The Concept and Significance of Labile and Over-all Protein Reserves of the Body

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Abstract
When dogs and rats were placed on a protein-free diet, the urinary excretion of nitrogen per day decreased rapidly during the first few days on the diet. The nitrogen lost during this period has been attributed to the so-called labile protein reserves of the body. After the animals had lost about 8 per cent of their body nitrogen, the decrease in the rate of daily urinary excretion of nitrogen was retarded until the animals lost about 25 per cent of their body nitrogen, after which the daily urinary excretion of nitrogen became relatively constant. Thus endogenous excretion of nitrogen could not be considered a constant until the animals were severely depleted. The rats lost 50 per cent of their body proteins before they succumbed to the rigors of the protein-free diet. Although there was a major loss of protein from the liver and part of the viscera during the first few days of the protein-free diet, the muscles and skin contributed the major part of the nitrogen lost in the urine during this period of protein depletion. It was concluded that the muscles and skin represent the major sources of protein reserves in the body. There was a correlation between the protein: DNA and RNA:DNA ratios of various tissues, which suggested that the decrease in cellular protein was the result of a lowered rate of protein biosynthesis. The change in daily urinary excretion of nitrogen could be explained as an attempt by the body to reach a new steady state for protein metabolism, and therefore the so-called labile protein reserve could be considered as part of the over-all reserves of the body.