In this chapter an attempt has been made to set the scene in terms of identifying and defining basic principles such as classification of amino acids, unique effects of deficiency

Fig. 1.1. Three-compartment models for 3-methylhistidine (3-MH) kinetics in selected animal species (Rathmacher, 2000). Tracer doses of labelled 3-methylhistidine are used in investigations with these models.

and utilization of isomers and precursors of neurotransmitters. Since the previous edition (D'Mello, 1994), significant advances have been made in our understanding of the multifunctional roles of amino acids. These have been highlighted in this chapter and pursued in greater depth in other parts of this book. Emerging issues include the role of specific amino acids, such as leucine, in molecular signalling and cysteine in modulation of immune function. In addition, a number of issues have reached a stage enabling a more mature assessment of their metabolic or nutritional significance. The use of 3-methyl-

histidine is a case in point, whereas the role of homocysteine emerges as an issue worthy of future research, particularly with respect to breeding animals.

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