Evaluation of modified [35S]methionine and [35S]casein preparations as supplements for sheep

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Abstract
1. Doses of L-[35S]methionine (2 g) and [35S]casein (20 g) were given to sheep in the diet or via the abomasum, and the patterns of 35S-labelling in blood, wool and excreta were studied during the 7 d following administration of the dose.2. Doses given via the abomasum resulted in substantial labelling of the plasma proteins and wool; only small amounts of the dose were recovered in the urine and faeces. In contrast, doses given in the diet resulted in much less labelling of plasma proteins and wool and ingreater losses in excreta. These results provide the basis of a rapid system for testing the effectiveness of various methods of protecting methionine and casein from degradation in the rumen.3. From the patterns obtained with N-formyl-DL-[35S]methionine and [35S]polymethionine (mol. wt 7–8 × 104) it appears unlikely that these compounds would stimulate wool growth if given as dietary supplements.4. [35S]Casein which had been treated with an 8 % aqueous solution of formaldehyde gave results which showed that the casein had been protected from destruction in the rumen without markedly reducing its subsequent digestibility. Treatment of [35S]casein with excess, formalde-hyde (a 40 % aqueous solution) gave a product which was completely indigestible.5. It is concluded that the radioisotope technique could be applied to other [35S]amino acids or their derivatives and to other [35S]proteins, and should also be applicable to larger ruminants such as cattle.

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