Studies on the Role of Copper in Mammalian Pigmentation : Preliminary Report

Abstract

It has been known since 1931 that copper is an essential dietary factor for maintaining the color of fur in rats (1). It has been assumed that copper acted as a local catalyst for melanin formation (2), but there was no clear-cut experimental evidence in support of this hypothesis. 400 - ©-0 DOPA+17X10~5MCU DOPA+1.8X104MMn DOPA+1.8XiO4Fe DOPA CONTROL 350 - / 300 - / §250 tXJ a: UJ I to - 7 / COLORSME i ® / / 150 MB / 100 j / / 50 • i .i i 60 120 180 240 TIME IN MINUTES Fig. 1 330 To investigate the possible role of copper in the process of pigmentation, various metallic salts were added to buffered dopa solutions. The reaction mixture was kept at 37°C and the darkening of the solutions was measured colorimetrically. The order of catalytic activity of the various heavy metal ions was: Fe < Pb < Mn < Ni < Co < Cu. Zn had very little effect, Mg and Hg none (Fig. 1.). 157 158 THE JOURNAL OF INVESTIGATIVE DERMATOLOGY In another series of experiments copper determinations were carried out on pigmented biological material, such as hair from rabbits, guinea pigs and rats with mottled white and colored spots. The copper content, as determined by the diethyldithiocarbamate method 200 LU S 100 Q 2 8 50 0-O DOPA + BLACK HAIR ASH (71 MG.) ®-© DOPA + WHITE HAIR ASH ( 71 MG. DOPA CONTROL 120 80 TIME IN MINUTES Fig. 2 240 TABLE 1 Copper determinations in Harding-Passey melanomas and in melanin prepared from the tumor WET WEIGHT DRY WEIGHT OF Cu IN /iG/GM DRY Cu IN /iG/GM NO. OF TUMOR TUMOR WATER CONTENT FAT-FREE TUMOR TISSUE MELANIN grams grams per cent 1 7.10 1.44 79.7 11.5 ----- 2 6.75 1.39 79.4 13.5 ----- 3 5.26 - 79.0 12.5 50.4 4 5.20 - - 26.2 337.9 5 4.60 0.937 79.7 15.1 - 6 4.28 0.907 78.8 17.5 163.0 7 2.45 0.460 81.3 22.4 - 8 1.95 0.420 78.5 26.5 - 9 0.52 - - 59.2 294.0 (3), was compared in white and dark hair samples of the same animal. In twelve out of seventeen comparative experiments, black and grey hair contained considerably more copper than white hair from the same animal. The additional copper content in the black or grey hair was of sufficient magnitude to exert a strong catalytic activity on the autoxidation PRELIMINARY AND SHORT REPORTS 159 of dopa. This was shown by dissolving the ash of the same amount of dark and white hair from the same animal in hydrochloric acid; the solution then was added to buffered dopa solutions. Fig. 2 shows a typical experiment in which the ash of black hair catalyzed dopa oxidation to a much higher degree than did the ash of the same amount of white hair from the same animal. No difference was found when the copper content of white and red-brown hair from the same guinea pig was compared. However, red-brown guinea pig hair contained more than twice as much iron as the white hair. This finding is of interest in view of the fact that an iron containing red pigment was isolated from human red hair (4). In isolated human epidermis the copper content was considerably higher than in the corium. There seemed to be no significant difference between the copper content of white and Negro epidermis. Harding-Passey melanomas of mice of the dba strain contained less copper than the kidneys and spleens of the same animals. Larger and older tumors had a lower copper content than those recently implanted. The melanin prepared from the melanomas with boiling NaOH and extensive purification contained four to thirteen times more copper than the tumor tissue from which it originated (Table 1). These findings strongly support the previously advanced theory that copper acts as a local catalyst of pigment formation, probably as part of an oxidative enzyme (5, 6). It is also possible that copper furthers pigmentation indirectly by oxidizing sulfhydryl compounds which inhibit pigmentation in vitro (7) and probably also in vivo (8, 9). Detailed data will be published elsewhere.