canavanine, occurs widely in unbound form in various legume plants of the sub-family Papillonoideae (Bell et al., 1978) and abundantly in jack bean (Canavalia ensiformis (L). DC), constituting up to 63 g/kg dry weight of the seed (Ho and Shen, 1966). Canavanine, a structural analogue of arginine, was first isolated from jackbean by Kitagawa and Tomiyama (1929). Canavanine is believed to exert its toxic influence by virtue of its structural similarity with the nutritionally indispensable amino acid, arginine. Canavanine may antagonize arginine and interfere with Ribonucleic Acid (RNA) metabolism (Rosenthal, 1982). Canavanine has been demonstrated to reduce feed intake of non-ruminants but this was observed only at the equivalent of about 300 g/kg dietary level of raw jackbean Tschiersch, 1962). Saponins: Saponins are a heterogeneous group of naturally occurring foam-producing triterpene or steroidal glycosides that occur in a wide range of plants, including pulses and oil seeds such as kidney bean, chickpea, soybean, groundnut, lupin and sunflower Liener, 1980; Price et al., 1987; Jenkins and Atwal, 1994). It has been reported that saponins can affect animal performance and metabolism in a number of ways as follows: erythrocyte haemolysis, reduction of blood and liver cholesterol, depression of growth rate, bloat (ruminants), inhibition of smooth muscle activity, enzyme inhibition and reduction in nutrient absorption Cheeke, 1971). Saponins have also been reported to alter cell wall permeability and therefore produce some toxic effects when ingested (Belmar et al., 1999). Saponins have been shown to bind to the cells of the small intestine thereby affecting the absorption of nutrients across the intestinal wall (Johnson et al., 1986). The effect of saponins on chicks have been reported to reduce growth, feed efficiency and interfere with the absorption of dietary lipids, cholesterol, bile acids and vitamins A and E (Jenkins and Atwal, 1994).