The role of inhibitors in the fluorescent staining of benign naevus and malignant melanoma cells with 9-amino acridine and acridine orange.

Guanidinobenzoatase is a trypsin-like protease capable of degrading fibronectin. An inactive form of guanidinobenzoatase is present on the surface of benign naevus cells and these cells stain very weakly with 9-aminoacridine, a known competitive inhibitor of guanidinobenzoatase. Malignant melanoma and metastatic malignant melanoma cells exhibit strong surface staining with 9-aminoacridine and also exhibit strong staining of cytoplasmic RNA with acridine orange. These simple fluorescent techniques have been used to distinguish benign naevus cells from malignant melanoma cells in human skin sections. This difference in cell surface staining with 9-aminoacridine has been demonstrated to be caused by the presence or absence of an inhibitor. The inhibitor can be displaced from the cell surface enzyme and then replaced by an affinity purified inhibitor obtained from fresh liver homogenates. It is proposed that the inhibition or control of cell surface guanidinobenzoatase may be one of the regulatory mechanisms by which benign naevus cells are prevented from developing into malignant melanoma cells.