As indicated above, the primary direction of arginine metabolism in mammals occurs via the urea cycle, enabling the disposal of excess N from amino acids. However, the peripheral metabolism of arginine is also of considerable biochemical and physiological significance. Thus, the action of arginine decarboxylase permits many organisms to synthesize putrescine and other polyamines. In animals, putrescine is produced solely by the action of ornithine decarboxylase (ODC). Although the specific functions of polyamines await elucidation, recent studies suggest that these compounds are essential for normal growth and development in all living organisms, and may regulate RNA synthesis and stabilize membrane structures. Polyamine production appears to be an indispensable feature of all tissues actively engaged in protein synthesis. Arginine uptake by the mammary gland from the blood supply substantially exceeds the quantities of this amino acid secreted in milk. This is generally attributed to the need to synthesize non-essential amino acids, particularly proline, within the gland itself. However, the excess uptake of arginine may also reflect the need for polyamine synthesis by tissues actively synthesizing proteins in the mammary gland. Polyamine synthesis is an important focal point for the action of antinutritional factors. Thus in lectin-induced hyperplastic growth of the small intestine, levels of putrescine, spermidine, spermine and cadaverine are markedly enhanced (Pusztai et al., 1993). On the other hand, the growth-retarding effect in chicks fed C. ensiformis has been attributed to inhibition of polyamine synthesis (Chapter 7). The non-protein amino acid, canavanine, present in this legume is metabolized to canaline, a potent inhibitor of ODC (D'Mello, 1993).
A striking feature of arginine relates to the synthesis of NO. The biosynthesis of NO involves the oxidation of arginine by NADPH and 02 via the action of NO-synthases. It is now established that NO plays a key role in vasorelaxation, neurotransmission, immuno-competence, male reproductive performance and gut motility (Moncada et ai, 1991). It is suggested in Chapter 7 that dietary canava-nine may inhibit NO synthesis through its structural antagonism with arginine. Enneking et ai (1993) arrived at a similar conclusion from their studies on canavanine-induced feed intake inhibition in pigs.
Homocysteine is a key intermediate in SAA metabolism, positioned at the juncture between remethylation to methionine and transsulphuration to cystathionine, yielding cysteine and taurine. The importance of homocysteine in human health was highlighted at the first conference on this amino acid (Rosenberg, 1996). It is now well recognized that plasma homocysteine levels are higher than normal in patients with coronary, cerebrovascular or peripheral arterial occlusive disease (Malinow, 1996). Other investigators suggested a link between homocysteine and neural tube defects (Mills et al., 1996; Rosenquist and Finnell, 2001). Furthermore, circulating concentrations of this amino acid may be of diagnostic value in assessing vitamin B12 status in humans (Stabler et al., 1996; Cikot et al., 2001). In pigs, prolonged vitamin B12 deficiency is associated with hyper homo-cysteinaemia (Stangl et al., 2000a) whereas in cattle a similar effect has been reported in long-term moderate deficiency of Co (Stangl et al., 2000b). Co is required for ruminal synthesis of vitamin B12. It is clear that much more effort is required to elucidate the role of homocysteine in farm animals, particularly pregnant ruminants and sows.
A number of essential amino acids have been implicated in immune function. Cysteine may function as an immunoregulatory signal between macrophages and lymphocytes. It has been proposed that release of this amino acid by macrophages enhances intracellular concentrations of the cysteine-containing tripeptide, glutathione (GSH) in lymphocytes. The latter is important for T-cell activity. Miller et al. (2000)
observed that cysteine infusion into the aboma-sum of sheep appeared to influence certain facets of immune response, including antibody responses to non-parasitic antigens. However, the exact role of cysteine in ovine immune function remains elusive. Swain and Johri (2000) indicated that the methionine requirement for optimum antibody production in broiler chickens was greater than that for optimum growth. Other reports suggest that dietary cysteine and the BCAA in particular may exert specific effects in the modulation of immune responses in broiler chickens (Takahashi et ai, 1997; Konashi et ai, 2000). Clearly, there is a need to undertake further studies to elucidate the exact role of SAA and BCAA as regulators of the immune »/stem.
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