Protein Complexes as Biochemically Active Systems 



185 



As pointed out above, there are a number of indications that the activity of 

 certain enzymes is altered in the presence of nucleic acids. The study of these 

 phenomena has in the main centred around the role of ribonucleic acid and has 

 been carried out, for the most part, at pH values below the isoelectric point of 

 the enzyme and the substrate. In these conditions, as is known, insoluble nucleo- 

 proteins are precipitated, this being the result of the formation of salt-hke hnks 

 between the acid and basic groups of the reactants. We thought that it might be 

 preferable to study the effect of deoxyribonucleic acid (DNA) on the proteolytic 

 process under conditions in which the interaction of DNA with the enzyme does 

 not cause precipitation. Therefore we conducted our experiments at pH values 

 above the isoelectric point of trypsin and serum albumin, i.e., at the optimum 

 values for proteolysis (phosphate buffer, pH 8-8-4). The DNA used was obtained 



Fig. 5. The kinetic curves for the proteolysis of serum albumin by trypsin (I) 

 and by trypsin with DNA (II). 



from calf's thymus. The molecular weight of the DNA, determined viscosi- 

 metrically [29] was o-8-i-5-io6. The kinetics of proteolysis were determined by 

 the modified spearophotometric method of Kunitz at 400 mix. 



Fig. 5 gives the kinetic curves for the proteolysis of serum albumin by trypsin 

 (curve I) and trypsin with DNA (curve II). Here the difference between the 

 initial and subsequently measured value of extinction (the latter in percentages 

 of the initial value) is plotted against the time in hours. It is apparent that in the 

 presence of DNA the proteolytic process is retarded and the degree of hydrolysis 

 decreased. 



To gain an insight into the mode of inhibition of proteolysis by DNA we 

 studied the effect of preliminary incubation of the enzyme or substrate with 

 DNA. The curves obtained are shown in Fig. 6. Curve I is for serum albumin 



13 



