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Excess activation of glutamate receptors and production of free radicals including nitric oxide may result in severe and irreversible damage to the mammalian central nervous system (CNS), but endogenous defense systems that protect neurons from these insults are poorly understood. Here, we purified and isolated a neuroprotective substance, which has been named ''serofendic acid,'' from a lipophilic fraction of FCS based on the ability to protect rat primary cortical neurons against nitric oxide cytotoxicity. Mass spectrometry and NMR spectroscopy revealed the chemical structure of serofendic acid (15-hydroxy-17-methylsulfinylatisan-19-oic acid) as a sulfur-containing atisane-type diterpenoid, which is unique among known endogenous substances. Synthetic serofendic acid exhibited potent protective actions on cortical neurons against cytotoxicity of a nitric oxide donor as well as of glutamate, although it did not show appreciable influences on glutamate receptor-mediated responses in these neurons. Electron spin resonance analysis demonstrated that serofendic acid had no direct scavenging activity on nitric oxide radicals but was capable of inhibiting the generation of hydroxyl radical, a presumed ''executor'' radical in the nitric oxide-mediated neurotoxic cascade. These findings suggest that serofendic acid is a low-molecularweight bioactive factor that promotes survival of CNS neurons, probably through the attenuation of free radical-mediated insults. G lutamate, a major excitatory neurotransmitter in the central nervous system (CNS), is well known to exert detrimental effects on CNS neurons under pathophysiological conditions. Glutamate neurotoxicity is associated with various neurological disorders, including hypoxic-ischemic brain injury (1, 2), Alzheimer's disease (3, 4), Huntington's disease, and Parkinson's disease (5). CNS neurons are also vulnerable to insults caused by reactive oxygen species (ROS) and nitric oxide (NO), and these radicals are considered to play crucial roles in glutamate neurotoxicity associated with ischemic brain injury and a wide range of neurodegenerative disorders (6-9). In cultured cortical neurons, glutamate induces Ca 2ϩ inf lux via the N-methyl-Daspartate (NMDA) subtype of glutamate receptor channels. Elevation of intracellular Ca 2ϩ triggers the formation of NO in NO synthase (NOS)-containing neurons. NO then diffuses out of NOS-containing cells and reaches the surrounding neurons, where it exerts potent cytotoxic actions with the aid of ROS (10-13). In fact, either pharmacological inhibition of NOS activity or scavenging of NO markedly attenuates glutamateinduced neurotoxicity (14, 15), suggesting that NO is a key molecule in NMDA receptor-mediated glutamate neurotoxicity in the cerebral cortex.Considering the crucial roles of glutamate as a major neurotransmitter despite its robust neurotoxic actions, one would assume that various kinds of regulatory mechanisms in the CNS might serve to maintain physiological functions and prevent the pathogenic effects of glutamate. In th...