Widespread cerebral deposition of a 40- 43-amino acid peptide called the amyloid β-protein (Aβ) in the form of amyloid fibrils is one of the most prominent neuropathologic features of Alzheimer's disease. Numerous studies suggest that Aβ is toxic to neurons by free radical-mediated mechanisms. We have previously reported that melatonin prevents oxidative stress and death of neurons exposed to Aβ. In the process of screening indole compounds for neuroprotection against Aβ, potent neuroprotective properties were uncovered for an endogenous related species, indole-3-propionic acid (IPA). This compound has previously been identified in the plasma and cerebrospinal fluid of humans, but its functions are not known. IPA completely protected primary neurons and neuroblastoma cells against oxidative damage and death caused by exposure to Aβ, by inhibition of superoxide dismutase, or by treatment with hydrogen peroxide. In kinetic competition experiments using free radical- trapping agents, the capacity of IPA to scavenge hydroxyl radicals exceeded that of melatonin, an indoleamine considered to be the most potent naturally occurring scavenger of free radicals. In contrast with other antioxidants, IPA was not converted to reactive intermediates with pro-oxidant activity. These findings may have therapeutic applications in a broad range of clinical situations.
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