On the measurement of protein turnover in animal cells.

The double isotope technique of Arias, Doyle, and Schimke ((1969) J. Biol. Chem. 244, 3303) with radioactive leucine as precursor has been modified to yield accurate values for rate constants of protein degradation. The advantages of the method are that it is both rapid and reproducible in that the protein whose half-life is being measured is isolated only once from one experimental organism and that the values obtained for the rate constants of degradation are comparable to those measured with [guanidinoJ4C]arginine, and that unlike the latter precursor, radioactive leucine is extensively incorporated into liver protein. The turnover of the population of total soluble proteins and six proteins isolated in homogeneous form from rat liver was studied by means of the double isotope method. The majority of liver proteins are degraded with half-lives between 2 and 8 days. There is a general correlation between subunit size and rate of degradation. The subunits of the multimeric proteins d-aminolevulinate dehydratase, catalase, ferritin, and lactate dehydrogenase are degraded at the same rate as the protein in its quaternary configuration. No intermediate states in the degradation of the subunits were found in the biologically active proteins.