Proteins are oxidatively modified by a large number of reactive species including reactive oxygen species, lipid peroxidation-derived aldehydes, and reducing sugars. Among divergent oxidative modifications, introduction of carbonyl groups such as aldehyde, ketone, and lactam into the amino acid side chains of proteins is a major hallmark for oxidative damage to proteins, and is termed “protein carbonylation”. Detection and quantification of protein carbonyls is commonly performed to determine the level of oxidative stress in the context of cellular damage, aging and several age-related disorders. This review focuses on the molecular mechanisms and biological implications of protein carbonylation, and also presents current analytical approaches for determining and characterizing carbonylated proteins.

Pyrroloquinoline quinone (PQQ), an aromatic tricyclic o-quinone, was identified initially as a redox cofactor for bacterial dehydrogenases. Although PQQ is not biosynthesized in mammals, trace amounts of PQQ have been found in human and rat tissues because of its wide distribution in dietary sources. Importantly, nutritional studies in rodents have revealed that PQQ deficiency exhibits diverse systemic responses, including growth impairment, immune dysfunction, and abnormal reproductive performance. Although PQQ is not currently classified as a vitamin, PQQ has been implicated as an important nutrient in mammals. In recent years, PQQ has been receiving much attention owing to its physiological importance and pharmacological effects. In this article, we review the potential health benefits of PQQ with a focus on its growth-promoting activity, anti-diabetic effect, anti-oxidative action, and neuroprotective function. Additionally, we provide an update of its basic pharmacokinetics and safety information in oral ingestion. Graphical abstract Structure and redox reaction of pyrroloquinoline quinone (PQQ).