Hutson N J, Mortimore G E
J Biol Chem. 1982 Aug 25;257(16):9548-54.
The suppression of proteolysis that normally accompanies cytoplasmic growth was investigated in livers of mice that had lost approximately 40% of their protein content during 48 h of starvation. The deficit was fully restored after 24 h of refeeding, and the net regain was linear between 12 and 24 h. Rates of protein breakdown were determined from (a) differences between synthesis and the net change in total liver protein, and (b) rates of valine release during 15-min in situ perfusions in the presence of cycloheximide. With appropriate correction for the turnover of a short lived pool, both procedures gave the same results; rates at 12 and 24 h of refeeding were decreased 90% over values in fed controls, an effect which accounted for 93% of protein regrowth. Measurements of degradable, intralysosomal protein revealed that sequestration of cytoplasmic protein by lysosomes was correspondingly decreased. Because the ratio of intracellular proteolysis to internalized protein was the same during refeeding as in earlier experiments where autophagy was the dominant process, the uptake of cytoplasmic proteins by lysosomes appears to be an obligatory step in proteolysis at all levels of regulation. The 20-fold range in rates of degradation exhibited by the mouse hepatocyte thus provides this cell with an unusual capability for regulating its protein content against relatively small changes in protein synthesis.
在饥饿48小时期间蛋白质含量损失约40%的小鼠肝脏中，研究了通常伴随细胞质生长的蛋白水解抑制作用。再喂食24小时后，蛋白质缺乏完全恢复，净恢复量在12至24小时之间呈线性关系。蛋白质分解速率通过以下两种方法确定：（a）合成与肝脏总蛋白净变化之间的差异，以及（b）在存在放线菌酮的情况下，原位灌注15分钟期间缬氨酸释放速率。对短命池的周转进行适当校正后，两种方法得出相同结果；再喂食12小时和24小时时的速率比喂食对照降低了90%，这一效应占蛋白质再生的93%。对可降解的溶酶体内蛋白的测量表明，溶酶体对细胞质蛋白的隔离相应减少。由于再喂食期间细胞内蛋白水解与内化蛋白的比例与早期以自噬为主导过程的实验相同，因此溶酶体对细胞质蛋白的摄取似乎是各级调控蛋白水解过程中的一个必要步骤。因此，小鼠肝细胞所表现出的20倍降解速率范围为该细胞提供了一种不同寻常的能力，可根据蛋白质合成中相对较小的变化来调节其蛋白质含量。
