Tyrosinase activity and ocular characteristics in Asinara white donkeys

The population of white asses of the Asinara isle (Italy) is characterised by white hair and coat, pink colour of glabrous areas of the body and skin, partial pigmentation of the iris. Our previous studies allowed to classify this phenotype as the expression of an oculocutaneous albinism. Aiming to go deeper into the knowledge of this kind of albinism in the population, the ophthalmic examination was performed and the tyrosinase activity of the hair bulb was studied in 5 animals of different age. The fundus of eye was observed an photographed with the portable hand held fundus camera (Kowa) and it showed a complete lack of pigment both in the retinal pigment epithelium and in the choroid. The lack of pigment makes transparent these two layers thus the vascular structure of the choroid becomes visible, both in the non tapetal fundus and in correspondence of the tapetal fundus, which is transparent too. Specific biochemical tests with L-tyrosine and L-DOPA, were carried aiming to evaluate the tyrosinase activity of the hair bulb, a marked pigmentation of the bulb was obtained when using DOPA, but the melanin was not produced at all when using the tyrosine. This means that the synthesis of melanin has a metabolic block of the conversion from tyrosine to DOPA which is the not active the next reaction which leads to the production of melanin.