Incorporation of abomasal and intravenous doses of [35S]cystine and [35S]methionine into wool

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Abstract
Summary The fate of abomasal and intravenous doses of [35S]cystine and [35S]methionine was investigated in sheep to determine their relative efficacy as supplements for wool growth. Administration of L-[35S]cystine and L-, DL- and D-[35S]methionine via the abomasum produced similar patterns of35S labelling in blood plasma. Only 10–16% of the administered35S was excreted in urine over 7 days and 22–32% of the dose was incorporated into wool over 21 days. All isomers were utilized by the sheep with similar efficiency and it was concluded that D- and L-methionine and L-cystine are equally effective as supplements for wool growth at optimal dose rates. Abomasal supplementation with a large amount of methionine (10 g/day) resulted in the excretion of a high proportion of the35S in urine (55% of the dose over 7 days) and low incorporation into wool. A comparison of the extent of incorporation of35S into wool from intravenous doses of L-[35S]cystine and L-[35S]methionine in the same sheep, and in sheep from flocks selected for high and low wool production, showed that incorporation from methionine averaged 80% of that from cystine. It was concluded that, at moderate (2–3 g/day) methionine availability, methionine is utilized for synthesis of wool with about 80% of the efficiency of cystine, conversion of methionine to cysteine occurring via the transulphuration pathway.