Metabolic Conversions

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The metabolic relationships between methionine and cysteine are well established (Fig.

The first study in which one of the sulphur amino acids was added to the diet of pigs was described by Bell et al. (1950). These authors reported that the addition of methionine at 2.0 g kg 1 to a semipurified diet in which soybean meal provided the sole source of protein improved biological value. The basal soybean meal diet contained 100 g kg-1 protein and 0.7 g kg 1 methionine, but the cystine content of this diet was not given. The biological value of the methionine-supplemented diet and the weight gain of the pigs fed this diet were equal to those of pigs fed a whole egg protein diet with 100 g kg-1 protein and 2.7 g kg 1 methionine. This study was the first to show that methionine was an essential amino acid for pigs. Previous research had shown that methionine was essential for rats, mice, chicks and humans.

One year later, Shelton et al. (1951) fed a semipurified diet containing 210 g kg-1 protein, 1.0 g kg-1 methionine, and 0.1 g kg 1 cysteine to growing pigs. They supplemented the diet with either methionine, cystine, or a mixture of the two amino acids. Supplementation with 6.0 g kg 1 cystine

Methionine

Homocysteine

S-Adenosylmethionine

S-Adenosylhomocysteine

Fig. 8.3. Metabolic pathways of sulphur amino acids.

Cysteine increased weight gain from 43 to 163 g day \ whereas supplementation with 5.0 g kg 1 methionine increased weigh gain to 572 g day Furthermore, supplementation with 2.0 g kg-1 methionine and 3.0 g kg-1 cystine was as effective as supplementation with 5.0 g kg-1 methionine or 5.0 g kg-1 methionine plus 6.0 g kg 1 cysteine. These results confirmed the essentiality of methionine and placed cystine in the 'conditionally dispensable' category. These early results indicated that the total sulphur amino acid requirement of pigs weighing approximately 20 kg was 6.0 g kg-1 of the diet and that all of this could be provided by methionine or half could be provided by methionine and half by cystine. Although these conclusions were based on very limited numbers of animals, and there was no evidence of statistical analysis, current estimates of sulphur amino acid requirements are remarkably similar to these early values.

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