CATALYTIC LECTIN (LECZYME) FROM BULLFROG (RANA-CATESBEIANA) EGGS - MECHANISM OF TUMORICIDAL ACTIVITY

Catalytic lectins (leczymes) of frog eggs are sialic acid-binding lect ins that have intrinsic RNase activity. They inhibit tumor cell prolif eration ill vitro and ill vivo, although their cytotoxic mechanism rem ains unclear. RNase A has no tumoricidal activity. It is hypothesized that leczymes bind to cell surface sialoglycoconjugate receptors, ente r the cell, and subsequently degrade RNA. In order to investigate the cytotoxic mechanism of cSBL, a leczyme from Rana catesbeiana eggs, we established cSBL-resistant clone RC-150 from mouse leukemia P388 cells . cSBL-treated P388 cells showed extensive RNA degradation over the co urse of 1 h, whereas cSBL-treated RC-150 cells showed no RNA degradati on even over the course of 24 h. Treatment of P388 cells with cSBL led to decreased concentration of intracellular Ca2+, decreased protein k inase A activity, and increased protein kinase G activity. Incubation with cSBL decreased glutathione levels and enhanced glutathione-S-tran sferase (GST) activity in P388 cells, but had no effect on RC-150 cell s. We conclude that cSBL-specific degradation of RNA occurs in cSBL-se nsitive tumor cells, that cSBL leads to alteration of signal transduct ion and an intracellular protein kinase cascade reaction, and that int ernalized cSBL is detoxified by GST or thioltransferase. Our findings support a bifunctional model in which a leczyme is both an adhesive pr otein (binding to sialoglycoconjugates) and an enzyme (displaying RNna se activity).