It is not quite clear so far to what extent the requirement for total non-essential N can be influenced by the presence or absence of different non-essential amino acids. There are a number of studies indicating that some amino acids, commonly classified as non-essential, may have essential character (Breuer et al., 1964; Newburg et al., 1975; Ball et al., 1986; Roth et al, 1994a) whereas some others are inferior as sources of non-specific N (Sugahara and Ariyoshi, 1967b; Allen and Baker, 1974). Therefore, both the specific requirements for non-essential amino acids and the value of these amino acids in supplying the organism with non-specific nitrogen should be taken into account when studying the optimum E:T ratio and formulating amino acid diets.
Results of studies aimed at identification of non-essential amino acids needed for normal performance have been controversial. The requirement for proline has been demonstrated in rats (Breuer et al., 1964; Heger et al., 1987), chicks (Sugahara and Ariyoshi, 1967a; Graber and Baker, 1973) and piglets (Ball et al., 1986; Kirchgessner et al., 1995), but in other experiments no effect of proline was found (Samuels et al., 1989; Chung and Baker, 1993). This discrepancy might be due to the differences in dietary levels of metaboli-cally related amino acids. Rogers et al. (1970) found in rats that if two amino acids of the pro-line-arginine-glutamate group were omitted from the diet, growth was markedly reduced. However, if the diet contained sufficient amounts of arginine and glutamate, the lack of proline had no effect on growth. Contrary to this finding, the adverse effects of proline deficiency in chicks were not eliminated even when the diet was supplemented with glutamate
(Bhargava et al., 1971), arginine or ornithine (Austic, 1976). Glutamate plays an important role in protein metabolism since it serves as the main medium in amino-N exchange and is readily converted into other non-essential amino acids. There are some studies indicating the need for glutamate in rats (Hepburn and Bradley, 1964), chicks (Graber and Baker, 1973; Maruyama et al., 1976) and piglets (Roth et al., 1994a). It seems, however, that its presence in the diet is not necessary if the diet contains sufficient amounts of proline and arginine (Rogers et al., 1970; Heger et al., 1987).
The asparagine requirement has been demonstrated in growing rats by Breuer et al. (1966), Rogers and Harper (1965), Frankel et al. (1973) and Newburg et al. (1975), although Salmon (1964) and Ranhotra and Johnson (1965) failed to observe any requirement. In the study by Rogers et al. (1970) asparagine clearly stimulated growth in one experiment, yet was without any effect in another experiment. Newburg et al. (1975) investigated the effects of asparagine, aspartic acid, glutamine and glutamate in all possible combinations on growth of weanling rats, and concluded that only asparagine is essential regardless of the presence or absence of any combination of other related amino acids. No asparagine requirement has been reported for adult rats or other species.
Glycine is regarded as a good source of non-specific nitrogen (Allen and Baker, 1974) but there are no indications of its requirements for mammalian species (Heger et al., 1987; Roth et al., 1994a). A positive effect of glycine addition to diets with a high concentration of essential amino acids observed in kittens (Taylor et al., 1996) probably resulted from the ability of glycine to alleviate the toxic effects associated with excessive intake of sulphur amino acids (Benevenga and Steele, 1984). In poultry, glycine has been classified as a semi-essential amino acid by some authors (Graber and Baker, 1973; NRC, 1994). An increased demand for glycine supply in poultry is associated with the synthesis of uric acid as the primary end product of protein catabolism in avian species that may consume a considerable amount of glycine, particularly when fed high-protein diets.
To date, it is not known whether there is a specific requirement for non-essential and semi-essential amino acids for maintenance. It is generally believed that, owing to the low needs for total N under maintenance conditions, both non-essential and semi-essential amino acids can be readily synthesized from their precursors. However, as discussed earlier, the maintenance requirement for nonessential N is relatively high and the synthesis of some non-essential or semi-essential amino acid(s) may not be sufficient to meet endogenous losses and specific requirements not related to protein metabolism. Indeed, preliminary studies with pigs (J. Heger, L. Krizovâ and K. SimeCek, unpublished observations) at near-maintenance conditions have shown that the deletion of arginine, proline or glutamate from a purified diet resulted in a significant decrease in N retention even though the concentration of total non-essential N was maintained at a sufficiently high level.
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