ENZYMES ATTACKING NUCLEIC ACIDS 557 



state of knowledge: the only bonds known to be susceptible to the action 

 of enzymes in nucleic acids or polynucleotides are the phosphoric ester 

 linkages. All known deaminases or enzymes involving the glycosidic bonds 

 act only on mononucleotides or their cleavage products. The degradation 

 of nucleic acid in the laboratory to substances such as GuUand's deamino- 

 ribonucleic acid or Chargaff's apurinic acid has so far no analogy in the 

 living organism. Obviously, confirmation or refutation of this statement 

 will be an important topic of future research. The validity of this generali- 

 zation would mean that the polymerization of mononucleotides would be 

 the exclusive mechanism of the biosynthesis of the nucleic acids and the 

 only determining factor for the sequence of the various nucleotide groups. 

 It would mean that actions such as the enzymic exchange of purine groups 

 would take place exclusively at the mononucleotide or nucleoside level. 



The degrees of specificity of the enzymes of nucleic acid metabolism 

 show widely different ranges. Many of these enzymes catalyze reactions 

 not only of natural substrates, but also of analogous substances. Such 

 cases are of particular interest when biosynthetic enzyme reactions are 

 inv^olved. The observations concerning the incorporation of antimetabo- 

 lites into important cell constituents of the intact organism are paralleled 

 by the behavior of isolated enzyme fractions. The concept of such incor- 

 poration is becoming increasingly important in the explanation of physio- 

 logical effects of certain antimetabolites. 



The number of different enzymic mechanisms in the field of nucleic acid 

 metabolism is too large to fit into the original system of nomenclature 

 proposed by Levene and Medigreceanu.^ The present terminology is some- 

 what arbitrary because some original designations are now applied with a 

 modified meaning. Levene and Medigreceanu introduced the collective 

 term "nucleases" for all enzymes involved in the metabolism of nucleic 

 acids or their degradation products or precursors. In contrast to this broad 

 application of the term "nucleases,"- the terms "ribonuclease" and "de- 

 oxyribonuclease"- — introduced by Dubos and Thompson^ and by Kunitz,^^ 

 respectively — are now usually limited to the designation of enzymes which 

 catalyze the cleavage of the phosphoric ester bonds interlinking the nucleo- 

 tide groups of nucleic acids or polynucleotides.* 



' P. A. Levene and F. Medigreceanu, J . Biol. Cheni. 9, 389 (1911). 



2 O. Hoffmann-Ostenhof, Advances in Enzymol. 14, 219, 225 (1953). 



» R. Dubos and R. H. S. Thompson, J. Biol. Chem., 124, 501 (1938). 



3» M. Kunitz, J. Gen. Physiol, 33, 349 (1950). 



* H. S. Loring and F. H. Carpenter suggested replacement of the term "ribonu- 

 clease" by the term "ribonucleinase" [J. Biol. Chem. 150, 381 (1943)], but the 

 customary use of the shorter term is continued in the literature. The reviewer 

 feels that, owing to the overlapping specificities of manj' nucleases, it will be next 

 to impossible to devise a framework of nomenclature which will not be riddled 

 with inconsistencies bj' future observations. 



