Rats were fed for six consecutivedaysonadiet containingtheproteinsourcesunder test at a protein level of 20%. Protein synthesis by skeletal muscle ribosomes was measured in vitro. Synthetic activity was estimated per unit ribosomal RNA and per g of wet weight of muscle. In separate experiments the nitrogen efficiency ratio (n.e.r.) of the protein sources was determined after 21 days feeding at a 10% protein level.As measured by protein synthesis the nutritional quality of lyophilised baker's yeast Saccharomyces cerevisiae increased after mechanical disintegration. A further improvement was achieved by feeding yeast protein concentrates.Judged by these determinations the nutritional quality of the three yeast preparations was higher than that of the low-quality protein, wheat gluten but did not reach the level of the high-quality protein, casein supplemented with methionine.The metabolic utilisation of spray-dried microalga Scenedesmus obliquus increased after mechanical disintegration from a value similar to that of wheat gluten to the level of methionine-supplemented casein.Lyophilised Spirulina platensis had a nutritional quality between that of wheat gluten and casein supplemented with methionine. Addition of the limiting amino acid methionine to air-dried baker's yeast or drum-dried Scenedesmus obliquus and Spirulina platensis stimulated the metabolic utilisation of the micro-organisms.Supplementation of the diet with methionine increased the nitrogen efficiency ratio of air-dried yeast and of drum-dried Scenedesmus obliquus but had no effect on drum-dried Spirulina platensis. The ratios were significantly lower than those of casein supplemented with methionine.There was no difference in the nitrogen efficiency ratio between disintegrated yeast and yeast protein concentrate but the ratio of both was higher than that obtained with lyophilised yeast. The ratio was similar to that of casein without the addition of methionine.
I. Rats were given diets containing zoo g/kg of a complete or incomplete amino acid mixture or of high-or low-quality proteins. After 6 d the amino acid-incorporating activity of ribosomes from skeletal muscle and liver was studied.z. The level of isotope incorporation relative to ribosomal RNA was similar for casein supplemented with methionine and for a complete amino acid mixture with the composition of whole-egg protein. Per wet weight of tissue there was a significant decrease after fecding with the complete amino acid mixture.3 . There was a significant decrease in activity after feeding with amino acid mixtures deficient in lysine, methionine or tryptophan. In skeletal muscle, but not in liver, the ribosomal activity was less than that obtained with wheat gluten. Activity per wet weight of both tissues was less than that obtained with wheat gluten.4. Refeeding with methionine for I d resulted in complcte restoration of ribosomal activity and activity per wet weight in skeletal muscle. j. After lysine deficiency, protein synthesis per unit wet weight of both tissues and ribosomal activity in liver were not restored after z d of refeeding. Recovery of ribosomal activity in skeletal muscle was complete after I d.
1. The amino acid incorporating activity of skeletal muscle ribosomes was studied in rats under various nutritional conditions using labelled amino acids.2. Ribosomes were obtained from rats that were given a protein-free diet for 5 d followed by a high-protein diet containing casein, gelatin or wheat gluten for 16.5 h and from others that were given one of these protein-containing diets or one containing protein from polished rice for 6 d.3. The level of isotope incorporation relative to RNA was somewhat higher when the protein source given for 16.5 h was a good-quality protein such as casein than with gelatin or wheat gluten, but fine discrimination between proteins was not considered to be feasible with this system.4. In the rats that were given the protein-containing diets for 6 d the differences were more pronounced and the amino acid incorporating activity was correlated with the biological value of the protein.
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