Degradation of alpha‐melanocyte stimulating hormone (alpha‐MSH) by CALLA/endopeptidase 24. 11 expressed by human melanoma cells in culture

AbstractThe common acute lymphoblastic leukemia antigen (CALLA) is identical to human endopeptidase 24.11 (E‐24.11) and is expressed on certain human melanoma lines. This work was conducted in order to investigate whether alphamelanocyte‐stimulating hormone (alpha‐MSH) could be a substrate for E‐24.11, its degradation leading to the negative alpha‐MSH radiobinding assay results observed with some CALLA‐positive cell lines. We used 3 human melanoma cell lines (GLL‐19, Mel Juso and G361) which lack receptors to alpha‐MSH and express CALLA, and, as a control, one CALLA‐negative melanoma cell line (HBL) with specific receptors for alpha‐MSH. Radioimmunoassays give evidence that alpha‐MSH was degraded in the presence of the 4 melanoma cell lines and that disappearance of the peptide was significantly reduced by phosphoramldon in 2 lines (GLL‐19 and G361). Upon incubation of alpha‐MSH with GLL‐19 and G361 cell membranes, 3 degradation products were completely abolished in the presence of phosphoramidon. Amino acid content analysis of alpha‐MSH fragments produced by purified E‐24.11 permitted identification of 6 peptide bonds in the sequence of alpha‐MSH susceptible to cleavage by the enzyme. It is concluded that alpha‐MSH is a substrate in vitro for purified E‐24.11 and for the enzyme present on the human melanoma cell lines GLL‐19 and G361, expressing a high level of endopeptidase activity. However, hydrolysis of alpha‐MSH by this enzyme does not seem to represent the main factor responsible for the apparent absence of receptors for the hormone on some cell lines.