The plasminogen activator (PA) content of metastatic malignant melanoma was determined in Triton X-100 extracts of 11 surgical specimens and adjacent normal tissue, using azocaseinolysis with added plasminogen. The mean PA content of the tumors was 8.4 ± 10 (SD) CTA u/g tissue (6 times that of the surrounding tissue), lower than found earlier in lung, colon and breast tumors. Inhibition by goat IgG against purified human urokinase showed that the predominant activator was of the UK type, as was the case with the tumors examined earlier, except those of the prostate. This is in contrast with recent reports which showed that human melanoma-derived cells secrete into the culture fluid almost exclusively the “vascular type” PA (Wilson et al., Cancer Res. 40, 933, 1980; Roblin and Young, Ibid. 40, 2706, 1980). While this type of PA was present in all extracts here examined, in 3 of them only trace amounts (<1%) could be found. The vascular type PA, when present, could be inhibited by rabbit IgG against human melanoma cell culture-derived vascular PA (kind gift of Dr. E. Dowdle). SDS-gel electrophoresis in conjunction with fibrin-agar overlay zymography showed multiple activator bands ranging in mol. wt. from 100,000 to 30,000. All but the 70,000 mol. wt. vascular type PA band were inhibited by inclusion of anti-UK IgG into the fibrin-agar mixture.The discrepancy between the tumor extracts and the culture media may be due to a selective advantage of vascular type PA-secreting cells in culture, or to the turning off of the UK gene in the culture environment. It is also possible that production of vascular type PA is suppressed iii vivo.

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