Non-specific depurination activity of saporin-S6, a ribosome-inactivating protein, under acidic conditions

Among five ribosome-inactivating proteins tested only saporin-S6 could effi ciently release the adenine from adenosine 20 of the synthetic oligoribonuc leotide (SRD RNA) mimic of the sarcin/ricin domain of rat 28S rRNA with a K -m of 9 mu M and a k(cat) of approximately 0.4 min(-1) at pH 7.6. The optim al pH for the depurination activity of saporin-Se is 5.0. However, saporin- S6 lost its site-specificity of depurination on SRD RNA around the optimal pH. The non-specific depurination activity of saporin-S6 was dependent on t he enzyme concentration and pH conditions. These results are valuable to un derstand the diversity and the depurination mechanism of ribosome-inactivat ing proteins.