64 NUCLEOPROTEINS AND ISfUCLEIC ACIDS 



The nucleoproteins are defined as conjugated proteins in which 

 the nucleic acid functions as the prosthetic group and is Unked 

 to the protein by electrostatic attraction or by secondary valence 

 forces, and sometimes by both. It is, of course, necessary to make 

 a clear distinction between the deoxypentose and the pentose 

 nucleoproteins. In the deoxypentose nucleoproteins^ the electro- 

 static contribution often is very pronounced, owing to the fact 

 that the deoxyribonucleic acids frequently are found in as- 

 sociation with proteins carrying strong positive charges, such as 

 the protamines or histones. Such complexes with basic proteins 

 are dissociated comparatively easily; and it is, for this reason, 

 sometimes difficult to decide whether such a nucleoprotein really 

 existed in the living cell or whether its isolation was the result of 

 a fortuitous combination of solutes that happened to be present 

 in the same extract. 



In decided contrast to the deoxyribonucleohistones and pro- 

 tamines, most ribonucleoproteins^' ^ are very difficult to untie. 

 This impedes the isolation of intact ribonucleic acids, since such 

 preparations are so difficult to free of protein that, when this is 

 accomplished, they can no longer be regarded as truly represent- 

 ative specimens. It would seem that electrostatic links are only 

 of secondary importance in the architecture of most pentose 

 nucleoproteins. It may be considered as a consequence of this 

 stubborn resistance to dissociation on the part of the pentose 

 nucleoproteins that nucleotide analyses carried out on the total 

 conjugated nucleoprotein have, on the whole, given much more 

 satisfactory results than those performed on isolated pentose 

 nucleic acids^. 



Of the representatives of the deoxynucleoprotein family it is 

 the nucleohistone of calf thymus that has been studied more than 

 any other and that is — probably quite wrongly — considered as 

 the prototype of the deoxypentose nucleoproteins. Less work has 

 been done on the nucleoprotamines, as they occur, for instance, 

 in fish sperm; and very little is known about microbial deoxy- 

 nucleoproteins. As to the pentose nucleoproteins, cytoplasmic 

 preparations mostly derived from the microsome fraction"^ and 



