Effect of Feeding Carbohydrate or Fat on Incorporation of 14C-Phenylalanine in vivo and in vitro into Rat Liver and Muscle Protein

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Abstract
The incorporation of phenylalanine-U-14C into liver, muscle protein and expired CO2 was determined in rats fed either carbohydrate or fat in order to elucidate the biochemical mechanism underlying the “protein sparing” action of dietary carbohydrate and fat. The rate of incorporation of this radioactive amino acid into these tissue proteins in vivo was compared by calculating (i) relative specific activity: the ratio of specific activity of the protein to that of the acid-soluble fraction in muscle and liver and (ii) recovery ratio: the ratio of radioactivity recovered in total muscle and/or liver protein versus that recovered in expired CO2. The incorporation of phenylalanine-U-14C into protein increased in both liver and muscle of rats fed either carbohydrate or fat and the ratio of radioactivity recovered in muscle protein to that in liver protein increased. Feeding either carbohydrate or fat caused a significant increase in rate of incorporation of this radioactive amino acid by liver slices as well as isolated diaphragm compared with that in fasted rats. The overall results indicate that feeding either carbohydrate or fat to fasted rats causes an increase in reutilization of endogenous amino acids and that this may be one of the possible mechanisms underlying the “protein sparing” action of dietary carbohydrate and fat.