Biosynthesis of Amino Acids and Nutritional Classifications

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The title of this book now reflects the fact that, in addition to traditional farm animal species, additional chapters have been included to address the amino acid nutrition of cats and fishes. In light of this, it is necessary to revisit the classification of amino acids in the context of the physiological state of each species in question, the productive and health management goals and how this influences our nutritional decisions (see Chapter 1). There are 20 amino acids commonly found in animal proteins, and all of these are incorporated into proteins as the L-isomer. As nutritionists, we use the terms essential and indispensable, and non-essential and dispensable, interchangeably. An important qualifier here is that these categories were initially established for growth, indeed rapid growth. For growing mammals, birds and fishes, the essential amino acids 'required' in the diet will be dominated by the composition of the proteins accreted (e.g. skeletal proteins in growing animals and fishes) or secreted (e.g. milk proteins in lactating animals, eggs in birds). The original definition by W.C. Rose and colleagues (Borman et al., 1946) had this in mind when they defined an essential amino acid as, 'One which cannot be synthesized by the animal organism out of materials ordinarily available to the cells at a speed commensurate with the demands for normal growth'. Table 1.1 shows the nutritional classifications of the 20 amino acids commonly found in animal tissue proteins plus non-protein amino acids that fall into conditional classifications.

Nutritional essentiality has sometimes been misinterpreted to mean that the animal is incapable of synthesizing it. In order to clarify, it must be pointed out that in mono-gastric species the context of nutritional essentiality relates directly to the diet whereas in ruminants essentiality relates to the supply of amino acid leaving the rumen, which is composed mostly of microbial proteins with variable contributions from undigested feed proteins. In the core essential group, the branched-chain amino acids, methionine and phenylalanine can be synthesized by animals from their corresponding keto-acids. However, the keto-acids can only be derived de novo from the original parent amino acid via transamination (see Chapter 4), and so there is no new or net synthesis, unless the corresponding keto-acid is provided in the diet. Racemic mixtures of d- and l-isomers of amino acids can provide a cheap source of supplemental amino acid to balance diets because the keto-acid that is produced following d-oxidase degradation has no racemic centre and so it can be ream-inated to yield the l-isomer. Similarly, keto-acids can be supplemented in the diet to yield the corresponding l-isomer. The effectiveness of these supplementation strategies probably depends on the relative rates and affinities of the aminotransaminases and competing oxidative pathways. The same holds true for the original l-isomer. In light of this potential competition, it has been suggested that if the balance could be tilted in favour of transamination (i.e. reamination of the keto-acid), amino acids (their keto-acids) could be 'protected' from irrevocable catabolism (Lobley et al., 2001). The main source of amino donors would be from glutamate or glutamine, both of which are found in millimolar concentrations within tissues. Transamination occurs with most amino acids except for lysine (Fig. 4.14) and threonine, whereas histidine transamination occurs to a limited extent.

Methionine can also be synthesized via remethylation of homocysteine but this also does not represent new synthesis since homocysteine itself originates from methionine. The methionine analogue 4-thiomethyl-2-hydroxy-butanoic acid has been used effectively in pigs, poultry, and dairy cows. The advantage here is that the analogue is not toxic, as is methionine, and it readily diffuses across and into tissues. In sheep (Wester et al., 2000), the analogue is converted into methionine via transamination, with the greatest contribu tions to tissue methionine occurring in the kidneys (22%) where there is abundant transaminase activity, followed by liver (14%) and the gastrointestinal tract (5-12%). The mammary gland is also a site of conversion with 20% of milk protein methionine derived from the analogue (Lobley and Lapierre, 2001). The ability of specific tissues to convert the analogue into methionine will depend on the relative activities of the transaminases and dehydrogenases (oxidation) within the tissues and, presumably, the adequacy of amino donors.

A recent finding that may influence requirements is that threonine and lysine, and to a lesser degree other amino acids, can be synthesized by microbes in the lumen of the small intestines and large bowel. On a gross basis this contribution may account for 1-20% of maintenance intake needs (Torrallardona et al., 1996). At intakes well above maintenance, as in growing animals, this contribution is probably <5%. The intestinal microbial synthesis of amino acids is influenced by diet composition where the availability of precursors (ammonia, urea, amino acids, non-starch carbohydrates) in the small intestines determines the extent of microbial growth.

Rose (Borman et al, 1946) defined nonessential amino acids as those that can be synthesized by the animal from materials normally available, and at a rate commensurate with normal growth. Reeds (2000) points out that, from a metabolic standpoint, the only truly non-essential amino acids are glutamate and serine. These can be synthesized from non-amino nitrogen (ammonium ions) and appropriate carbon skeletons derived from intermediates of glycolysis (3-phospho-glycerate) and the tricarboxylic acid cycle (a-ketoglutarate). This should not be taken to infer that glutamate is not required in the diet. Indeed, diets devoid of glutamate have been shown to depress growth (Rose et al., 1948), suggesting that under growing conditions the material normally available or its rate of synthesis may be limiting. All other non-essentials derive their amino group or carbon skeleton from other amino acids. In this connection, glutamate and serine play a central role because they are the primary precursors for non-essential amino acid synthesis.

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