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

2015; Taylor & Francis; Volume: 14; Issue: 4 Linguagem: Inglês

10.4081/ijas.2015.3976

1828-051X

Maria Grazia Cappai, Maurizio Picciau, Giuseppa Nieddu, Ilaria Sogos, Raffaele Cherchi, Walter Pinna,

Polyamine Metabolism and Applications

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 <em>Asinara</em> 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 <em>Asinara</em>’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 <em>Asinara</em>’s white donkey. These results definitely point to the fact that <em>Asinara</em>’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 <em>Asinara</em>’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.