The effect of in vivo and in vitro x-irradiation on melanosomal tyrosinase activity was measured in mouse B-16 melanomas. Tyrosinase activity increased from 48-120 hours after x-irradiation of B-16 melanomas in vivo with 1000 R, reaching a peak of 225% of control at 96 hours. In vitro, x-irradiation of melanosomes produced no change in tyrosinase activity immediately after irradiation. However, incubation of irradiated melanosomes at 4°C for 20 hours in air produced a 4 fold increase in tyrosinase activity at doses between 125 and 375 krads. In vitro, the x-ray-induced increase in tyrosinase activity was reduced by chloroquine or by incubation in nitrogen instead of air. X-rays may damage the melanosomal membrane permitting an increased amount of tyrosine to reach the tyrosinase molecule within the melanosome; this mechanism could contribute to the increased tyrosinase activity produced by x-irradiation.

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