Dietary Protein Level and the Turnover Rate of Tissue Proteins in Rats

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Abstract
The effect of varying levels of dietary protein on the turnover rates of the combined proteins of several tissues in rats was studied. The rate of loss of isotope from the proteins of each tissue labeled following injection of DL-methionine-S35 was measured in young rats fed diets containing zero to 60% of casein for 3 weeks. The tissues examined were liver, kidney, plasma, spleen, intestinal mucosa, heart, muscle and testis. The apparent turnover rate of the proteins of each tissue was calculated from the slope of the response curve of the specific activity obtained plotting against the first, fourth and seventh day following injection of labeled methionine. Among these tissue proteins, the turnover rates of intestinal mucosa, plasma, kidney, spleen and liver proteins were affected by the protein level in the diet, and were increased with increments of the protein level. However, no further increase was observed by increasing the protein level above 25 or 40%. The response curves of the changes in turnover rates of these proteins plotted against the protein level were similar to that of body weight plotted in a similar way.