Protein turnover of breast muscle in germ-free and conventional chicks

The effect of the gut microflora on protein turnover in pectoral muscle (M. pectoralis profundus) was studied by means of dietary infusion of L-[U-14C]phenylalanine and of massive dose injection of L-[4-3H]phenylalanine in chicks fed on a semi-purified casein-gelatin (SCG) diet until 19 d of age, and in those subsequently given either a nitrogen-free (NF) diet or NF supplemented with methionine and arginine (MA) for a further 9 d. Time-course changes in radioactivity released in expired carbon dioxide during the 8 h infusion period showed that isotopic equilibrium was reached in 4 h with the SCG diet and in 5 h with the MA diet. However, with the protein-deprived chicks given the NF diet, isotopic equilibrium was not achieved since radioactivity in CO2 increased linearly throughout. On feeding the NF diet, fractional protein synthesis rate and the absolute amount of protein synthesized in chick breast muscle were reduced. These reductions were partially alleviated by supplementing the NF diet with methionine and arginine. The fractional degradation rate of breast muscle was increased in chicks given the NF diet, while the absolute amount of protein degraded was decreased. The addition of methionine and arginine counteracted these changes brought about by protein starvation. Generally speaking, the presence of the gut microflora had little, if any, effect on protein turnover rate in chick-breast muscle.