Amino Acid Supplementation of Proteins and Protein Hydrolysates Four Figures

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Abstract
The addition of up to 40 mg N/kg as methionine to acid hydrolysates of casein given orally at maintenance levels in adult dogs increases nitrogen retention. Negative nitrogen balance and serious physical deterioration develop quickly upon increasing the level of methionine supplementation above 40 mg N/kg. The addition of up to 40 mg N/kg as methionine to a minimum lactalbumin diet or of up to 70 mg N/kg to a minimum casein diet has little effect on nitrogen retention. In both cases additional methionine causes toxic symptoms. In general, under these conditions, the addition of more than 50% of the total amino acids as methionine creates an imbalance and seriously impairs utilization. It appears that the toxic effect of methionine-induced imbalance cannot be directly ascribed to its labile methyl group or sulfur content. Excess choline chloride, cystine, and cysteine were fed without adverse effect. Glycine and alanine may be added to casein, to hydrolysates of casein, and to lactalbumin as complementary sources of nitrogen. These amino acids added individually or together have no influence on nitrogen retention. When the limiting amino acid of the protein or mixture is also furnished (as, for example, methionine to a casein diet), nitrogen retention is much increased. Large amounts of phenylalanine added to a casein diet are apparently metabolized without difficulty. The tyrosine formed may be voided in the urine in quantities sufficient to exceed its solubility therein. The addition of tyrosine and leucine is without effect on nitrogen utilization. Tyrosine is poorly absorbed and at high intake levels a large proportion is eliminated in the feces.