The mouseblocus 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 con‐firm 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 dopachr
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