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Thymidine kinase (EC 2.7.1.21) is an enzyme supporting DNA synthesis under conditions of increased cell proliferation. Although it has proved to be a useful marker for various malignant diseases, it has not been tested in malignant melanoma. Thymidine kinase activity was measured by means of a radioenzymic assay in two classical animal models of melanoma disease--B16 and Cloudman S91 melanoma-bearing mice. Tumour cell proliferation was assessed histochemically by measuring the expression of proliferating cell nuclear antigen (PCNA). Tumour cytosolic specific thymidine kinase activity was found to be higher in less pigmented Cloudman S91 melanoma than in differentiated, ie pigmented B16 melanoma, relative to the proliferative activity of the two tumours. Serum thymidine kinase levels were increased in melanoma-bearing animals of both types compared with healthy mice; this also reflected the efficacy of the therapy: cyclophosphamide-treated B16 melanoma-bearing mice in which the tumour development was slowed down had significantly lower serum enzyme levels in comparison with the non-treated group and the same levels compared with control, healthy mice. Our results suggest that serum thymidine kinase levels might be used as a marker to follow the effect of melanoma therapy.

The procedure for isolating soluble melanoprotein from human malignant melanoma was described. Electron microscopy confirmed the absence of any contaminating organized particles. The melanoprotein was brown hygroscopic substance with 8.8% of melanin, easily soluble in water. The solution revealed general absorption with a shoulder at 280 nm. Chemical composition was typical for a melanoprotein complex with characteristic high sulphur content (2.1%) and abundance in zinc (210 mug Zn/g dry sample). Sedimentation and electrophoretic properties indicated relative homogeneity. Similar substance could also be isolated from Harding-Passey mouse melanoma. Such soluble melanoproteins have been needed for their broad exploitation in tumor immunology.