Bufadienolides preferentially inhibit aminopeptidase N among mammalian metallo-aminopeptidases; relationship with effects on human melanoma MeWo cells

Bufadienolides are steroids that inhibit Na+/K+-ATPase; recent evidence shows that bufalin inhibits the activity of porcine aminopeptidase N (pAPN). We evaluated the selectivity of some bufadienolides on metallo-aminopeptidases. Among the enzymes of the M1 and M17 families, pAPN and porcine aminopeptidase A (pAPA) were the only targets of some bufadienolides. psi-bufarenogin, telocinobufagin, marinobufagin, bufalin, cinobufagin, and bufogenin inhibited the activity of pAPN in a dose-dependent manner in the range of 10(-7)-10(-6) M. The inhibition mechanism was classical reversible noncompetitive for telocinobufagin, bufalin and cinobufagin. Bufogenin had the lowest Ki value and a non-competitive behavior. pAPA activity was inhibited by psi-bufarenogin, cinobufagin, and bufogenin, with a classical competitive type of inhibition. The models of enzyme-inhibitor complexes agreed with the non-competitive type of inhibition of pAPN by telocinobufagin, bufalin, cinobufagin, and bufogenin. Since APN is a target in cancer therapy, we tested the effect of bufadie-nolides on the MeWo APN+ human melanoma cell line; they induced cell death, but we obtained scant evidence that inhibition of APN contributed to their effect. Thus, APN is a selective target of some bufadienolides, and we suggest that inhibition of APN activity by bufadienolides is not a major contributor to their antiproliferative properties in MeWo cells.