L-Methionine and L-Cystine Requirements of the Growing Rat

TWO experiments were conducted to re-evaluate the methionine requirement of the growing rat fed an amino acid diet containing zero, intermediate and excessive levels of cystine and to study the interrelationship between methionine and cystine in meeting the total sulfur amino acid requirement of the growing rat fed an amino acid diet. When the diet contained 0, 0.15, 0.30 and 0.45% L-cystine, the calculated requirements for methionine were 0.51, 0.36, 0.18 and 0.17%, respectively, of the diet. Therefore, the minimum (absolute) methionine requirement was 0.17%) of the diet. The calculated total methionine plus cystine requirement was 0.47% of the diet, and cystine could supply 64% of this total. Due to the differences in molecular weights and moles of sulfur between methionine and cystine, the total moles of sulfur required from sulfur amino acids was calculated. The sulfur required from sulfur bearing amino acids was 3.34 × 10− moles per 100 g of diet. When this was converted back to a percent amino acid basis, it was calculated that the growing rat required 0.50% total sulfur amino acids if provided by methionine alone, but only 0.43% if methionine was present at an absolute requirement of 0.17% of the diet and cystine was included at a maximum usable level of 0.26% of the diet.