DHICA

DHICA is the acronym for 5,6-dihydroxyindole-2-carboxylic acid (molecular formula: C9H7NO4). In the absence of thiol compounds, DOPAquinone undergoes intramolecular cyclization and oxidation to form DOPAchrome, which is then converted to 5,6-dihydroxyksyindole (DHI) or 5,6-dihydroxyindole-2-carboxylic acid (DHICA). This synthesis pathway is catalyzed by a multienzyme complex consisting of the tyrosine-related protein(TYRP1), the dopachrome tautomerase (see DCT gene below) and the P protein (see melanosomal transmembrane protein below). The end product of this synthesis pathway is the polymerization product eumelanin (see melanin below).

Melanins are natural pigments of skin, hair and eyes and can be divided into two main types: brown to black eumelanin and yellow to reddish-brown pheomelanin. The biosynthesis of melanins takes place in melanosomes, which are specialized cytoplasmic organelles of melanocytes - dendritic cells located in the basal layer of the epidermis, the choroid of the eye, the hair follicles as well as in the inner ear, the central nervous system and the heart. Melanogenesis is a multi-stage process and begins with the conversion of the amino acid L-tyrosine into DOPA-quinone. Eumelanin is formed by polymerization of DHI and DHICA and their quinones. Melanogenesis is regulated by physical and biochemical factors.

Orlow SJ et al. (1992) Synthesis and characterization of melanins from dihydroxyindole-2-carboxylic acid and dihydroxyindole. Pigment Cell Res 5:113-121.