The 5,6-dihydroxyindole-2-carboxylic acid (DHICA) oxidase activity of human tyrosinase.
Melanin synthesis in mammals is catalysed by at least three enzymic proteins, tyrosinase (monophenol dihydroxyphenylalanine:oxygen oxidoreductase, EC 1.14.18.1) and tyrosinase-related proteins (tyrps) 1 and 2, whose genes map to the albino, brown and slaty loci in mice, respectively. Tyrosinase catalyses the rate-limiting generation of L-dopaquinone from L-tyrosine and is also able to oxidize L-dopa to L-dopaquinone. Conversely, mouse tyrp1, but not tyrosinase, catalyses the oxidation of the indolic intermediate 5,6-dihydroxyindole-2-carboxylic acid (DHICA) into the corresponding 5,6-indolequinone-2-carboxylic acid, thus promoting the incorporation of DHICA units into eumelanin. The catalytic activities of the human melanogenic enzymes are still debated. TYRP1 has been reported to lack DHICA oxidase activity, whereas tyrosinase appears to accelerate DHICA consumption, thus raising the question of DHICA metabolism in human melanocytes. Here we have used two different approaches, comparison of the catalytic activities of human melanocytic cell lines expressing the full set of melanogenic enzymes or deficient in TYRP1, and transient expression of TYR and tyr genes in COS7 cells, to demonstrate that human tyrosinase actually functions as a DHICA oxidase, as opposed to the mouse enzyme. Therefore, human tyrosinase displays a broader substrate specificity than its mouse counterpart, and might be at least partially responsible for the incorporation of DHICA units into human eumelanins.
Olivares
C
C
Department of Biochemistry and Molecular Biology, School of Medicine, University of Murcia, Apto 4021, Campus Espinardo, 30100 Murcia, Spain.
Jiménez-Cervantes
C
C
Lozano
J A
JA
Solano
F
F
García-Borrón
J C
JC
D000595:Amino Acid Sequence; D000818:Animals; D001483:Base Sequence; D019556:COS Cells; D002457:Cell Extracts; D017931:DNA Primers; D006801:Humans; D008543:Melanins / Q000096:biosynthesis; D008545:Melanoma / Q000201:enzymology / Q000378:metabolism / Q000473:pathology; D008562:Membrane Glycoproteins; D008969:Molecular Sequence Data; D014442:Monophenol Monooxygenase / Q000737:chemistry / Q000378:metabolism*; D010084:Oxidation-Reduction; D010088:Oxidoreductases; D011506:Proteins / Q000737:chemistry / Q000378:metabolism*; D017386:Sequence Homology, Amino Acid; D014407:Tumor Cells, Cultured
D002457:Cell Extracts; D017931:DNA Primers; D008543:Melanins; D008562:Membrane Glycoproteins; D011506:Proteins; D010088:Oxidoreductases; C489799:TYRP1 protein, human; C064056:tyrosinase-related protein-1; D014442:Monophenol Monooxygenase