The N-terminus of Histone H3 is proteolytically processed in aged chicken liver. A histone H3 N-terminus specific endopeptidase (named H3ase) has been purified from the nuclear extract of aged chicken liver. By sequencing and a series of biochemical methods including the demonstration of H3ase activity in bacterially expressed GDH, it was established that the H3ase activity was a moonlighting protease activity of glutamate dehydrogenase (GDH). However, the active site for the H3ase in the GDH remains elusive. Here, using cross-linking studies of the homogenously purified H3ase, we show that the GDH and the H3ase remain in the same native state. Further, the H3ase and GDH activities could be uncoupled by partial denaturation of GDH, suggesting strong evidence for the involvement of different active sites for GDH and H3ase activities. Through densitometry of the H3ase clipped H3 products, the H3ase activity was quantified and it was compared with the GDH activity of the chicken liver nuclear GDH. Furthermore, the H3ase mostly remained distributed in the perinuclear area as demonstrated by MNase digestion and immuno-localization of H3ase in chicken liver nuclei, as well as cultured mouse hepatocyte cells, suggesting that H3ase demonstrated regulated access to the chromatin. The present study thus broadly compares the H3ase and GDH activities of the chicken liver GDH.

组蛋白 H3 的 N 末端在老化鸡肝中经过蛋白水解加工。从老化鸡肝的核提取物中纯化出一种组蛋白 H3 N 末端特异性内肽酶（称为 H3ase）。通过测序和一系列生化方法，包括证明细菌表达的GDH中的H3ase活性，确定H3ase活性是谷氨酸脱氢酶（GDH）的兼职蛋白酶活性。然而，GDH 中 H3ase 的活性位点仍然难以捉摸。在这里，通过对同质纯化的 H3ase 进行交联研究，我们发现 GDH 和 H3ase 保持相同的天然状态。此外，H3ase 和 GDH 活性可以通过 GDH 的部分变性来解偶联，这表明有强有力的证据表明 GDH 和 H3ase 活性涉及不同的活性位点。通过对 H3ase 剪切的 H3 产物进行光密度测定，对 H3ase 活性进行定量，并将其与鸡肝核 GDH 的 GDH 活性进行比较。此外，通过 MNase 消化和 H3ase 在鸡肝细胞核以及培养的小鼠肝细胞中的免疫定位证明，H3ase 主要分布在核周区域，这表明 H3ase 对染色质的进入受到调节。因此，本研究广泛比较了鸡肝 GDH 的 H3ase 和 GDH 活性。