J. Soc. Cosmetic Chemists, 21, 77-83 (Feb. 4, 1970) The Red Pigmentary System and Its Relation to Black Melanogenesis* PETER FLESCH, M.D., Ph.D.* Synopsis--Identification of the chromophores of PIGMENTS Of red hair and feathers as derived from two molecules each of cysteine and dopa explains many unique properties of the red PHEOMELANOPROTEINS: their easy separation into protein and nonprotein parts the loose, patterned electron microscopic appearance of the "red melanosomes" and the tyrosinase- and dopa-posivity of the "red melanocytes." As the initial step in the synthesis of black and red pigments (tyrosine --• dopa) is the same, it is postulated that pigment formation may be switched from black to red or red to black. An experimental switch mechanism of this type may be produced by converting red rabbit hair to black by local irritation from rubbing physiologic switches occur in agouti animals (hamster, rat, mouse). Further evidence is presented in support of the theory that iron is an essential part of red pheomelanoprotcins and their structure is probably: (cysteinyldopa) 2-derivative-Fe (III)- protein. INTRODUCTION The past three years have greatly expanded our knowledge of the red pigments of hair and feathers. At present, their chemical composi- tion is better known than that of the black melanins and great advances have been made in clarifying the pathways leading to their formation. This publication summarizes these recent developments and discusses the interrelation between black and red pigmentary biosynthetic processes. CHEMISTRY Because of their amphoteric nature and reactivity with hot acids, the red pigments may be isolated in two forms: (a) With weak acids or alkalies at room temperature we may obtain the orange-colored proto- pigments (PP) which have no characteristic absorption bands in the visi- ble part of the spectrum and do not display indicator properties (b) Hot acids extract the characteristic purplish-pink compounds (trichosiderin or feather siderin) with absorption bands in an acid medium--maximum * This investigation was supported by Public Health Service Grant No. 1 RO1 AM 13141 from the Division of Research Grants, National Institutes of Health, Bethesda, Md. * Deceased July 1, 1969. Study conducted at Department of Dermatology, University of Pennsylvania, School of Medicine, Philadelphia, Pa. 19104. 77

78 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS at about 535 m/• (1). The siderins have indicator properties (red to yellow at pH 2). The siderins also may be obtained by heating PP with acids (2), which causes decarboxylation of the pigment portion of the molecule (3). Like all melanocyte-produced pigments, the red pigments consist ot• a protein attached to a nonprotein pigment molecule (chromophore or pyrrotrichol) (4). However, in contrast to the black melanoproteins, the link between pigment and protein is loose and hence the chromophores can be separated by splitting the crude siderins with iron-binding anions (e.g., thiocyanate) followed by dialysis and gel filtration (2), or by a combination of gel filtration with passage through ion-exchange resins. Using the latter method, several chemists (3, 5) isolated a relatively pure chromophore that contained sulfur. They suggested that the chro- mophore may result from a combination of dopa with cysteine (6). When they reacted dopa with cysteine in the presence of an oxidizing agent, a mixture was obtained from which a compound could be isolated which had the same IRS as the chromophore. It was concluded that cysteinyldopa was a precursor of the red pigments (7). After prelimi- nary studies with a model system, Prota et al. fully elucidated the reactions that take place when cysteine combines with dopa (8), and identified the red pigment chromophore as derived from two molecules each of cysteine and dopa (3) (Fig. 1). OH OH NH2- •CH- C H• / •'"•//•"S '/' •S '/•"'•'/"•CHz-•CH-NH• COOH cOOH Figure 1. Structure of chromophore of red pigment (3) This structure does not account for a number of closely related chro- mophore derivatives which can be extracted from hair and feathers (4, 6). Nevertheless, clarification of the basic chemistry of the red pigments has, thrown light on many aspects of red pigmentation which will be dis- cussed below. I,ess is known about the two other components of red pigments: pro- tein and ferric ion. Proteins isolated from different species had a re- markably similar amino acid composition. A minimum molecular weight of 6700 to 8300 was suggested (2). In contrast to the black melanoproteins (9, 10), neither the proteins isolated by us nor the pro- teins analyzed by others (11) contained cystine or dopa. These amino