Abstract
Tyrosinase activity is increased at specific stages of development in Xenopus laevis oocytes in mature females by an injection of 1000 units of human chorionic gonadtropin (HCG). Enzyme activity is stimulated slightly in stage II oocytes, greatly (5- to 6-fold) in stage III and early stage IV oocytes, slightly in late stage IV, and not at all in stage V oocytes. Tyrosinase activity has been localized cytochemically in oocytes by the DOPA-reaction. The DOPA-reaction product is found in the distal cisterna of the Golgi complex and in an anastomosing network of smooth-surfaced tubules associated with the Golgi complex. No reaction product is found in the clustered elements of smooth endoplasmic reticulum which gives rise to the premelanosomes. Substantial melanization of the premelanosomes does not occur until the DOPA-positive Golgi complexes move into proximity with the premelanosomes at the oocyte periphery. Biochemical assay of the isolated melanin granules shows that premelanosomes isolated from stage III and IV oocytes contain significant tyrosinase activity. This activity appears to decrease in the later stages of melanization. It is concluded that the metabolic activities leading to the formation of oocyte pigmentation are stimulated by gonadotropins and the degree of response to the stimulation is quantitatively regulated according to parameters typical of the specific stage of oocyte development.
Published Version
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