It is generally accepted that the profile of absorbed amino acids from the gut affects the utilization efficiency and this has led to a concept of an 'ideal' amino acid pattern (Fuller, 1992).
To determine the ideal pattern of amino acids is a complex challenge (see Chapters 9 and 13). The simplest approach depends on the amino acid profiles of the products, i.e. accretion of the body protein mass (weight gain), milk output, or wool growth. In growing animals protein deposition both in the body and in wool needs to be included. Since amino acid composition of body protein does not change significantly during growth from 25 kg to 55 kg live weight (MacRae et al., 1993), the composition of the whole body could be used for estimating protein accretion. However, a specific consideration has to be made for wool growth because the amino acid composition in wool is different from the tissue proteins (as shown in Table 17.1). In particular, high concentrations of Cys, Ser and Arg in wool suggest that amino acid profiles with high proportions of these amino acids could favour wool growth. Interestingly these amino acids are not essential and their availability for both anabolism and catabolism is influenced by biosynthesis de novo.
More realistically, the 'ideal' amino acid pattern should reflect the dynamic profile of protein turnover. The protein deposited in both the tissues and wool accounts for only a fraction of total protein synthesis. The majority of amino acids in the metabolic pool (in terms of quantity) is derived from basal protein turnover, the profile of which is determined by the relative contributions of each tissue/organ. For example, protein synthesis in the gut tissue accounts for 0.26 of whole-body protein synthesis and skeletal muscle accounts for 0.18 (Lobley, 1993). This compares to their contributions of about 0.05 and 0.5 to total protein content of the wholebody (MacRae et ah, 1993). The relatively high concentrations of sulphur-containing amino acids from the gut tissue compared to the whole carcass (57 vs. 27 g kg-1 protein; MacRae et al., 1993) coupled with a much high turnover rate in the gut tissue (Adams et al., 2000) indicate a relative increase in demand for the sulphur-containing amino acids.
A further factor that influences the 'ideal' amino acid pattern is the differences in oxidation of individual indispensable amino acids in the body. From an animal production point of the view, oxidation represents a net loss of amino acids from the body. A low oxidation rate means more amino acid would be available for protein synthesis. As seen in Table 17.3, relatively high rates of oxidation for Met, Lys and Cys would mean a higher requirement in the diet relative to the other amino acids.
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