Effects of inhibitors of protein degradation on the rate of protein synthesis in Chinese Hamster Ovary cells

In the absence of serum and amino acids, cultured Chinese Hamster Ovary cells released to the medium two thirds of the leucine produced by protein degradation. Because protein synthesis requires all the amino acids, the loss of leucine implies incomplete reincorporation of the other amino acids as well. Leupeptin (0.45 mg/ml) and chloroquine (up to 40 microM) inhibited protein breakdown by 21 and up to 41%, respectively, and resulted in proportional decreases in protein synthesis. Chloroquine abolished the stimulation of protein breakdown by amino acid deprivation. From the values of protein synthesis and leucine output with and without chloroquine, it is estimated that the stimulation of protein degradation not only permitted continuing protein synthesis but also increased amino acid output. In the presence of serum or amino acids protein breakdown was slower than in their absence and less sensitive to inhibition by chloroquine, but proportional effects on synthesis and degradation were still observed. It is suggested that protein degradation may be necessary for the maintenance of optimum intracellular concentrations of amino acids even in the presence of extracellular amino acids.