Protein Complexes as Biochemically Active Systems 



All 



preincubated with NaCl for 45 minutes at 37 °Cj after which the curve for 

 proteolysis by trypsin (added after preincubation) was plotted. Curve II is for 

 serum albumin preincubated with DNA, after which the course of proteolysis 

 by trypsin was plotted. 



Curve III is for trypsin preincubated with NaCl, after which substrate was 

 added and the proteolysis curve plotted then. In the case of curve IV the trypsin 

 was preincubated with DNA, after which proteolysis was plotted for serum 

 albumin. For curves II and IV the amount of DNA was the same and the DNA/ 

 protein ratio was equal to o-o66; the enzyme/substrate ratio in these experiments 

 was 1/70. 



1V2 2 2!/2 3 



Time, hr 



Fig. 6. The effect of preincubation of the enzyme or substrate with DNA on the 

 proteolysis of serum albumin by trypsin (see text) 



It follows from the experiment that marked inhibition of proteolysis occurred 

 only if there was prehminary incubation of the substrate with DNA, whereas 

 preliminary incubation of the enzyme with DNA had practically no effect on 

 the kinetics of proteolysis. The conclusion to be drawn from these results is 

 that DNA in the serum albumin-trypsin system affects not the enzyme, but 

 the substrate, producing inhibition of the proteolytic process. 



The author of the present paper also studied the influence of DNA on pro- 

 teolysis at various DNA/substrate ratios. These findings are given in Fig. 7, 

 where the curve for the proteolysis of serum albumin by trypsin in the absence 

 of DNA served as a control. The DNA/substrate ratio for curves I, II and III 

 is equal to 0-33, o-66 and 0-99 respectively. It was also found that at a DNA/ 

 substrate ratio equal to i/ioo proteolysis is likewise retarded appreciably. 



