Influence of Levels of Methionine and Cystine on the Total Sulfur Amino Acid Requirement of the Growing Rat

ONE hundred and sixty-eight male albino rats were used in three trials that were conducted concurrently to determine the L-methionine requirement of rats fed a crystalline amino acid diet devoid of cystine and the L-cystine requirements of rats fed crystalline amino acid diets containing 0.17 and 0.25% L-methionine. Each calculated requirement was similar regardless of response criterion used, i.e., average daily gain, gain/feed or protein efficiency ratio. The calculated methionine requirement was 0.49% of the diet when rats were fed a diet devoid of cystine. The calculated cystine requirements were 0.24 and 0.19% when rats were fed diets containing 0.17 and 0.25% L-methionine, respectively. Therefore, the total sulfur amino acid requirements of the growing rat expressed as a percentage of the diet were 0.49, 0.41 and 0.44% when the rats were fed diets devoid of cystine and containing 0.17 and 0.25% methionine, respectively. The larger the proportion of methionine used in meeting the total sulfur amino acid requirement, the greater the total sulfur amino acid requirement was for the growing rat. When expressed on a molar basis, there were no differences in the sulfur amino acid requirements between rats fed diets containing only methionine and those fed diets containing combinations of methionine and cystine.