Mortimore G E, Hutson N J
Acta Biol Med Ger. 1981;40(10-11):1577-85.
Long lived, resident proteins comprise more than 99% of the cytoplasmic protein content in rat liver. In perfusion experiments, their overall rate of degradation can be varied from 4.5-5.0%/h with glucagon or stringent amino acid depletion to 1.5%/h with additions of 4-10 times normal plasma amino acids. The aggregate volume of degradative lysosomal components and their content of degradable protein, estimated in homogenate experiments, were found to relate directly to rates of protein breakdown from maximal down to and including the basal state. The rate constant of autophagic vacuole regression, estimated stereologically after stopping their formation with amino acids, was 0.087/min (t1/2 = 8.0 min), and the turnover of cytoplasmic volume during deprivation, calculated from this constant and increases over basal in either newly induced autophagic vacuoles (AVi) or secondary degradative forms, agreed quantitatively with corresponding rates of protein turnover. The fact that the time courses of intralysosomal proteolysis in liver homogenates were identical, despite large differences in the quantity of internalized protein degraded, suggests that the rate constant of intralysosomal digestion in intact hepatocytes is invariant over the full range of regulation. Our findings thus support the hypothesis that all resident proteins are sequestered and degraded by the lysosomal-vacuolar system.
长寿的驻留蛋白占大鼠肝脏细胞质蛋白含量的99%以上。在灌注实验中，它们的总体降解速率可因胰高血糖素或严格的氨基酸耗竭而在4.5 - 5.0%/小时之间变化，而添加4 - 10倍正常血浆氨基酸时，降解速率则为1.5%/小时。在匀浆实验中估计的降解性溶酶体成分的总体积及其可降解蛋白的含量，被发现与从最大到包括基础状态的蛋白质分解速率直接相关。在用氨基酸停止自噬泡形成后，通过立体学方法估计的自噬泡消退速率常数为0.087/分钟（半衰期 = 8.0分钟），根据这个常数以及新诱导的自噬泡（AVi）或二级降解形式中相对于基础水平的增加量计算出的剥夺期间细胞质体积的周转率，在数量上与相应的蛋白质周转率一致。尽管内化降解的蛋白量存在很大差异，但肝脏匀浆中溶酶体内蛋白水解的时间进程是相同的，这一事实表明完整肝细胞中溶酶体内消化的速率常数在整个调节范围内是不变的。因此，我们的发现支持了所有驻留蛋白都被溶酶体 - 液泡系统隔离和降解的假说。
