Tyrosinase, the key enzyme in melanin synthesis, is expressed in murine brain

Tyrosinase is one of the key enzymes in mammalian melanin synthesis. The pigment is produced in two different cell types: the pigmented epithelial cell of the retina, and the melanocyte, a cell of neural-crest origin. We recently showed that a fusion gene between regulatory sequences of tyrosinase gene (tyr) and the beta-galactosidase gene (lacZ), when introduced into transgenic mice, resulted in embryonic expression in presumptive pigment cells but also in cells populations along the entire neural tube. This expression in the developing brain was striking, and we therefore asked whether this would still be detectable after birth. Transgenic mice carrying the tyr-lacZ fusion gene showed beta-galactosidase expression in adult brain. On Western blots, we detected tyrosinase-specific bands of 65-68 kDa in brain and eye. Using an affinity-purified antibody, we showed that detection of tyrosinase is specific and competed off by the presence of the cognate tyrosinase-derived peptide. However, neither tyrosine hydroxylase nor Dopa oxidase activity were detected in protein extracts of brain. We therefore suggest that tyrosinase is present in brain but either not functional or catalyzing different reactions compared to pigment cells.