Inhibition of Ribonuclease from Aspergillus saitoi by Nucleosides By MASACHIKA IRIE

1. Ribonuclease from Aspergillus saitoi [RNase M, EC 2.7.7.17] was found to be inhibited competitively by various nucleosides. Among four common ribonucleosides, the inhibitory effect was in the following order, A>C>G>U. 2. When the base was the same, deoxyribonucleoside was found to be less effective than ribonucleoside. It was suggested that the RNase M bound with 2'-OH group of ribonucleoside in some way. 3. a-Anomers of adenosine and guanosine were less effective than the natural /3-anomers. This indicates that the RNase M requires the nucleoside the proper orienta-tion of the sugar 2'-OH or 3'-OH and base moiety. 4. L-/!-Cytidine-5'-phosphate was less effective than D-/J-cytidine-5'-phosphate(CMP). From this result, it was suggested that the RNase M requires nucleosides the proper spacial relationship among base, 2'-OH and ether oxygen. 5. When mixed with nucleosides, the RNase M gave similar difference spectra to those observed by mixing with nucleotides. 6. pKi-pH profiles of adenosine, cytidine and guanosine were studied and it was found that the bends concaved side upwards at pH 6.0 observed in the case of pKm- or pXrpH profile of nucleotide were not observed. 7. Inactivation of the RNase M by iodoacetate was protected by the presence of adenosine, but not by photooxidation. 8. From the results obtained here, possible mode of binding of nucleosides with the RNase M was discussed. Ribonuclease from Aspergillus saitoi (RNase M) [EC 2. 7. 7.17] ribonucleate nucleotide-2'- MATERIALS AND METHODS transferase] has been known to be non-specific Enzyme — RNase M was purified from commercial with respect to bases (1). Previous Studies digestive " Molsin " according to the method reported on the precise substrate specificity of RNase M P ious ly ( /). The concentration of the RNase M using dinucleoside phosphates (XpY) as sub- w a * d e t c r m i n c d »P<ctrophotometrically at 280m? using 6 i j.v,. l, u ' i v * • extinction coefficient of 48,500.