570 G. SCHMIDT 



tions that the structures responsible for the comparatively low ultraviolet 

 absorption are mainly the purine nucleotide groups of the cores. 



Observations on the Kinetics of Ribonuclease. Substrates of well-defined 

 and simple structure, i.e., oligonucleotides containing per molecule only 

 one susceptible hnkage or a well-defined number of such linkages would 

 obviously be the most suitable materials for studies of the kinetics of 

 ribonuclease as well as for its assay. Such substances, e.g., certain dinucleo- 

 tides^i or cyclic nucleotides,^"'" became available only very recently, and 

 no detailed kinetic studies on these highly interesting substrates have been 

 published as yet. 



For these reasons, all available kinetic data are based on the action of 

 ribonuclease on ribonucleates. It is obvious that such data are of very 

 limited value for the understanding of the action of ribonuclease. Many of 

 the kinetic studies were carried out on commercial preparations of PNA. 

 Recent experiences have shown that commercial preparations of yeast 

 PNA must be considered as more or less degraded products which differ 

 in many properties such as the number of terminal groups per molecule 

 from PNA samples prepared by the mildest available procedures in the 

 laboratory. But even genuine nucleic acid samples offer very complex 

 conditions for kinetic studies since any ribonucleic acid preparation from 

 a given tissue probably consists of a mixture of different nucleic acids and 

 each individual PNA molecule contains a large number of linkages which 

 are hydrolyzed by ribonuclease. Although some important features are 

 common to all these linkages, they differ amongst each other in regard to 

 their structure. It is very possible that the various linkages hydrolyzed by 

 ribonuclease are cleaved at different rates so that the rate of ribonuclease 

 action as measured, e.g., by the rate of formation of acidic groups from 

 PNA, represents an overall rate resulting from a large number of individual 

 enzyme reactions each of which might have its own characteristic kinetics. 



According to the manometric measurements of J. A. Bain and H. P. 

 Rusch,^^ the effect of the concentration of ribonucleate on the rate of 

 ribonuclease action is very considerable; the maximal initial velocity is 

 only approached at a substrate concentration of 6.5% (Fig. 5). 



In addition, the curves of Figure 5 (according to Bain and Rusch^^) 

 demonstrate that relatively high substrate concentrations (at least 5%) 

 are required in order to obtain constant hydrolysis rates during the initial 

 phases of the enzyme reaction. At low substrate concentrations, the rates 

 are falling continuously even during the first seconds of incubation. 



This might suggest a strong competitive effect of the hydrolysis products 

 of ribonuclease action on the enzyme. Actual data concerning such effects, 

 however, are scanty and controversial. So far, only the effect of added 



