106 CYTOCHEMISTRY OF PROTEINS 



considerable variety of groups. Table IX shows the formulae 

 for a number of these reagents. These are being investigated 

 in collaboration with A. Loveless and L. G. Bell. The reagents 

 may be used in two ways. Thus the CHO group may be allowed 

 to react with the NH 2 groups of a protein. After this reaction 

 has gone to completion, the second reactive group (e.g., N0 2 or 

 NH 2 ) may be subjected to the procedure discussed in the previ- 

 ous section, to produce a colour. In other cases, the second re- 

 active group is allowed to react with the protein, the CHO group 

 being kept intact. The section is then exposed to reduced fuchsin, 

 which forms a purple colour with the aldehyde group. The fol- 

 lowing is an example of this type of reaction: 



Pr-NH 2 + CHO<^ ^>NH 2 -> Pr-NH-CHOH^ ^>NH< 



i 



Pr • NH • CHOH^ \N=N ■ OH 



Pr-NH-CHOH< >N=N< >NH; 



Blocking Agents 



As was pointed out above, chromogenic reagents are usually 

 able to react with several of the groups found in a tissue section. 

 Consequently, before the results obtained with a chromogenic 

 reagent can be analyzed, it is necessary to make studies on sec- 

 tions in which some of the reactive groups have been blocked with 

 non-chromogenic reagents. Table X shows a list of such reagents 

 and of the groups which they can conveniently be used to block. 



Some of the reagents given in Table X are able potentially to 

 react with a considerable number of the chemical groups of a 

 protein. But the actual range of their blocking activity can be 

 restricted by controlling the pH at which the blocking reaction 

 is carried out, and controlling the time of exposure to the 

 blocking agent. This is the case, for example, with phenyl iso- 

 cyanate and diazonium hydroxides. Some of the blocking re- 

 agents have the useful property of having a reversible action. 

 This is the case with Hg, ClCH 2 -0-CH 3 , PhSCOCl, ClCH 2 -0- 

 CH=CH 2 , PhAsO, PhHgOH. With these reversible reagents it 



