Age‐dependent changes in eumelanin composition in hairs of various ethnic origins

Abstract

SynopsisHair pigmentation is one of the most conspicuous phenotypes of humans. From a chemical point of view, however, data remain scarce regarding human hair pigmentation characteristics. To determine melanin content and composition in human eumelanic hair from individuals of different ethnic origins and at different ages, we collected hair from 56 subjects with eumelanic hair from each group of African-American, East Asian, and Caucasian origin. The 56 subjects consist of 14, seven each of males and females, each from four age classes of younger than 11, between 12 and 19, between 20 and 45, and older than 46. We analysed hair colour scale, total melanin value, and contents of pyrrole-2,3,5-tricarboxylic acid (PTCA) and pyrrole-2,3-dicarboxylic acid (PDCA). We measured age-dependent increases in the relative quantity of eumelanin in pigmented human hairs in the three ethnic groups. Regarding melanin composition, we observed an increase in the PDCA/PTCA ratio with age in African-American and Caucasian hairs until approaching the quite constant level of the ratio in East Asian hairs in the elderly individuals. Our results evidence differences in the content and composition of eumelanin in human hair among African-American, Caucasian and East Asian individuals. Furthermore, we show evidence of age-dependent changes in the quantity and quality of eumelanin in pigmented human hairs. In particular, the age-dependent modification of the PDCA/PTCA ratio, a marker for 5,6-dihydroxyindole units in eumelanin, suggests a chronological evolution of hair follicle melanocyte phenotype (e.g. decrease in dopachrome tautomerase expression).

Colorful DNA polymorphisms in humans

“…DCT expression has been proposed to affect age-dependent changes in eumelanin composition in hairs between African, European, and Asian populations [130]. Lao et al [37] showed a haplotype of DCT locus being highly frequent in Han Chinese (75%) but absent in the European and the African populations, suggestive for positive selection on DCT in East Asians as also confirmed by later studies [131,132].…”

Human pigmentation genes under environmental selection

“…This was determined by direct measurement of melanin content and type of eumelanin in a sample of African-American, East Asian and Caucasian individuals [85]. African and European hairs were found to have changes in the ratio of dihydroxyindole (DHI) to 5,6-dihydroxyindole-2-carboxylic acid (DHICA), favoring DHI more as people age.…”

“…African and European hairs were found to have changes in the ratio of dihydroxyindole (DHI) to 5,6-dihydroxyindole-2-carboxylic acid (DHICA), favoring DHI more as people age. However, the Asian samples seemed to have low DHICA levels throughout life, and did not have graying to the extent of other populations [85]. Since a major role of dopachrome tautomerase (DCT) is the isomerization of dopachrome to DHICA, it is plausible that these ethnic differences reflect differences in DCT expression.…”

Sexual Dimorphism in Melanin Pigmentation, Feather Coloration and Its Heritability in the Barn Swallow (Hirundo rustica)

“…Thus, melanic hues may result from a combination of variation in absolute and relative concentration of either melanin [26]–[28]. Moreover, ontogenetic variation can occur, with an age-related change in absolute as well as relative production of either melanin form [29], [30].…”