S-Carboxyethylcysteine (a constituent of Acacia seed) negatively affects casein protein utilization by rats.

OBJECTIVE Two rat bioassay experiments are reported. The first investigated the first limiting amino acid in Acacia colei and the second experiment investigated the effect of S-carboxyethylcysteine (CEC; a compound present in acacia seed) on protein use. METHODS In the first experiment, Wistar rats were fed A. colei seed supplemented with three levels of methionine, cysteine, and tryptophan (0.1%, 0.2%, and 0.4%). In the second experiment, the Wistar rats were fed CEC-incorporated casein diets. RESULTS Supplementation of A. colei with tryptophan had no significant effect on the protein efficiency ratio, cysteine showed the highest protein efficiency ratio value at the 0.4% level, and the protein efficiency ratio increased significantly with the increase in methionine content, making methionine the first limiting amino acid. The methionine-induced growth rate was suppressed by the incorporation of CEC, which also had a negative effect on the plasma amino acid levels. CONCLUSION The results indicated that methionine is the first limiting amino acid in A. colei and that CEC could affect the seed's protein use. Acacia colei seed can be used effectively as famine food only if it is complemented with other cereals known to be rich in sulfur amino acids.