Human TRP‐1 Has Tyrosine Hydroxylase but no DOPA Oxidase Activity

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Abstract
Human TRP‐1 has been immunopurified from normal human melanocytes cultured from black neonatal subjects and used to investigate the catalytic function of TRP‐1 for the two substrates, L‐tyrosine and L‐DOPA. Immunopurified TRP‐1 did not demonstrate DOPA staining on SDS/PAGE nor DOPA oxidase (DO) activity with either routine or modified assays. The purified TRP‐1 also demonstrated no tyrosine hydroxylase (TH) activity using the routine Pomerantz assay. However, there was apparent TH activity exhibited by immunopurified TRP‐1 under conditions with low tyrosine concentration (≤0.8 μCi/ml of 3H‐tyrosine), prolonged incubation time (i.e., overnight) and in the absence of the cofactor L‐DOPA. Using these latter specific conditions, TH activity was also detected in cell lysates from a tyrosinase‐negative albino melanocyte line which exhibited no TH activity with the routine Pomerantz assay. In addition, TH activity under low substrate assay conditions was not exhibited in a melanocyte line derived from a TRP‐1 deficient, Brown albino individual. However, the absence of TH in this Brown albino cell line could be compensated for by the addition of L‐DOPA to the assay. These results suggested that TRP‐1 has some tyrosine hydroxylase but no DOPA oxidase activity. We propose that one function of TRP‐1 is to modulate tyrosinase activity by making DOPA available as a cofactor to perpetuate the initial steps in melanogenesis.

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