Biochemical and histochemical activities of tyrosinase in the skins of normal and albino turbot scophthalmus maximus
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Albinism occurr frequently in hatchery-reared turbot, particularly in China. To elucidatethe mechanism of albinism, we comparedthe biochemical and histochemical activity of tyrosinase inthe skins of normal and albino turbot using substrate L-3,4-dihydroxyphenylalanine (L-dopa). It was foundthat: (1) tyrosinase activity existed in allthe skin extracts tested, including pigmented and non-pigmented skins from ocular or blind side of normal and albino turbot, andthe tyrosinase activity ofthe ocular skin extracts was significantly higherthanthat ofthe blind skin extracts from a single individual for both normal and albino turbot. Unexpectedly,the tyrosinase activity inthe extracts from albino skin ofthe ocular side of albino turbot (termed AOA skin) was about 56% higherthanthat from pigmented skin ofthe ocular side of normal turbot (termed PON skin); (2) histochemical staining showedthat tyrosinase activity was present only inthe PON skin but not inthe AOA skin, andthe white skin ofthe blind side of albino (termed WBA skin) and normal turbot (termed WBN skin). A large amount of positive black granules was formed inthe epidermal cells of PON skin but no black granules were formed inthe skins of AOA, WBA and WBN; (3) temperature and salinity have similar effects onthe tyrosinase activity of ocular pigmented and of albino skin extracts, andthe optimal temperature was 55∼60 °C and optimal salinity 26‰; however, different pH values had different effects onthese tyrosinase activities, andthe optimal pH value of ocular pigmented skin extracts was 7.0 and ocular albino skin extracts 8.0; allthethree activators tested (SDS, trypsin and zymosan) can increasethe activity of tyrosinase in both ocular pigmented and albino skin extracts of turbot. Butthe level of activation onthe ocular albino skin extracts of albino turbot was significantly higherthanthat on ocular pigmented skin extracts of normal turbot.
Onthe basis ofthese observations, it is suggestedthat a large amount of wild type tyrosinase is expressed in albino skin butthe tyrosinase activity is blocked by some unknown inhibitory factors andthe blocked activity of tyrosinase can be readily recovered bythe homogenization of skin tissues in vitro.the unknown inhibitory factors need further studying.
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