Tan Dun-Xian, Manchester Lucien C, Esteban-Zubero Eduardo, Zhou Zhou, Reiter Russel J
Department of Cellular and Structural Biology, Health Science Center, University of Texas, San Antonio, TX 78229, USA.
Molecules. 2015 Oct 16;20(10):18886-906. doi: 10.3390/molecules201018886.
Melatonin is a tryptophan-derived molecule with pleiotropic activities. It is present in almost all or all organisms. Its synthetic pathway depends on the species in which it is measured. For example, the tryptophan to melatonin pathway differs in plants and animals. It is speculated that the melatonin synthetic machinery in eukaryotes was inherited from bacteria as a result of endosymbiosis. However, melatonin's synthetic mechanisms in microorganisms are currently unknown. Melatonin metabolism is highly complex with these enzymatic processes having evolved from cytochrome C. In addition to its enzymatic degradation, melatonin is metabolized via pseudoenzymatic and free radical interactive processes. The metabolic products of these processes overlap and it is often difficult to determine which process is dominant. However, under oxidative stress, the free radical interactive pathway may be featured over the others. Because of the complexity of the melatonin degradative processes, it is expected that additional novel melatonin metabolites will be identified in future investigations. The original and primary function of melatonin in early life forms such as in unicellular organisms was as a free radical scavenger and antioxidant. During evolution, melatonin was selected as a signaling molecule to transduce the environmental photoperiodic information into an endocrine message in multicellular organisms and for other purposes as well. As an antioxidant, melatonin exhibits several unique features which differ from the classic antioxidants. These include its cascade reaction with free radicals and its capacity to be induced under moderate oxidative stress. These features make melatonin a potent endogenously-occurring antioxidant that protects organisms from catastrophic oxidative stress.
褪黑素是一种由色氨酸衍生而来的具有多种活性的分子。它几乎存在于所有或全部生物体内。其合成途径取决于所检测的物种。例如，植物和动物中色氨酸到褪黑素的途径有所不同。据推测，真核生物中的褪黑素合成机制是内共生作用导致从细菌遗传而来的。然而，目前微生物中褪黑素的合成机制尚不清楚。褪黑素代谢高度复杂，这些酶促过程由细胞色素C进化而来。除了酶促降解外，褪黑素还通过假酶和自由基相互作用过程进行代谢。这些过程的代谢产物相互重叠，通常很难确定哪个过程占主导。然而，在氧化应激下，自由基相互作用途径可能比其他途径更突出。由于褪黑素降解过程的复杂性，预计在未来的研究中会鉴定出更多新的褪黑素代谢产物。褪黑素在单细胞生物等早期生命形式中的原始和主要功能是作为自由基清除剂和抗氧化剂。在进化过程中，褪黑素被选为一种信号分子，将环境光周期信息转化为多细胞生物中的内分泌信息，以及用于其他目的。作为一种抗氧化剂，褪黑素具有一些与经典抗氧化剂不同的独特特性。这些特性包括其与自由基的级联反应以及在中度氧化应激下被诱导的能力。这些特性使褪黑素成为一种强大的内源性抗氧化剂，可保护生物体免受灾难性氧化应激的影响。
