Dietary Protein Quality Measured by in vitro Protein Synthesis in Rat Skeletal Muscle Ribosomes

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Abstract
For 6 consecutive days rats were fed a diet containing either 10% or 20% protein of varying biological values. Ribosomes of skeletal muscle were isolated. The RNA content per gram wet weight of muscle was higher in the ribosomal preparations of the casein-fed than of the wheat gluten-fed rats. This RNA was shown by polyacrylamide gel electrophoresis to be of ribosomal origin. The proteins present in the ribosomal preparations seemed unchanged when expressed per unit wet weight of muscle. The small alterations in protein due to changes in ribosomal content could not be detected by the analytical methods used. The amino acid-incorporating activity of the skeletal muscle ribosomes was measured. When diets containing 10% protein were fed the isotope incorporation relative to protein was highest with casein as the protein source, followed by barley and soya protein or wheat gluten in that order. These differences correlated well with their biological value but less well with their protein efficiency ratio. Calculations of the radioactivity values relative to ribosomal RNA gave no differences in activity between the groups of rats. When diets containing 20% protein were fed the effect on protein synthesis was more pronounced and apparent also relative to ribosomal RNA. In addition, the activity of liver ribosomes per RNA — but not per protein — was higher in the casein-fed than in the wheat gluten-fed rats. The amino acid-incorporating activity of skeletal muscle ribosomes was positively correlated with the biological value of the dietary protein both at a 10% and a 20% level while the agreement with the protein efficiency ratio was less marked.