The methionine-sparing effects of cystine are well documented (Chapter 8). The methionine-cystine conversion is of practical importance since many feeds contain more cystine than methionine. Consequently, diets may be relatively high in total SAA but low in methionine. In assessing the extent to which cystine may spare dietary methionine requirements, it is important to ascertain whether values being considered are on a weight or molar basis. In addition, it is necessary to recognize that cystine efficacy is considerably greater for maintenance than for new tissue growth. The higher cystine requirements for maintenance have been attributed, in part, to the high cystine content of hair and other keratin components. Methionine : cystine ratios suggested for pigs are 49:51 for protein accretion and 23:77 for maintenance (Chapter 8). For practical formulations, current recommendations are that not more than 50% of the SAA requirement of animals should be provided by cystine. Even in fish, the cystine replacement value for methionine varies between 40 and 60% on a sulphur basis (Chapter 23).
The SAA requirement for wool growth is high and generally above that supplied by exogenous sources (Chapter 17). Methionine supplementation enhances efficiency of protein utilization by promoting protein metabolism in the follicles and skin. Cysteine synthesized in the skin is quantitatively important in supporting wool growth; however, its supply is restricted by the methionine needs for polyamine synthesis. Methionine appears to be used relatively inefficiently for growth by cattle and sheep. In cattle (Chapter 18), this effect has been attributed to the role of methionine in providing methyl groups, whereas in sheep its utilization for cysteine synthesis may be more important (Chapter 17).
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