INTRACELLULAR LOCALIZATION OF TYROSINASE AND SITE OF MELANIN FORMATION IN MELANOCYTE

[摘要] Enzyme activities of fractionated cell components (melanosomes, mitochondria, smooth surface membranes, rough surface membranes and ribosomes) of Harding Passey and B-16 mouse melanomas were studied. Tyrosinase activity was found to be in melanosomes, smooth surface membranes, rough surface membranes and ribosomes. The smooth surface membranes, which is one of the components of small granules, showed significantly high tyrosinase activity. Such a distribution pattern of tyrosinase among cell particles supports the hypothesis proposed previously that in the melanin-forming cell, tyrosinase is synthesized in small granules, which are presumably ribosomes, and subsequently transferred through the endoplasmic reticulum (rough surface membranes) to the Golgi area where it is quantized. Each quantum then probably acquires a membranous envelope (smooth surface membranes) within which the tyrosinase is condensed and placed into or onto the network structure, i. e., melanosome. Incorporation studies of dopa [dihydroxyphenylalanine]-C14 into various cell particles isolated from mouse melanoma were carried out in order to clarify the site of melanin formation in melanocyte. Dopa-C14 incorporation occurred only in melanosomes. If the site of the dopa incorporation can be accepted as the site of melanin formation, it may be concluded that melanin is synthesized only in melanosomes. Although in mammals tyrosinase is generally believed to occur only in the presence of melanin, and vice versa, smooth surface membranes of melanocytes do not appear to incorporate dopa-C14 significantly despite the presence of tyrosinase activity. It is therefore suggested that melanosomes are the specific site of melanin formation in the melanocyte, and that the presence of tyrosinase activity is not necessarily related to the formation of melanin.