Not only fibroblasts but also melanoma cells express "leucine aminopeptidase" activity.

"Leucine aminopeptidase" (LAP, aminopolypeptidase, EC 3.4.11) activity has been recommended and widely used as a histochemical marker for the identification of contaminating LAP-positive fibroblasts in pigment cell cultures. Using a sensitive biochemical assay with L-leucyl-p-nitroanilide as a substrate we demonstrated that in vitro melanoma cells also exhibit LAP activity. Our comparison of four melanoma cell lines with four fibroblast lines showed that the differences in the enzyme activity were not qualitative but only quantitative. For this reason the specific antibodies, karyological analysis and electron microscopy are recommended as more reliable means in distinguishing fibroblasts from poorly differentiated pigment cells than the LAP-cytochemistry.