Catabolic Rate of Body Protein in Adult Rat over an Entire Period of Protein Depletion

In order to clarify the magnitude of different labile body proteins and the over-all catabolizable body protein, the catabolic rate of total body nitrogen in adult rat was measured by nitrogen balance method up to the time of death due to protein depletion.

The more labile body protein having 83.3% of fractional catabolic rate per day and occupying 2.8% of the whole body protein mainly represented the so-called protein reserves at the beginning of protein depletion. The less labile one, the remainder, namely 97.2%, having 0.79% of fractional catabolic rate almost wholly represented the exponential decrease of body protein during the first 40 days of protein depletion. Urinary and fecal nitrogen in this period showed a similar exponential decrease. In the next 40 days of the depletion, body protein decreased almost linearly giving the constant excretions of urinary and fecal nitrogen. In the last 40 days, it decreased drastically accompanied by a remarkable increase in urinary nitrogen.

After 118 days, on the average, the animals died of protein depletion at 35.7% level of the initial body nitrogen. Contributions of various organs to the total nitrogen deficit up to the time of death, were considerably different in different organs, where muscle was the greatest in total amount but with less catabolizability than the viscera, such as liver, pancreas and spleen.