Alterations in a wide array of physiological functions are a normal consequence of aging. Importantly, aged individuals exhibit an enhanced susceptibility to various degenerative diseases and appear less able than their young and adult counterparts to withstand (patho)physiological stress. Elucidation of mechanisms at play in the aging process would benefit the development of effective strategies for enhancing the quality of life for the elderly. It is likely that decrements in cellular and physiological function that occur during aging are the net result of numerous interacting factors. The current review focuses on the potential contribution(s) of free radical-mediated modifications to protein structure/function and alterations in the activities of two major proteolytic systems within cells, lysosomes and the proteasome, to the age-dependent accumulation of fluorescent intracellular granules, termed lipofuscin. Specifically, aging appears to influence the interplay between the occurrences of free radical-derived modifications to protein and the ability of cells to carry out critical proteolytic functions. We present immunochemical and ultrastructural evidence demonstrating the occurrence of a fluorescent protein cross-link derived from free radical-mediated reaction(s) within lipofuscin granules of rat cerebral cortex neurons. In addition, we provide evidence that a fluorophore-modified protein present in lipofuscin granules is the alpha subunit of F1F0-ATP synthase, a mitochondrial protein. It has previously been shown that protein(s) bearing this particular fluorescent cross-link are resistant to proteolysis and can inhibit the proteasome in a non-competitive fashion (J. Biol. Chem. 269 (1994a) 21639; FEBS Lett. 405 (1997) 21). Therefore, the current findings demonstrate that free radical-mediated modifications to protein(s) that lead to the production of inhibitor(s) of cellular proteolytic systems are present on specific protein components of lipofuscin. In addition, the mitochondrial origin of one of these proteins indicates specific intracellular pathways likely to be influenced by free radical events and participate in the formation of lipofuscin. The results of these studies are related to previous in vitro and in vivo observations in the field, thus shedding light on potential consequences to cellular function. In addition, future research directions suggested by the available evidence are discussed.

中文翻译：

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衰老，脂褐素形成和自由基介导的细胞蛋白水解系统抑制作用。

多种生理功能的改变是衰老的正常结果。重要的是，老年个体对各种退行性疾病表现出更高的易感性，并且与年轻人和成年人相比，承受（病理性）生理压力的能力较弱。阐明衰老过程中起作用的机制将有益于开发有效策略以提高老年人的生活质量。衰老过程中细胞和生理功能的下降很可能是许多相互作用因素的净结果。目前的评论集中在自由基介导的修饰对蛋白质结构/功能的潜在贡献以及溶酶体和蛋白酶体中两个主要蛋白水解系统活性的改变，称为脂褐素的荧光细胞内颗粒的年龄依赖性积累。具体而言，衰老似乎会影响自由基衍生的蛋白质修饰与细胞执行关键蛋白水解功能的能力之间的相互作用。我们目前的免疫化学和超微结构证据表明荧光蛋白交联的发生源自自由基介导的大鼠脑皮层神经元脂褐素颗粒内的自由基反应。此外，我们提供的证据表明，脂褐素颗粒中存在的经过荧光团修饰的蛋白质是F1F0-ATP合酶的线粒体蛋白质的α亚基。先前已显示带有该特定荧光交联的一种或多种蛋白对蛋白水解具有抗性并且可以以非竞争性方式抑制蛋白酶体（J. Biol.Chem.269（1994a）21639; FEBS Lett.405（ 1997）21）。因此，目前的发现表明，在脂褐素的特定蛋白质组分上存在自由基介导的对蛋白质的修饰，其导致细胞蛋白水解系统抑制剂的产生。另外，这些蛋白质之一的线粒体起源表明可能受自由基事件影响并参与脂褐素形成的特定细胞内途径。这些研究的结果与该领域以前的体外和体内观察结果有关，从而揭示了对细胞功能的潜在后果。此外，