Kinetic studies on turtle pancreatic ribonuclease: a comparative study of the base specificities of the B2 and P0 sites of bovine pancreatic ribonuclease A and turtle pancreatic ribonuclease.

Kinetic constants for the transesterification of eight dinucleoside phosphates CpX and UpX by bovine and turtle pancreatic ribonuclease were determined. Both ribonucleases have a preference for purine nucleotides at the position X. However, bovine ribonuclease, like other mammalian ribonucleases, prefers 6-amino bases at this site, while turtle ribonuclease prefers 6-keto bases. This difference in specificity at the B2 site may be explained by the substitution of glutamic acid at position 111 by valine in turtle ribonuclease. These results have been confirmed by inhibition studies with the four nucleoside triphosphates. Inhibition studies with pT and pTp showed that a cationic binding group (P0) for the 5'-phosphate of the pyrimidine nucleotides bound at the primary B1 site is present in turtle ribonuclease, although lysine at position 66 in bovine ribonuclease is absent in turtle ribonuclease. However, the side chain of lysine 122 in turtle ribonuclease is probably located in the correct position to take over the role as cationic P0 site.