The interaction between ribonuclease and mononucleotides as measured spectrophotometrically.

The action of pancreatic ribonuclease is inhibited by a number of mononucleotides (l), of which one of the most effective is cytidine 2’-phosphate (2). We have presented evidence (3) that cytidine 2’-phosphate is a competitive inhibitor of the action of ribonuclease upon either ribonucleic acid or cytidine 2’) 3’ cyclic phosphate, but that the dissociation constant of the cytidine 2’-phosphate-ribonuclease complex was significantly greater with ribonucleic acid than with a nucleoside cyclic phosphate as substrate. Because these results bear closely upon the question of the character of the catalytic site on the enzyme, it was of interest to study the binding of cytidine 2’-phosphate and other nucleotides to ribonuclease in the absence of substrate. We (4) and others (5) have found that the ultraviolet spectra of mixtures of ribonuclease with certain nucleotides are nonadditive functions of their constituent absorbancies and that the difference spectra thus obtained may be used as convenient indices of one type of enzyme-nucleotide interaction, entirely independent of enzymatic activity. This paper summarizes some of our findings.