Adverse Effects of Amino Acids
AA is assumed that any surplus ingested by animals is disposed of without adverse effects. The ruminant has endowed a detoxification mechanisms by microbial metabolism of amino acids within the rumen. However, there is now evidence demonstrating that amino acids may precipitate profound deleterious effects in diverse classes of farm animals. These manifestations conform with the three categories of imbalances, antagonisms and toxicities
1. Amino Acid Imbalance A change in the pattern of amino acids in the diet depressions in food intake and growth. The primary manifestation of adverse effects was a depression in food intake which consequently also decreased intake of the limiting amino acid, leading to reduced growth.
A diets containing of groundnut meal to provide crude protein (CP) and supplemented with or without methionine plus lysine. The result assumed minimal amino acid needs of the young chick were considered to be satisfied at the high inclusion rates of either methionine, or lysine. Several studies with pigs indicate that amino acid imbalances may occur at the tissue level.
Leibholz et al. (1986) observed that the concentration of free lysine in plasma of pigs increased 1 2 h after feeding a diet containing pure lysine. whereas the circulating of other amino acids originating from the protein bound fraction of the diet peaked at 2 6 h postprandial. In pigs fed once daily, this lack of synchrony in absorption would precipitate an amino acid imbalance at the cellular level. Under these circumstances growth and efficiency of dietary nitrogen (N) would be impaired, but the deleterious effects could be offset by more frequent feeding. 5
The efficiency of utilization of free lysine supplements for growth of pigs fed once daily was only 0.43 0.67 of values recorded with pigs fed in six time intervals. In contrast, no such benefit occurred on feeding the unsupplemented control diet more frequently. The benefits of increased feeding frequency and lysine supplementation to improvements in N utilization.
The question of imbalances in cattle nutrition is relevant, A basal diet co-limiting in supplies of methionine and lysine, cows fed this diet ate less dry matter and produced less milk and lactose than unsupplemented controls. The models suggested that the basal diet may have been limiting in intestinally absorbable lysine, isoleucine or histidine.
1.2 Effects on food intake In rats fed an imbalanced diet reduced their food intake within 3 6 h. The amino acids arriving in the portal circulation after consumption of an imbalanced diet stimulate synthesis or suppress breakdown of protein in the liver leading to greater retention of the limiting amino acid.
Since these events occur within a few hours of ingestion, it suggested that changes in plasma amino acids may provide the metabolic signal of neurotransmitters in the brain. Feeding imbalanced diets reduced production of noradrenaline in of rats,but not in chicks. 10
1.3 Effects on nutrient utilization N retention efficiency declined from 0.60 to 0.44 on an imbalancing amino acid diet of rat. Indicating that the effects of imbalance are mediated via reductions in appetite. Langer and Fuller (1994) demonstrated that the additional of leucine, isoleucine and valine to a diet limiting in methionine increased N efficiency in growing pigs. 11
2. Amino Acid Antagonisms A deleterious interaction between structurally similar amino acids. Antagonisms may be precipitated by a wide range of analogues occurring naturally in crop plants as non-protein amino acids. 2.1 Branched-chain amino acid antagonisms Maize byproducts, sorghum, and blood meal contain disproportionate quantities of branched-chain amino acids. 12
effects. leucine valine antagonism Excess leucine in a diet marked limiting in valine, and growth depression in young chicks. Valine supplementation reversed this effect. In turkey poults, the supplementation effects of leucine and valine, valine in plasma was reduced as levels of leucine increased. 13
The lysine arginine antagonism Addition of excess lysine to each of the first-limiting in methionine, tryptophan,histidine or threonine precipitated a severe growth depression in chicks which, in every case,was reversed by arginine supplementation. Alterations in arginine:lysine ratios may be beneficial in heat stress in broilers. 14
Antagonisms induced by non-protein amino acids Non-protein amino acids may occur in all parts of the plant, but the seed is the most concentrated source such as legumes and brassicae. Mimosine Mimosine may be regarded as a structural analogue of tyrosine and its neurotransmitter derivatives, dopamine, noradrenaline. The adverse properties of mimosine are include disruption of reproductive function, loss of hair and wool and even death. Tyrosine may reverse the deleterious effects of mimosine. 15
In 16
Some rumen bacteria are capable of detoxifying both forms of degraded mimosine ;DHP(3,4-DHP), 2,3-DHP. Ruminants in Australia, the USA and Kenya had the goitrogenic effects if high intakes of Leucaena. In Central America, Hawaii, and Indonesia, ruminants possess the bacteria that are required for DHP degradation, which accounts for the absence of Leucaena toxicity. 17
Analogues of sulphur-containing amino acids In plants, where the sulphur atom is replaced by selenium, are manifestations of acute selenium poisoning. S-methylcysteine sulphoxide (SMCO) occurs in forage and root brassica crops (turnip). The adverse effects of SMCO occur after its metabolism by rumen bacteria to dimethyl disulphide. A severe haemolytic anaemia,the liver becoming swollen, pale and necrotic appears within 1 3 weeks in animals fed mainly on brassica forage. 18
Analogues of arginine Of the three analogues of arginine( indospicine, homoarginine, canavanine), canavanine is more widely distributed and present in higher concentrations in leguminous seeds. Canavanine (jack bean, JB), a structural analogue of Ornithine, contributes the toxicity for young chicks. Since the urea cycle is non-functional in avian species, they are unable to synthesize arginine; consequently, readily death to the adverse effects of canavanine in jack beans 19
20
Arginine supplementation enhanced weight gain and food intake, but lysine supplementation markedly decreased on growth and utilization. These results support the existence of a canavanine arginine antagonism analogous to that between lysine and arginine. 21
Amino Acid Toxicity The acute growth depressions caused by excesses of some individual amino acids. At 40 g/ kg diet, methionine is the most growth-depressing amino acid, however at this inclusion level, leucine, isoleucine, and valine do not impair growth when added to practical diets for pigs and poultry. Scherer and Baker (2000) suggest that excess methionine may increase vitamin B6 requirements of young chicks. 22
Dietary excesses of tryptophan or tyrosine administered for 5 days prior to slaughter reduced stress in pigs, a response attributed to increased hypothalamic concentrations of several neurotransmitters. Oral or intraruminal administration of tryptophan or the indole to cattle can cause respiratory distress and pulmonary lesions Tryptophan toxicity depends on the balance between metabolic activation of 3-methylindole and its conjugation with glutathione. 23
Conclusions Amino acid imbalances are in diets fed to non-ruminants, causing reductions in growth and N utilization. Antagonisms occur due to adverse ratios of lysine and arginine and of the branched-chain amino acids in some common feedstuffs. Adverse effects may arise from the presence of non-protein amino acid analogues such as mimosine and SMCO in brassica forages. Although ruminants are normally less susceptible to dietary amino acid imbalances and toxicities. 24