CHARACTERIZATION OF A MELANOSOMAL TRANSPORT-SYSTEM IN MURINE MELANOCYTES MEDIATING ENTRY OF THE MELANOGENIC SUBSTRATE TYROSINE

In this study, we identify a transport system for tyrosine, the initia l precursor of melanin synthesis, in the melanosomes of murine melanoc ytes. Melanosomes preloaded with tyrosine demonstrated countertranspor t of 10 mu M [H-3]tyrosine, indicating carrier-mediated transport. Mel anosomal tyrosine transport was saturable, with an apparent K-m for ty rosine transport of 54 mu M and a maximal velocity of 15 pmol of tyros ine/unit of hexosaminidase/min. Transport was temperature-dependent (E (alpha) = 7.5 kcal/mol) and showed stereospecificity for the L-isomer of tyrosine. Aromatic, neutral hydrophobic compounds (such as tryptoph an and phenylalanine), as well as the small, bulky neutral amino acids (such as leucine, isoleucine, and methionine) competed for tyrosine t ransport. Tyrosine transport was inhibited by the classical system L a nalogue, 2-aminobicyclo[2.2.1]heptane-2-carboxylic acid and by monoiod otyrosine, but not by cystine, lysine, glutamic acid, or 2-(methylamin o)isobutyric acid. Tyrosine transport showed no dependence on Na+ or K +, and did not require an acidic environment or the availability of fr ee 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 t o the function of the melanosome since tyrosine is the essential subst rate required for the synthesis of the pigment melanin.