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Volume 271, Number 8, Issue of February 23, 1996 pp. 4002-4008
©1996 by The American Society for Biochemistry and Molecular Biology, Inc.
Characterization of a Melanosomal Transport System in Murine Melanocytes Mediating Entry of the Melanogenic Substrate Tyrosine

(Received for publication, August 18, 1995; and in revised form, October 17, 1995)

S. Brian Potterf ,  Jacqueline Muller ,  Isa Bernardini ,  Frank Tietze ,  Takeshi Kobayashi ,  Vincent J. Hearing ,  William A. Gahl

In this study, we identify a transport system for tyrosine, the initial precursor of melanin synthesis, in the melanosomes of murine melanocytes. Melanosomes preloaded with tyrosine demonstrated countertransport of 10 µM [^3H]tyrosine, indicating carrier-mediated transport. Melanosomal tyrosine transport was saturable, with an apparent K for tyrosine transport of 54 µM and a maximal velocity of 15 pmol of tyrosine/unit of hexosaminidase/min. Transport was temperature-dependent (E = 7.5 kcal/mol) and showed stereospecificity for the L-isomer of tyrosine. Aromatic, neutral hydrophobic compounds (such as tryptophan and phenylalanine), as well as the small, bulky neutral amino acids (such as leucine, isoleucine, and methionine) competed for tyrosine transport. Tyrosine transport was inhibited by the classical system L analogue, 2-aminobicyclo[2.2.1]heptane-2-carboxylic acid and by monoiodotyrosine, but not by cystine, lysine, glutamic acid, or 2-(methylamino)-isobutyric acid. Tyrosine transport showed no dependence on Na or K, and did not require an acidic environment or the availability of free thiols. These results demonstrate the existence of a neutral amino acid carrier in murine melanocyte melanosomes which resembles the rat thyroid FRTL-5 lysosomal system h. This transport system is critical to the function of the melanosome since tyrosine is the essential substrate required for the synthesis of the pigment melanin.



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