564 



G. SCHMIDT 



(RNA-ase) 



(RNA-ase) 



-^ (RNA-ase) 



Fig. 4. Interlinkages in oligo- and polynucleotides and in ribonucleic acids cleaved 

 by ribonuclease I. [Modified from the scheme of W. E. Cohn et al. in "Phosphorus 

 Metabolism" (McElroy and Glass, eds.), Vol. 2, p. 344. Johns Hopkins Press, Balti- 

 more, 1952.] 



These physicochemical changes are explained by observations of Brown, 

 Dekker, and Todd^^ and of Markham and Smith" that cychc 2',3'-nucleo- 

 side phosphoric acid diesters are- intermediaries in the cleavage of the 

 internucleotide bonds by ribonuclease.^^ In a second step the cyclic pyrim- 

 idine nucleotides are hydrolyzed by ribonuclease to ordinary 3 '-nucleotides 

 (or nucleotide groups). The diphasic enzymic cleavage of the internucleo- 

 tide bonds is a peculiarity of the action of ribonuclease I on ribonucleic acid 

 and ribopolynucleotides. These substrates have the structural requirements 

 for such a reaction mechanism because they have 2 '-hydroxy groups in 

 the vicinal position to the corresponding 3'-phosphoryl groups of the 

 internucleotide bonds (Fig. 4). The action of deoxyribonuclease on deoxy- 

 ribonucleate causes only the usual volume contraction of 2 cm. per equiva- 

 lent and is not accompanied by an intermediary phase of dilatation. 



"D. M. Brown, C. A. Dekker, and A. R. Todd, /. Chem. Soc. 1952, 512. 



" R. Markham and J. D. Smith, Biochem. J. 52, 552 (1952). 



" L. A. Heppel, P. R. Whitfeld, and R. Markham, Biochem. J. 56, Proc. iii (1954). 



