Since the first edition of this book, considerable advances have been made in elucidating the processes involved in the metabolism of amino acids in animals. These developments as reviewed in several chapters in the present updated edition, rely on some basic appreciation of intermediary metabolism of important amino acids. To avoid unnecessary repetition, outline pathways are collated in this chapter, commencing with key reactions and following up with more advanced features of metabolism of particular relevance to chapters in this edition.
Inter-organ flux of amino acids is important in determining the profile that reaches the ultimate sites of utilization (Seal and Parker, 2000). Amino acids cannot be stored as free molecules and must follow anabolic routes to peptides, proteins, hormones and other bioactive molecules (Fig. 4.1) or cata-bolic pathways to ammonia (fish), urea (mammals) or uric acid (birds and reptiles). The terms 'ammoniotelic', 'ureotelic' and 'uricotelic' are used to indicate the forms of N excretion in the respective groups of animals. Despite the disparate end products of amino acid catabolism in these animals, the basic features outlined in the next section
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apply in common to all species covered in this book. Furthermore, rumen microbes display these key reactions as well as the innate capacity to synthesize all of the essential amino acids (Chapter 15).
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