Tyrosine hydroxylase isoenzyme I is present in human melanosomes: a possible novel function in pigmentation

Both human epidermal melanocytes and keratinocytes have the full capacity for de novo synthesis of 6(R) L-erythro 5,6,7,8, tetrahydrobiopterin, the essential cofactor for the rate limiting step in catecholamine synthesis, via tyrosine hydroxylase. Catecholamine synthesis has been demonstrated in proliferating keratinocytes of the epidermis in human skin. This study presented herein identified for the first time the expression of tyrosine hydroxylase isozyme I mRNA within the melanocyte. The location of the enzyme was demonstrated in both the cytosol and melanosomes of human epidermal melanocytes, using immunohistochemistry and immunofluorescence double staining as well as immunogold electron microscopy. High-performance liquid chromatography (HPLC) analysis of pure melanosomal extracts from the human melanoma cell line, FM94, confirmed the production of L-dopa within these organelles. In addition, enzyme activities for both tyrosine hydroxylase and tyrosinase were measured in the same preparations, by following the catalytic release of tritiated water from L-[3,5-3H]tyrosine. The melanosomal membrane location of tyrosine hydroxylase together with tyrosinase implies a coupled interaction, where L-dopa production facilitates the activation of tyrosinase. Our results support a direct function for tyrosine hydroxylase in the melanosome via a concerted action with tyrosinase to promote pigmentation.