THE SITE OF BIOSYNTHESIS OF MAMMALIAN TYROSINASE

[摘要] The various components of a homogenate of B-16 mouse melanoma have been isolated by centrifugation, examined individually for tyrosinase activity, RNA, phospholipid and protein-nitrogen content and studied under the electron microscope. A majority of the tyrosinase activity present in the original homogenate was found in the ''large-granule fraction, but a certain amount of tyrosinase activity was also regularly present in the small-granule fraction. One type of particle isolated from the small-granule fraction by highspeed centrifugation over a density gradient resembled ribosomes under the electron microscope; it also contained a high concentration of RNA and phospholipid as well as tyrosinase activity. The rate of in vivo incorporation of leucine-C14 into the soluble tyrosinase isolated from both the small-granule fraction and the large-granule fraction has also been determined. The pattern of incorporation of leucine-C14 by the soluble tyrosinase of these two fractions is adequate proof of their precursor-end-product relationship. The data obtained in this study of the morphology and biochemistry of cell components and the in vivo incorporation of leucine-C14 by soluble tyrosinase are all compatible with the hypothesis that tyrosinase is synthesized in small granules, which are presumably ribosomes, and subsequently transferred to and stored in melanosomes.