Ser concentration in wool protein is as high as 83 g kg"1, double the concentration in the whole-body (Table 17.1). Wool growth of 10 g day"1 needs 0.83 g day"1 Ser. Ser is also a co-substrate with Met for Cys de novo synthesis (Cooper, 1983). Assuming that 0.75 of the

Cys used for wool is derived from transsulphuration (Souri et al., 1996), 0.64 g day"1 Ser would be required for the synthesis of the 1 g Cys contained in 10 g wool protein. So the total Ser for 10 g of wool growth would be about 1.5 g day"1. Ser concentrations in plasma are consistently reduced by supplementation with Met, possibly due to increased demands for protein synthesis and for transsulphuration. This has been reported in sheep (Reis et al., 1990), cattle (Campbell et al., 1997) and humans (Frontiera et al., 1994), indicating that a supplement of Met results in the depletion of Ser. However, infusion of Ser at 10 g day"1 into the jugular vein in Merino lambs (about 35 kg) for 7 days did not promote the transsulphuration rate and wool growth rate (Liu et al., 2000). Probably the de novo synthesis is sufficient to support wool production, as indicated by the high concentration in the skin compared to some other amino acids (e.g. 835, 38 and 166 mmol kg"1 wet tissue respectively for Ser, Met and Cys).

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