Melatonin Targets Mitochondrial Redox Homeostasis: Optimizing the Intracellular Microenvironment.

The discovery of melatonin as a multifunctional free radical scavenger and its possible synthesis in the mitochondrial matrix of peripheral eukaryotic somatic cells highlights a critical new perspective on the importance of this indole. Experimental evidence supporting these findings is substantial, but there are still lingering questions whether melatonin is a direct radical scavenger in vivo and whether it is synthesized in the mitochondrial matrix. We systematically analyze the innovative experimental approaches that support melatonin's radical scavenging actions and assess the compelling data supporting its production in mitochondria. Melatonin concentrations are reportedly higher in this organelle than in other cellular compartments. Proteins for the enzymes required to convert serotonin to melatonin are present in the mitochondrial matrix and purified mitochondria synthesize melatonin. In the mitochondrial matrix, melatonin is likely located within the "damage radius" of highly reactive oxygen species. We also summarize novel actions of melatonin associated with its regulation of membrane fluidity, determine the molecular composition of membrane lipid rafts, and modulate liquid-liquid phase separation and biomolecular condensates intracellularly. If the findings discussed herein continue to be validated, melatonin would be in an optimal position to function as an antioxidant and may be a key driver in the context of preserving mitochondrial redox homeostasis and disease mitigation.