Cutaneous metabolic pathway of tyrosine as a precursor to melanin in Asinara’s white donkey, Equus asinus L., 1758

Abstract

L-tyrosine (L-Tyr) is a conditionally indispensable amino acid for Equids. The biologic role of L-Tyr is ubiquitous, being involved in the production of hair and skin pigments. The white donkeys of Asinara island display a peculiar white coat, with life-long hypopigmentation of skin, hairs and eyes. Our results underline how the white phenotype is due to a disorder of melanin biosynthesis, detected in skin biopsies and on hair bulbs through immunohistochemical and enzymatic assays. The metabolic pathway of L-Tyr is compatible with the clinical frame of ocular-cutaneous albinism (OCA). The enzymatic activity in hair bulbs for the conversion of L-Tyr to L-3,4-dihydroxyphenilalanine (L-DOPA) tested negative, but positive for L-DOPA conversion into eumelanin in the 100% of the Asinara’s white donkey. This is the case of a negative-tyrosinase albinism, which contemporarily excludes dietary deficiencies of phenylalanine (Phe) or L-Tyr and potential incapability of Tyr intestinal uptake or Tyr conversion from Phe. The tyrosinase (TYR) residual activity positively and significantly (P=0.012) correlates (P=0.897) with the albino phenotype of the Asinara’s white donkey. These results definitely point to the fact that Asinara’s white donkey population owes its phenotype to TYR residual activity, thus on genetic basis. From a comparative point of view, the albino form of the Asinara’s white donkeys is similar to the human OCA type 1, from which however differs for positively testing to L-DOPA conversion, upon incubation of hair bulbs in L-DOPA solutions.

Keywords

• Donkey
• L-DOPA
• Melanin
• Oculo-cutaneous albinism
• Phenylalanine