Pyrroloquinoline quinone (henceforth pqq) is a small quinone molecule which has the ability to be a redox agent, capable of reducing oxidants (an antioxidant effect) and then being recycled by glutathione back into an active form. It appears to be quite stable as it can undergo several thousand cycles before being used up, and it is novel since it associates with protein structures inside the cell (some antioxidants, mostly notably carotenoids like β-carotene and Astaxanthin, are located at specific areas of a cell where they exert proportionally more antioxidant effects due to proximity; PQQ seems to do this near proteins like carotenoids do so at the cell membrane).
PQQ is present in nanomolar quantities in most foods and in most animal tissuse. It promotes growth, mitochondrial function, and development in vivo and stimulates cell proliferation and viability in culture. It is a potent antioxidant that scavenges superoxide and peroxinitrite. It is neuroprotectant against 6-hydroxydopamine-induced and hypoxia/ischemia-induced neurotoxicity. PQQ conjugates to α-synuclein in vitro, inhibiting the formation of amyloid fibrils.