Many methods have been described in the literature for determining the activity of ribonuclease (McDonald, 1955); most of these employ ribonucleic acid as substrate and are not suitable for kinetic investigations because it is not clear which

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Ross, Mathias & Rabin, 1962) provide evidence that the catalytic site of ribonuclease contains two imidazole groups; one of these is required in the acid form and the other in the base form. For the hydrolysis of cytidine 2',3'-phosphate, the shifts in the pK values of the active site on binding the substrate indicate that the acid site, but not the base site, is involved in binding the substrate. The acid site is probably hydrogen-bonded to the 2'oxygen atom of the cyclic phosphate and this accounts for the much stronger binding of cytidine 2'-phosphate compared with that of the 3'phosphate. There must be additional sites of enzyme-substrate binding which do not ionize over the pH range 4-8-5, since K., is not identically

Holton for his collaboration in the study of changes in the extinction of heart-muscle preparation during activation. It is a pleasure to thank Miss Valerie Francis for her technical assistance.

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In the degradation of ribonucleic acid by pancreatic,-ribpnuclease, uridine and cytidine 2':3'

In addition to hydrolysing RNA, bovine pancreatic ribonuclease splits esters of pyrimidine nucleoside 3'-phosphates, including dinucleotides. For a series of 3':5'-linked dinucleotides of general structure CpN, where N is a 5' linked nucleoside, kcat for the release of N varies enormously with the precise structure of N. Structural studies have been interpreted to indicate that the group N interacts with a subsite, B2, on the enzyme that comprises Gln69, Asn71 and Glu111. We report studies by site-directed mutagenesis that indicate that Gln69 is not involved in productive interactions with any of the dinucleotide substrates and that Asn71 is an important component of subsite B2 for all dinucleotide substrates tested. Glu111 appears to be functionally involved in catalysis for dinucleotide substrates solely when N is guanosine.