Tyrosinase activity in FRM and M14 melanoma cells under control conditions, in cells treated with ConA and in HPV-16 E5 expressing cells. Tyrosinase activity was measured in FRM and M14 melanoma control cells (CTR), in cells treated with ConA (+ ConA) and in HPV-16 E5 expressing cells (+ E5). Cells were lysed by sonication as described in Materials and Methods, Enzymatic activity was assayed by measuring the amount of [3H] labelled water produced after incubation for 2 h at 37°C in reaction buffer containing [3H] tyrosine. Results are given as nmoles [3H]2O formed/h/mg protein. The mean ± SD of four independent experiments are depicted. Statistical comparison was made using the non parametric Mann – Whitney test. (*) = p < 0.05; (**) = p < 0.005. CTR cells did not show enzyme activity. Treatment with V-ATPase inhibitor or E5 expression restored the catalytic activity of the enzyme with the E5 oncogene associated with higher levels of activity.