A ribonuclease A variant with low catalytic activity but high cytotoxicity

Onconase(TM), a homolog of ribonuclease A (RNase A) with low ribonucleolyti c activity, is cytotoxic and has efficacy as a cancer chemotherapeutic. Her e variants of RNase A were used to probe the interplay between ribonucleoly tic activity and evasion of the cytotoxic ribonuclease inhibitor protein (R I) in the cytotoxicity of ribonucleases. K41R/G88R RNase A is a less active catalyst than G88R RNase A but, surprisingly, is more cytotoxic. Like Onco nase(TM), the K41R/G88R variant has a low affinity for RI, which apparently compensates for its low ribonucleolytic activity. In contrast, K41A/G88R R Nase,which has the same affinity for RI as does the K41R/G88R variant, is n ot cytotoxic. The nontoxic K41A/G88R variant is a much less active catalyst than is the toxic K41R/G88R variant. These data indicate that maintaining sufficient ribonucleolytic activity in the presence of RI is a requirement for a homolog or variant of RNase A to be cytotoxic. This principle can gui de the design of new chemotherapeutics based on homologs and variants of RN ase A.