Potterf S B, Muller J, Bernardini I, Tietze F, Kobayashi T, Hearing V J, Gahl W A
Laboratory of Cell Biology, NCI, National Institutes of Health, Bethesda, Maryland 20892, USA.
J Biol Chem. 1996 Feb 23;271(8):4002-8. doi: 10.1074/jbc.271.8.4002.
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 microM [3H]tyrosine, indicating carrier-mediated transport. Melanosomal tyrosine transport was saturable, with an apparent Km for tyrosine transport of 54 microM and a maximal velocity of 15 pmol of tyrosine/unit of hexosaminidase/min. Transport was temperature-dependent (Ea = 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.
在本研究中，我们在小鼠黑素细胞的黑素小体中鉴定出一种用于酪氨酸（黑色素合成的初始前体）的转运系统。预加载酪氨酸的黑素小体表现出对10微摩尔[³H]酪氨酸的反向转运，表明存在载体介导的转运。黑素小体酪氨酸转运具有饱和性，酪氨酸转运的表观米氏常数（Km）为54微摩尔，最大速度为15皮摩尔酪氨酸/己糖胺酶单位/分钟。转运依赖温度（活化能Ea = 7.5千卡/摩尔），并且对酪氨酸的L-异构体具有立体特异性。芳香族、中性疏水化合物（如色氨酸和苯丙氨酸）以及小的、体积较大的中性氨基酸（如亮氨酸、异亮氨酸和蛋氨酸）竞争酪氨酸转运。酪氨酸转运受到经典系统L类似物2-氨基双环[2.2.1]庚烷-2-羧酸和单碘酪氨酸的抑制，但不受胱氨酸、赖氨酸、谷氨酸或2-（甲氨基）异丁酸的抑制。酪氨酸转运不依赖于Na⁺或K⁺，也不需要酸性环境或游离巯基的存在。这些结果证明了小鼠黑素细胞黑素小体中存在一种中性氨基酸载体，它类似于大鼠甲状腺FRTL-5溶酶体系统h。由于酪氨酸是合成色素黑色素所需的必需底物，所以这种转运系统对黑素小体的功能至关重要。
