Neuroprotection by minocycline due to direct and specific scavenging of peroxynitrite

Minocycline prevents oxidative protein modifications and damage in disease models associated with inflammatory glial activation and oxidative stress. While the drug has been assumed to act by preventing the upregulation of proinflammatory enzymes, we probed here its direct chemical interaction with reactive oxygen species. The antibiotic did not react with superoxide or NO radicals, but peroxynitrite (PON) was scavenged in the range of around 1 μM minocycline and below. The interaction of pharmacologically-relevant minocycline concentrations with PON was corroborated in several assay systems, and significantly exceeded the efficacy of other antibiotics. Minocycline was degraded during the reaction with PON, and the resultant products lacked antioxidant properties. Minocycline’s antioxidant activity extended to cellular systems, as it prevented neuronal mitochondrial DNA damage and glutathione depletion. Maintenance of neuronal viability under PON stress was shown to be solely dependent on direct chemical scavenging by minocycline. We chose alpha-synuclein (ASYN), known from Parkinsonian pathology, as biologicallyrelevant target in chemical and cellular nitration reactions. Submicromolar concentrations of minocycline prevented tyrosine nitration of ASYN by PON. Mass spectrometric analysis revealed that minocycline impeded nitrations more effectively, than methionine oxidations and dimerizations of ASYN, which are secondary reactions under PON stress. Thus, PON scavenging at low concentrations is a novel feature of minocycline and may help to explain its pharmacological activity.