THE MELANOSOME - A DISTINCTIVE SUBCELLULAR PARTICLE OF MAMMALIAN MELANOCYTES AND THE SITE OF MELANOGENESIS

[摘要] Suspensions of large granules from B-16 mouse melanoma and from the retinal pigment-epithelium of the chick embryo have been prepared in isotonic sucrose and centrifuged at high speed over a specific-gravity gradient. The different tractions thus obtained have been examined for suc-cinoxidase, glutamate oxidase and tyrosinase activity and have also been studied under the electron microscope. The granules which have the typical appearance of mitochondria under the electron microscope contain a high concentration of succinic oxidase and glutamate oxidase, both of which are typical mitochondrial enzymes. These granules are found in the gradient tube in a density layer where mitochondria are characteristically found, whatever their biological source. The granules which, under the electron microscope, had the typical appearance of melanin granules have been found to contain the bulk of the tyrosinase activity. The authors conclude that melanin granules and mitochondria are distinct cytoplasmic constituents of the melanin-forming cell and that each of these two constituents is equipped with its own characteristic enzymes. The term melanosome is proposed for the distinctive, enzymically active particle which is the site of melanin formation and is located only within the cytoplasm of the melanocyte. Formation of melanin in a melanocyte is the same process as synthesis of melanin granules.