The mouse brown (b) locus protein functions as a dopachrome tautomerase.
The mouse b locus controls black/brown coat coloration. Its product, the b-protein or TRP-1, has significant homology to tyrosinase, and this has led to suggestions that the b-protein is itself a melanogenic enzyme. In order to investigate its function, we have used lines of mouse fibroblasts stably expressing the b-protein. We were unable to confirm previous reports that the b-protein has tyrosinase or catalase activity, but detected stereospecific dopachrome tautomerase activity in b-protein-expressing fibroblasts. This dopachrome tautomerase binds to Concanavalin A-Sepharose, and the major product of its action on L-dopachrome is 5,6-dihydroxyindole-2-carboxylic acid, as expected for the mammalian enzyme. Since this activity is not present in untransfected fibroblasts we conclude that the b-protein has dopachrome tautomerase activity. Further supporting evidence comes from the analysis of melanin metabolites produced by fibroblasts expressing tyrosinase alone, or in combination with the b-protein. Culture medium from the line expressing both proteins contains significant amounts of methylated carboxylated indoles, such as 6-hydroxy-5-methoxyindole-2-carboxylic acid, which would be expected in cells with an active dopachrome tautomerase. The levels of these compounds in medium from cells expressing tyrosinase alone are approximately 20-fold lower, and not significantly above background. Hence, it appears that the b-protein acts as a dopachrome tautomerase in vivo as well as in vitro.
Winder
A J
AJ
Sir William Dunn School of Pathology, University of Oxford, United Kingdom.
Wittbjer
A
A
Odh
G
G
Rosengren
E
E
Rorsman
H
H
D016475:3T3 Cells; D000818:Animals; D015153:Blotting, Western; D002374:Catalase / Q000378:metabolism; D005455:Fluorescent Antibody Technique; D006023:Glycoproteins / Q000235:genetics / Q000378:metabolism*; D045563:Indolequinones; D007211:Indoles / Q000378:metabolism; D019746:Intramolecular Oxidoreductases; D007535:Isomerases / Q000378:metabolism*; D008543:Melanins / Q000378:metabolism; D008562:Membrane Glycoproteins; D051379:Mice; D014442:Monophenol Monooxygenase / Q000378:metabolism; D010088:Oxidoreductases; D011809:Quinones / Q000378:metabolism; D014162:Transfection
D006023:Glycoproteins; D045563:Indolequinones; D007211:Indoles; D008543:Melanins; D008562:Membrane Glycoproteins; D011809:Quinones; C017408:6-hydroxy-5-methoxy-2-indolylcarboxylic acid; C001123:dopachrome; C030692:5,6-dihydroxy-2-indolylcarboxylic acid; D010088:Oxidoreductases; D002374:Catalase; C079621:Tyrp1 protein, mouse; D014442:Monophenol Monooxygenase; D007535:Isomerases; D019746:Intramolecular Oxidoreductases; C042501:dopachrome isomerase