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. 2022 Nov;35(6):622-626.
doi: 10.1111/pcmr.13062. Epub 2022 Aug 26.

5,6-Dihydroxyindole eumelanin content in human skin with varying degrees of constitutive pigmentation

Sandra Del Bino  1 , Shosuke Ito  2 , Juliette Sok  1 , Kazumasa Wakamatsu  2
Affiliations

5,6-Dihydroxyindole eumelanin content in human skin with varying degrees of constitutive pigmentation

Sandra Del Bino et al. Pigment Cell Melanoma Res. 2022 Nov.

Abstract

Human skin contains two distinct components: brown to black, insoluble eumelanin and light colored, alkaline-soluble pheomelanin. Eumelanin consists of 5,6-dihydroxyindole (DHI) and 5,6-dihydroxyindole-2-carboxylic acid (DHICA) moieties, while pheomelanin consists of benzothiazine (BT) and benzothiazole (BZ) moieties. These melanin monomer units can be quantitatively analyzed through specific degradation products by high-performance liquid chromatography (HPLC). Alkaline hydrogen peroxide oxidation (AHPO) of eumelanin gives rise to pyrrole-2,3,5-tricarboxylic acid (PTCA) and pyrrole-2,3-dicarboxylic acid (PDCA) as specific degradation products of the DHICA and DHI moieties, respectively. BZ moiety in pheomelanin can be analyzed as thiazole-2,4,5-tricarboxylic acid (TTCA). By reductive hydrolysis with hydroiodic acid, BT moieties in pheomelanin can be analyzed as 4-amino-3-hydroxyphenylalanine (4-AHP). As a recently improved AHPO-HPLC method enabled a better characterization of PDCA, this prompted us to address the question of DHI to DHICA ratio in human skin samples with varying degrees of constitutive pigmentation ranging from very light to dark. Results showed for the first time the ratio of 4 moieties: DHI 35%, DHICA 41%, BZ 20%, and BT 4%. The ratio is constant regardless of the degree of pigmentation. The high content of DHICA moiety may impart an antioxidant property to the epidermis melanin.

Keywords: 5,6-dihydroxyindole; 5,6-dihydroxyindole-2-carboxylic acid; eumelanin; pheomelanin; skin.

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Conflict of interest statement

SDB and JS are full time employees of L'Oreal, Research and Innovation, Aulnay, France. SI has a consulting contract with L'Oreal, Research and Innovation. KW declares no conflict of interest.

Figures

FIGURE 1
FIGURE 1
Alkaline hydrogen peroxide oxidation (AHPO) and hydroiodic acid (HI) hydrolysis of eumelanin and pheomelanin. AHPO of the 5,6‐dihydroxyindole (DHI) moiety gives pyrrole‐2,3‐dicarboxylic acid (PDCA) and pyrrole‐2,3,5‐tricarboxylic acid (PTCA), while 5,6‐dihydroxyindole‐2‐carboxylic acid (DHICA) moiety gives PTCA. The benzothiazole (BZ) moiety of pheomelanin gives thiazole‐2,4,5‐tricarboxylic acid (TTCA) and thiazole‐4,5‐dicarboxylic acid (TDCA). Upon HI hydrolysis, the benzothiazine (BT) moiety gives 4‐amino‐3‐hydroxyphenylalanine (4‐AHP) and 3‐amino‐4‐hydroxyphenylalanine (3‐AHP). Taken from Ito et al. (2020).
FIGURE 2
FIGURE 2
DHICA % from PDCA/PTCA ratio versus DHICA % from PTCA/A500 ratio (a). Total melanin content by spectrophotometry versus total melanin content by HPLC (b). p values were obtained with JMP 10 software (SAS Institute Inc.) to analyze bivariate fit. When the outliner (72% versus 29%) in (a) was omitted, the correlation became much better with R 2 = .5253 and p = .008.
FIGURE 3
FIGURE 3
Melanin content in the different skin color groups. Contents (μg/mg) of DHI eumelanin (DHI‐EM), DHICA eumelanin (DHICA‐EM), benzothiazole‐pheomelanin (BZ‐PM) benzothiazine‐pheomelanin (BT‐PM) in the epidermis of very light to dark skin (a). Contents (% composition) in DHI‐EM, DHICA‐EM, BZ‐PM, and BT‐PM (b).
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