Lebherz H G
J Biol Chem. 1975 Sep 25;250(18):7388-91.
The results of a double isotope experiment using 3H- and 14C-labeled leucine as precursors of protein synthesis demonstrated that the aldolase C to A subunit transition which is associated with chick skeletal muscle development involves the preferential synthesis of different aldolase isoenzymes. This developmental system was used to test for subunit exchange between aldolase tetramers in vivo. In a second double isotope experiment, it was found that the 14C:3H ratios of A and C subunits derived from the same heterotetramer were essentially identical, while the isotope ratios of the same subunit type derived from different isoenzymes were considerably different. Had subunit exchange between the isoenzymes occurred, A subunits of a given heterotetramer would have been expected to have higher isotope ratios than the corresponding C subunits. Therefore, these data suggest that subunit exchange between aldolase tetramers does not occur in vivo, at least not in skeletal muscle to an appreciable extent. The results of the present study suggest that all aldolase tetramers are constructed at the time of the initial assembly of newly synthesized subunits, that is, "new" tetramers would not be generated by subunit exchange between already constructed tetramers. In addition, the present work suggests that the degradation of all four subunits of an aldolase tetramer are coupled inasmuch as the subunits would not be reincorporated into other tetramers. Thus, in contrast to some other proteins, it appears that the subunits of the aldolase tetramer turn over coordinately.
一项使用³H和¹⁴C标记的亮氨酸作为蛋白质合成前体的双同位素实验结果表明，与鸡骨骼肌发育相关的醛缩酶C到A亚基的转变涉及不同醛缩酶同工酶的优先合成。该发育系统用于测试体内醛缩酶四聚体之间的亚基交换。在第二项双同位素实验中，发现来自同一异源四聚体的A和C亚基的¹⁴C:³H比值基本相同，而来自不同同工酶的相同亚基类型的同位素比值则有很大差异。如果同工酶之间发生了亚基交换，那么给定异源四聚体的A亚基的同位素比值应该比相应的C亚基更高。因此，这些数据表明醛缩酶四聚体之间在体内不会发生亚基交换，至少在骨骼肌中不会达到可观的程度。本研究结果表明，所有醛缩酶四聚体都是在新合成亚基的初始组装时构建的，也就是说，“新”的四聚体不会通过已构建的四聚体之间的亚基交换产生。此外，目前的研究表明，醛缩酶四聚体的所有四个亚基的降解是耦合的，因为这些亚基不会重新掺入其他四聚体中。因此，与其他一些蛋白质不同，醛缩酶四聚体的亚基似乎是协同周转的。
