Sulfur amino acid requirements of the growing rat fed eight percent dietary protein

Abstract

Methionine and methionine plus cystine requirements of the growing rat fed equivalent to 8% dietary protein were determined. Diets formulated to be adequate in all nutrients except sulfur amino acids (SAA) and providing 4% protein from ANRC casein and equivalent to 4% casein protein from amino acids (excluding methionine and cystine) were supplemented with graded levels of L-methionine. The diet providing 0.34% total SAA (0.32% L-methionine + 0.02% L-cystine) gave the best feed/gain ratio, relative net protein ratio (RNPR), liver protein utilization (LPU) and plasma amino acid acid parameters. When similar ANRC casein + crystalline amino acid diets providing 0.44% total SAA but varying in ratio (by weight) by methionine/cystine were fed to rats, optimal feed/gain ratio, RNPR and plasma amino acid parameters were obtained when cystine replaced 33 to 60% of dietary L-methionine. Inclusion of L-cystine at the expense of L-methionine in these 8% protein diets improved overall rat performance and utilization of dietary methionine. Supplementation of the casein + amino acid basal diet with methionine sulfoxide, methionine sulfone (oxidized forms of methionine) or cysteic acid (oxidized form of cysteine/cystine) to provide 0.34% total SAA, indicated that relative to methionine (100), methionine sulfoxide was completely available and methionine sulfone (68) and cysteic acid (55) were less available. Correction for methionine and cystine digestibilities in ANRC casein suggests that the SAA requirements for the growing rat are 0.33% of diet or 4.1% of dietary protein when 8% protein diets are fed.