Inhibition of hepatocytic protein degradation by methylaminopurines and inhibitors of protein synthesis.

Nutritional control of protein degradation in isolated rat hepatocytes can take place in the absence of protein synthesis. Suppression of degradation by amino acids (step-up) is unaffected and the enhanced degradation seen upon amino acid deprivation (step-down) is only partially inhibited by cycloheximide at a concentration (10(-3) M) which inhibits protein synthesis virtually completely. Protein degradation per se is, however, inhibited by cycloheximide as well as by puromycin, apparently at least in part by mechanisms additional or unrelated to their effect on protein synthesis. Several puromycin analogues (methylaminopurines) are stronger inhibitors of protein degradation than of protein synthesis, most notably puromycin aminonucleoside and 6-dimethylaminopurine riboside (N6,N6-dimethyladenosine). The latter compounds appear to specifically inhibit cellular autophagy, since neither the degradation of endocytosed protein (asialofetuin) nor the extralysosomal (amino acid-, propylamine- and leupeptin-resistant) degradation are affected.