Protein turnover during metabolic arrest in turtle hepatocytes: role and energy dependence of proteolysis

Hepatocytes from the western painted turtle (Chrysemys picta bellii) are capable of a coordinated metabolic suppression of 88% during 10 h of anoxia at 25°C. The energy dependence and role of proteolysis in this suppression were assessed in labile ([³H]Phe-labeled) and stable ([¹⁴C]Phe-labeled) protein pools. During anoxia, labile protein half-lives increased from 24.7 ± 3.3 to 34.4 ± 3.7 h, with stable protein half-lives increasing from 55.6 ± 3.4 to 109.6 ± 7.4 h. The total anoxic mean proteolytic suppression for both pools was 36%. On the basis of inhibition of O₂ consumption and lactate production rates by cycloheximide and emetine, normoxic ATP-dependent proteolysis required 11.1 ± 1.7 μmol ATP · g^-1 · h^-1 accounting for 21.8 ± 1.4% of total cellular metabolism. Under anoxia this was suppressed by 93% to 0.73 ± 0.43 μmol ATP · g^-1 · h^-1. Summation of this with protein synthesis ATP turnover rates indicated that under anoxia 45% of total ATP turnover rate was directed toward protein turnover. Studies with inhibitors of energy metabolism indicated that the majority of energy dependence was found in the stable protein pool, with no significant inhibition occurring among the more labile proteins. We conclude that proteolysis is largely energy dependent under normoxia, whereas under anoxia there is a shift to a slower overall proteolytic rate that is largely energy independent and represents loss mostly from the labile protein pool.