Organic Substances 113 



t> 



either for the aromatic nucleus or for phenoHc functions or 

 for the guanidine grouping. 



The shades given by any of the reagents to be mentioned 

 cannot be compared for intensity and sharpness with those 

 of the better color reactions in histochemistry. Some of the 

 reagents will react only with unfixed ( or incompletely fixed ) 

 proteins and, even so, give shades so pale and indistinct as to 

 make the test virtually worthless. With some of the other 

 tests, the interpretation of the results is vague. With the ex- 

 ception of the Millon and Sakaguchi reactions, which do give 

 valuable information, the methods in this group will be men- 

 tioned for the record rather than because of their usefulness. 



1. Millon's reaction^ is one of the oldest tests for protein 

 substances. Actually, the reagent is specific for phenols and, 

 in the case of amino acids, for tyrosine. 



Of the several modifications published,^ Pollister's appears 

 to be the most reliable. 



Method 



Any good fixation is suitable. Incubate sections at 30°- 

 37° C. in a solution containing 5 per cent mercuric acetate 

 and 15 per cent trichloroacetic acid. After 5-10 minutes add 

 about one-tenth volume of a 1 per cent solution of sodium 

 nitrite and incubate sections for another 25 minutes. Rinse 

 sections directly in 70 per cent alcohol, dehydrate, and 

 mount. Tyrosine-containing proteins are stained in a shade 

 of pink to brick-red. The shade is rather transparent, and it 

 is advisable, for better visibility, to use sections not thinner 

 than 10 jx. 



2. The Sakaguchi test^ is specific for derivatives of guani- 

 dine in which at least one hydrogen in each of the amino 



4. Millon, E.: Compt. rend. Acad, sc, 28:40, 1849. 



5. Bensley, R. R., and Gersh, L: Anat. Rec, 57:217, 1933; Serra, J. A., 

 and Queiroz Lopes, A.: Port, acta biol., 1:51, 1945; Pollister, A. W.: Rev. 

 d'hematol., 5:527, 1950. 



6. Sakaguchi, S.: J. Biochem. (Japan), 5:25, 1925. 



