Essential amino acids

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Higher animals require a core of nine amino acids for maintenance and productive purposes (Table 1.1). The need for these amino acids arises from the inability of all animals to synthesize the corresponding carbon skeleton or keto acid. These amino acids are classified as 'indispensable' or 'essential' and provision of these nutrients is mandatory. Non-ruminants will receive the essential amino acids via the diet, but ruminants may also acquire substantial amounts of these amino acids through the digestion of microbial protein synthesized in the rumen. Those amino acids which animals are able to synthesize are termed 'dispensable' or 'non-essential'.

All mammals require the core of nine essential amino acids, but may also respond to dietary arginine and possibly proline as well, during the early phases of rapid growth. Endogenous synthesis of arginine from gluta-mate/glutamine and proline plays an important role in the provision of arginine in the pig during the neonatal and immediate post-weaning phases (Wu et al., 1997). However, it has been estimated that 40% of the arginine requirements of the rapidly growing pig must be supplied by the diet. This need arises partly because the vast majority of arginine synthesized in the urea cycle is catabolized in the liver by an active arginase within this pathway. Consequently, insufficient arginine is exported for the rapid growth of extra-hepatic tissues. It is relevant to recall in this context that sow colostrum and milk are markedly deficient in arginine (Wu and Knabe, 1994).

The cat is unique among mammals in its requirement for arginine as an essential component of the diet. Indeed, Morris and Rogers (1978) observed that one meal without arginine was sufficient to precipitate mortality in cats. Other effects included emesis, tetanic spasms and hyperammonaemia. It was concluded that the inability of the cat to synthesize ornithine is the basis of the dietary need for arginine. The cat also has a specific requirement for taurine which is directly related to its role in the prevention of retinal degeneration.

Poultry have an absolute requirement for arginine arising from a non-functional urea cycle. This dependence translates into acute sensitivity to natural structural analogues of arginine (Chapter 14).

Of the two aromatic amino acids required for protein synthesis and other functions, only phenylalanine is considered to be essential. Tyrosine is regarded as dispensable as it is readily synthesized from phenylalanine. Although

Table 1.1. Nutritional classification of amino acids.

Essential

Table 1.1. Nutritional classification of amino acids.

Essential

Common core

Additional species-related requirements

Conditionally non-essential

Non-essential

Lysine

Arginine (cats, poultry, fish)

Cyst(e)ine

Glutamate

Histidine

Taurine (cats)

Tyrosine

Glutamine

Leucine

Arginine

Glycine

Isoleucine

Proline

Serine

Valine

Alanine

Methionine

Aspartate

Threonine

Asparagine

Tryptophan

Phenylalanine

this conversion is irreversible, the presence of tyrosine in the diet may reduce the requirement for phenylalanine. However, this sparing effect of tyrosine is limited and, consequently, a minimum quantity of dietary phenylalanine should always be ensured. With regard to growing poultry, for example, at least 58% of the total aromatic amino acid requirement should be supplied in the form of phenylalanine. An analogous situation exists between methionine and cyst(e)ine (see Chapter 8). The unique relationship between tryptophan and the B-complex vitamin, nicotinamide, represents another facet of the multifunctional roles of amino acids.

It is now widely acknowledged that high-yielding animals will not achieve their genetically determined potential if the dietary N is supplied exclusively in the form of the essential amino acids. Additional N is required and highly effective sources of this non-specific N include glutamate, alanine and diammonium citrate. However, the most effective source is a mixture of the non-essential amino acids. Consequently, although animals have specific dietary requirements for the essential amino acids, some combination of the dispensable amino acids should also be provided in order to maximize performance. The issue of essential to non-essential amino acid ratios is reviewed in Chapter 6.

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