Tyrosinase-Catalyzed Oxidation of the Leukoderma-Inducing Agent Raspberry Ketone Produces (<i>E</i>)‑4-(3-Oxo-1-butenyl)-1,2-benzoquinone: Implications for Melanocyte Toxicity

The exposure of human skin to 4-(4-hydroxyphenyl)-2-butanone (raspberry ketone, RK) is known to cause chemical/occupational leukoderma. RK has a structure closely related to 4-(4-hydroxyphenyl)-2-butanol (rhododendrol), a skin whitening agent that was found to cause leukoderma in the skin of consumers in 2013. Rhododendrol is a good substrate for tyrosinase and causes a tyrosinase-dependent cytotoxicity to melanocytes, cells that are responsible for skin pigmentation. Therefore, it is expected that RK exerts its cytotoxicity to melanocytes through the tyrosinase-catalyzed oxidation to cytotoxic <i>o</i>-quinones. The results of this study demonstrate that the oxidation of RK by mushroom tyrosinase rapidly produces 4-(3-oxobutyl)-1,2-benzoquinone (RK-quinone), which is converted within 10–20 min to (<i>E</i>)-4-(3-oxo-1-butenyl)-1,2-benzoquinone (DBL-quinone). These quinones were identified as their corresponding catechols after reduction by ascorbic acid. RK-quinone and DBL-quinone quantitatively bind to the small thiol <i>N</i>-acetyl-l-cysteine to form thiol adducts and can also bind to the thiol protein bovine serum albumin through its cysteinyl residue. DBL-quinone is more reactive than RK-quinone, as judged by their half-lives (6.2 min vs 10.5 min, respectively), and decays rapidly to form an oligomeric pigment (RK-oligomer). The RK-oligomer can oxidize GSH to GSSG with a concomitant production of hydrogen peroxide, indicating its pro-oxidant activity, similar to that of the RD-oligomer. These results suggest that RK is cytotoxic to melanocytes through the binding of RK-derived quinones to thiol proteins and the pro-oxidant activity of the RK-oligomer

Eumelanin/Pheomelanin ratio of <i>A. cahirinus</i> from “Evolution Canyon” I, Israel.

<p>(A) in the opposite slopes, (B) across stations. Means with the same letter are not significantly different according to Waller-Duncan k-ratio t-test.</p

Contents of eumelanin and pheomelanin in hairs of spiny mice (<i>Acomys cahirinus</i>) from “Evolution Canyon” I, Israel.

<p>(A) in the opposite slopes, (B) across stations. Means with the same letter are not significantly different according to Waller-Duncan k-ratio t-test.</p

Habitat divergence in “Evolution Canyon”.

<p>(A) photo showing part of the “African” slope (AS)/south-facing slope (SFS) of EC, characterized by light terra rossa soil with a stretch of grass cover that generates a yellow-brownish background. (B) Photo of part of the “European” slope (ES)/north-facing slope (NFS), characterized by dark terra rossa soil and shady, humus-laden dark background.</p

“Evolution Canyon” (EC), Nahal Oren, Israel.

<p>(A) aerial view and (B) cross-section showing the opposing slopes. Note the two habitat types; dry, open habitat (with light colored background) on the “African” slope and humid, closed habitat (with dark-colored background) in the “European” slope.</p

Pelage color variation between sexes (species and populations' pooled data).

<p>Pelage color variation between sexes (species and populations' pooled data).</p

Adaptation of Pelage Color and Pigment Variations in Israeli Subterranean Blind Mole Rats, <i>Spalax Ehrenbergi</i>

<div><p>Background</p><p>Concealing coloration in rodents is well established. However, only a few studies examined how soil color, pelage color, hair-melanin content, and genetics (i.e., the causal chain) synergize to configure it. This study investigates the causal chain of dorsal coloration in Israeli subterranean blind mole rats, <i>Spalax ehrenbergi</i>.</p><p>Methods</p><p>We examined pelage coloration of 128 adult animals from 11 populations belonging to four species of <i>Spalax ehrenbergi</i> superspecies (<i>Spalax galili, Spalax golani, Spalax carmeli</i>, and <i>Spalax judaei</i>) and the corresponding coloration of soil samples from the collection sites using a digital colorimeter. Additionally, we quantified hair-melanin contents of 67 animals using HPLC and sequenced the <i>MC1R</i> gene in 68 individuals from all four mole rat species.</p><p>Results</p><p>Due to high variability of soil colors, the correlation between soil and pelage color coordinates was weak and significant only between soil hue and pelage lightness. Multiple stepwise forward regression revealed that soil lightness was significantly associated with all pelage color variables. Pelage color lightness among the four species increased with the higher southward aridity in accordance to Gloger's rule (darker in humid habitats and lighter in arid habitats). Darker and lighter pelage colors are associated with darker basalt and terra rossa, and lighter rendzina soils, respectively. Despite soil lightness varying significantly, pelage lightness and eumelanin converged among populations living in similar soil types. Partial sequencing of the <i>MC1R</i> gene identified three allelic variants, two of which were predominant in northern species (<i>S. galili</i> and <i>S. golani</i>), and the third was exclusive to southern species (<i>S. carmeli</i> and <i>S. judaei</i>), which might have caused the differences found in pheomelanin/eumelanin ratio.</p><p>Conclusion/Significance</p><p>Darker dorsal pelage in darker basalt and terra rossa soils in the north and lighter pelage in rendzina and loess soils in the south reflect the combined results of crypsis and thermoregulatory function following Gloger's rule.</p></div

Patterns of variation across species of mole rats in Israel.

<p>A) genetic variation, B) phenotypic variation, and C) environmental variation.</p

Neuromelanins of Human Brain Have Soluble and Insoluble Components with Dolichols Attached to the Melanic Structure

<div><p>Neuromelanins (NMs) are neuronal pigments of melanic-lipidic type which accumulate during aging. They are involved in protective and degenerative mechanisms depending on the cellular context, however their structures are still poorly understood. NMs from nine human brain areas were analyzed in detail. Elemental analysis led to identification of three types of NM, while infrared spectroscopy showed that NMs from neurons of substantia nigra and locus coeruleus, which selectively degenerate in Parkinson’s disease, have similar structure but different from NMs from brain regions not targeted by the disease. Synthetic melanins containing Fe and bovine serum albumin were prepared to model the natural product and help clarifying the structure of NMs. Extensive nuclear magnetic resonance spectroscopy studies showed the presence of dolichols both in the soluble and insoluble parts of NM. Diffusion measurements demonstrated that the dimethyl sulfoxide soluble components consist of oligomeric precursors with MWs in the range 1.4–52 kDa, while the insoluble part contains polymers of larger size but with a similar composition. These data suggest that the selective vulnerability of neurons of substantia nigra and locus coeruleus in Parkinson’s disease might depend on the structure of the pigment. Moreover, they allow to propose a pathway for NM biosynthesis in human brain.</p> </div

Pelage color variation among populations living in different soil types.

<p>Pelage color variation among populations living in different soil types.</p