THE PREFERENTIAL CATABOLISM OF RECENTLY SYNTHESIZED PROTEIN DURING STARVATION

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Abstract
During conditions of negative N balance, the N15 concentrations of all tissue proteins examined, except those of incisor dentin and hair, failed to increase with fasting time, regardless of whether the initial activity was above or below the "average isotope concentration." Differential centrifugation of livers from starved, cortisone-treated rats revealed that all fractions retained more protein N than did corresponding fractions from starved normal controls. Concomitantly, the N15 concentrations of various corresponding fractions were shown to be essentially the same in both groups of animals. It was therefore concluded that recently deposited liver protein in the cortisone-treated, starved rat was derived from nitrogenous compounds of higher N15 concentrations than that of average total carcass proteins. These and other data were interpreted to signify that recently "synthesized" protein was preferentially catabolized under the conditions of these experiments.