An excess of ROS in the intracellular environment can cause harmful effects on structures such as nucleic acids (DNA and RNA), enzymes, structural proteins, and cell membrane lipids. The quinone–hydroquinone and quinone–semiquinone interconversions, with ROS generation, are responsible for the wide range of biological activities of menadione and its derivatives. Additionally, the menadione semiquinone radical can participate in another redox cycle, such as, the Fenton reaction, also resulting in the production of hydroxyl and hydroperoxyl radicals ( Figure 3). −), hydrogen peroxide (H 2O 2), hydroxyl radical (.In case of naphthoquinones such as menadione, the quinone–semiquinone or quinone–hydroquinone interconversion generates reactive oxygen species (ROS), such as superoxide anion (O 2 In the presence of oxygen, the reduced species is oxidized back to the quinone, thus completing the cycle. The main characteristics of the quinones (Q) redox cycle, comprises the one-electron reduction to generate a semiquinone intermediate (SQ) and the two-electron reduction leading to hydroquinone (HQ), in NAD(P)H oxidase-dependent processes. ![]() The presence of an α,β-unsaturated diketone in the quinone structures allows them to accept electrons through reduction processes, followed by oxidation, thus establishing a redox cycle. In these cases, the redox cycle of menadione, followed by reactive oxygen species (ROS) generation, resulted from the interactions between nucleophilic biomolecules and 1,4-naphthoquinonic nucleus of menadione and its derivatives. The action of menadione in live organisms is not restricted to its use as a biosynthetic precursor to vitamins K 1 and K 2, a variety of studies has shown a wide range of biological activities of menadione, such as anticancer, antibacterial, antifungal, antimalarial, antichagasic, and anthelmintic effects. The MSB favors the formation of prothrombin and speeds up blood coagulation, improving its antihemorrhagic activity when compared to the natural vitamins K. Menadione sodium bisulfite complex (MSB, 13) and menadiol (vitamin K 4, 14), in turn, are two water-soluble derivatives converted in the body, to menadione. Moreover, in adult humans, vitamin K 1 can be converted into vitamin K 2, a process that requires menadione as intermediate. In humans, the menadione–vitamin K 2 conversion occurs after its alkylation in the liver. In animals, menadione can be converted in vitamin K 2 in the intestinal tract, by intestinal microbiota. Vitamins K, obtained through food, play an important role in maintaining animals’ physiology, by acting on blood clotting and regulating bone calcification. Menadione or 2-methyl-1,4-naphthoquinone ( 10), most known as vitamin K 3, is a naphthoquinone derivative exclusively synthetic, not found in nature, used as an important precursor to synthesize vitamins K 1 and K 2, being classified as a provitamin ( Figure 2). ![]() ![]() The naphthoquinone menadione has attracted a lot of attention. With their particular and quite interesting chemical properties and bioactivities, naphthoquinones have aroused great interest, mainly in the pharmaceutical field, where they have been widely used in the development of new and more efficient drugs. Quinones also received considerable attention due to their importance in microbial systems, once several studies have shown that structural variations in microbial quinones have chemotaxonomic significance and can be used in the classification and identification of various microbial species. Actually, quinones play important roles in several physiological processes in these organisms, such as photosynthesis and oxidative phosphorylation, as well as many other metabolic processes. In nature, quinones are biosynthesized as secondary metabolites by various organisms, from simple single-celled microorganisms to more complex beings, such as higher plants and animals. Naphthoquinones belong to the chemical family of quinones and are widely present in synthetic and natural products ( Figure 1).
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