Subcellular Tyrosinase Activity and Site of Melanogenesis in Melanocytes

Abstract

In mammals, melanin is synthesized in a specialized secretory cell known as the melanocyte. This cell is characterized by the presence of tyrosinase, which is responsible for the aerobic oxidation of L-tyrosine to dopa and to dopaquinone, which in turn is converted to indole-5,6-quinone. This monomer then is oxidized and polymerized to form a large polymer which is believed to be attached, through its quinone linkages, to amino or sulfhydryl groups of the protein matrix of the pigment granules [8, 10]. Recent electron microscopic studies show that the melanin granules appear to start in a region of the melanocyte associated with the Golgi membranes as hollow vesicles in which a tenuous material appears in the form of a folded lamella. This material is rapidly thickened and defined by the deposition of more dense material which is considered to be polymerized indole-5,6quinone, although the nature of this material is not yet thoroughly determined [2,22]. On the other hand, morphological and biochemical studies have revealed that in the secretory cell, the rough membranes and the smooth membranes which are main components of the small granule fraction when it is fractionated by biochemical procedures, were all engaged in the formation of secretory substances and in the secretory mechanisms [23].