The kinetics of disappearance of labeled leucine from the free leucine pool of rat liver and its effect on the apparent turnover of catalase and other hepatic proteins.

The specific radioactivity of free leucine has been measured in rat liver at various times after the injection of labeled leucine. The results of these measurements have served to calculate the effect of leucine reutilization on a previous measurement of the apparent turnover rate of catalase with labeled leucine as precursor. This effect was found to account entirely for the difference between the half-life of catalase observed in these experiments (3.5 days) and that determined with &aminolevulinate as precursor or by indirect methods (1.5 days). It is shown that in general, when the specific radioactivity of proteins is measured between 1 and 10 days after the injection of labeled leucine, all proteins with half-lives between 0.1 and 2 days will show apparent half-lives between 3 and 4 days. The limitations of using leucine for studies on protein turnover are discussed, and some suggestions are made for optimizing the design of such experiments.