Among the numerous pigments found in animals, mammalian melanins are a unique group of natural biopolymers. Melanin is a pigment synthesized and stored in specialized lysosome-related organelles - melanosomes, located internally in melanocytes (d'Ischia et al., 2015; Maranduca et al., 2019; Hida et al., 2020). In animals, the process of melanin synthesis, called melanogenesis, is a complex biosynthetic pathway of tyrosine oxidation and polymerization (Le et al., 2021; Videira et al., 2013). The main enzyme of this multistep process is tyrosinase. Melanins not only determine the colour of skin, hair and iris, can but also protect against ultraviolet (UV) radiation and toxic compounds by reducing and neutralizing reactive oxygen species (ROS). Melanins have also been shown to contain free radicals, which are involved in binding of many chemicals and medicinal compounds to the pigment (Raghunath et al., 2015; Delijewski et al., 2014; Mostert, 2021).

Free radicals make melanin paramagnetic, allowing it to be studied using magnetic field and microwaves (Sealy et al., 1982; Pukalski et al., 2020; Al Khatib et al., 2018, Al Khatib et al., 2020; Chodurek and Pilawa, 2022). Electron paramagnetic resonance spectroscopy (EPR) research has proven occurrence of o-semiquinone free radicals in melanin (Sealy et al., 1982; Chodurek and Pilawa, 2022; Zdybel et al., 2012). Concentration of free radicals depends on the type of natural melanin (Chodurek and Pilawa, 2022; Zdybel et al., 2017; Płonka and Rakoczy, 1997; Suwannarach et al., 2019; Melo et al., 2022) and it depends on the type of ligands contained in this polimer (Chodurek and Pilawa, 2022; Felix et al., 1978; Sarna et al., 2022; Żądło and Sarna, 2019; Zdybel et al., 2011; Buszman et al., 2005a, Buszman et al., 2005b; Beberok et al., 2014).

Streptomycin (STR) was one of the first drugs with well-established antituberculosis activity. As a water-soluble and broad-spectrum aminoglycoside antibiotic, STR inhibits protein synthesis through direct and tight interaction with 16S rRNA in the 30S ribosomal subunit. It is effective against gram-negative and gram-positive bacteria (Quinn et al., 2020; Mitic et al., 2006; Baran et al., 2023). Aminoglycosides have been found to cause ototoxicity and nephrotoxicity, in which free radicals play a major role (Le et al., 2023; Selimoğlu et al., 2003). Unlike nephrotoxic side effects, which are usually temporary, ototoxic side effects within the inner ear are irreversible. Aminoglycoside antibiotics can be both cochleotoxic and vestibulotoxic. Studies have confirmed that streptomycin predominantly damages vestibular hair cells (DiSogra, 2019; Reiter et al., 2011; Wrześniok et al., 2013a). In case of aminoglycosides ototoxicity, various ROS and other free radicals, formed in Fenton reaction, were identified in the vestibular organ (DiSogra, 2019; Selimoglu, 2007).

Previously, we have established that streptomycin, dihydrostreptomycin, amikacin, tobramycin, kanamycin bind to melanin in vitro (Wrześniok et al., 1999; Wrześniok et al., 2012), which may result in the accumulation of these medicines in melanin-containing tissues. Melanocytes are present in various parts of the inner ear, including the cochlea and vestibular organs (van Beelen et al., 2020). Therefore, disruption of homeostasis of these cells may be one of the mechanisms of aminoglycosides ototoxicity. The present study aimed to assess the effect of streptomycin on antioxidant status and melanogenesis in normal human melanocytes both darkly- and lightly-pigmented. In addition, the variation of free radical concentrations in streptomycin-treated melanocytes was examined.