High tyrosinase activity in albino Xenopus laevis oocytes

Tyrosinase was measured in oocytes of the recently described albino mutant (apap) of Xenopus laevis. Although these oocytes show no pigmentation and the eggs are known to contain no melanosomes, tyrosinase - which is probably the only enzyme necessary for melanin synthesis from tyrosine - was increased more than twofold relative to the wild type. Tyrosinase recovered from albino and wild type oocytes showed the same Km with respect to tyrosine, and this was not altered by previous gonadotrophin stimulation in vivo. The tyrosinase assay, based on [14]tyrosine incorporation into acid-insoluble products, was of greater sensitivity than previously described methods of the same type, through removal of low molecular weight material from the oocyte homogenate prior to incubation, and the use of tyrosine of high specific activity.