ENZYMES ATTACKING NUCLEIC ACIDS 587 



1' 2' 3' 4' 5' 



(Diesterase) 



/ 



(Diesterase) 



(Bone P-ase) P 



Fig. 8. Action of the phosphodiesterases of intestinal mucosa and of snake venom, 

 and of bone phosphomonoesterase on PNA. [From Volkin and Cohn."] 



hydrolyzed by snake venom phosphodiesterase because their terminal phos- 

 phomonoester groups are in the 5'-position; on the other hand, the ohgo- 

 nucleotides of digests of PXA with ribonuclease I which contain the termi- 

 nal secondary phosphoryl groups in the 3'-positions are hydrolyzed by snake 

 venom phosphodiesterase only after the removal of these groups by a pre- 

 liminary digestion with a phosphomonoesterase (bone phosphomonoesterase 

 or prostatic phosphatase). 



The question remains why undegraded PNA (prepared by mild pro- 

 cedures in the laboratory) is hydrolyzed by the enzyme without a preceding 

 monoesterase treatment^^^ despite its content of terminal 3'-phosphoryls 

 which amounts to about 6 to 10% of its total phosphorus. Possibly the in- 

 hibiting influence of these terminal groups decreases with increasing degrees 

 of polymerization of the substrates. 



The resistance of highly polymerized deoxyribonucleic acids toward 

 snake venom diesterase could be attributed either to their molecular size 

 or to structural reasons. No decision between these alternatives is possible 

 at present. 



It may well be that the action of the snake venom enzyme is a simple 

 hydrolysis; at least it is obvious that its action on DNA cannot involve an 

 intermediary formation of 2,3-cyclic nucleotide groups. 



Properties of Snake Venom Phosphodiesterases 



Correlation with toxicity. According to Taborda et al.,^^'' the average 

 phosphodiesterase concentration in venoms of various species shows a 



1" A. R. Taborda, L. C. Taborda, J. N. Williams, Jr., and C. A. Elvehjem, J. Biol. 

 Chem. 195, 207 (1952). 



