Introduction

The principal nitrogenous compounds present in cells are proteins and their amino acid precursors, nucleic acids and their nucleotide precursors, peptides such as glutathione, compounds derived from amino acids, such histamine, choline and creatine, and excretory compounds such as urea and uric acid. The dietary nitrogen requirements for birds can be met by the essential amino acids (Section 2.2) together with the nitrogen-containing B vitamins. A typical avian diet will generally contain a wider range of nitrogenous compounds, including the non-essential amino acids. Birds are able to synthesise all their body proteins, nucleic acids and other nitrogenous compounds from the minimum dietary requirements. The mechanisms of protein and nucleic acid synthesis are essentially the same in all vertebrates, are well described in many textbooks (Mathews & van Holde, 1990; Zubay, 1993) and are, therefore, not discussed in this chapter. The hormonal control of protein synthesis in the hen oviduct has been extensively studied, and this is discussed in Section 10.3.

The relative importance of anabolic and catabolic pathways varies between tissues and between growing and mature birds and is considered under the general heading of protein turnover (Section 5.2). The mechanism of protein degradation is much less well understood than that of protein synthesis, although much progress has been made recently on this front (see Section 5.3). The final stages of protein degradation leading to nitrogen excretion are quite distinct in birds compared with those in ureotelic vertebrates and are considered in Section 5.4.