PKOTEIN REACTIONS. 101 



with sodium nitroprusside and ammonia. This reaction is not given by 

 all proteins and is due to the cystine groups. 9, ABDERHALDEN and 

 SCHMIDT'S reaction with triketohydrindenhydrate which gives a blue 

 coloration on boiling. The triketohydrindenhydrate (also called " Nin- 

 hydrin") reacts with all compounds which have an amino group in 

 the a-position to the carboxyl, is according to ABDERHALDEN and 

 SCHMIDT 1 an excellent reagent for the detection of dialyzable amino- 

 acids and non-biuret giving amino-acid derivatives. They have been 

 able to detect by this reagent such non-biuret giving substances in the 

 dialysate on the dialysis of different animal fluids. They have also 

 determined the delicacy of this reagent with different amino-acids. 



The biuret reaction is not only given by protein substances, but also by many 

 other bodies. According to H. SCHIFF 2 this reaction occurs with those bodies 

 containing amino groups, CONH 2 , CSNH 2 , C(NH)NH 2 or also CH 2 NH 2 , united 

 either directly by their carbon atoms or by means of a third carbon or nitrogen 

 atom. As examples of such bodies we can mention several diamines or amino- 

 amides, such as oximide, biuret, glycinamide, a- and /3-aminobutyramide, aspartic- 

 acid amide, etc., although we are not certain as to the conditions necessary for the 

 bringing about of this reaction. The biuret reaction alone is therefore no proof 

 as to the protein nature of a substance for example, urobilin gives a very similar 

 color reaction and a protein substance can still retain its protein nature, as by 

 the action of nitrous acid or by a splitting off of ammonia, although it does not 

 give the biuret reaction. 



The delicacy of the various reagents differs for the different proteids, 

 and on this account it is impossible to give the degree of delicacy for 

 each reaction for all proteids. Of the precipitation reactions, HELLER'S 

 test (if we eliminate the peptones and certain proteoses) is recommended 

 in the first place for its delicacy, though it is not the most delicate reac- 

 tion, and because it can be performed so easily. Among the precipita- 

 tion reactions, that with basic lead acetate (when carefully and exactly 

 executed) and with alcohol and the reactions given under 6, are the most 

 delicate. The color reactions 1 to 4 show great delicacy in the order in 

 which they are given. 3 



No proteid reaction is in itself characteristic, and, therefore, in testing 

 for proteids one reaction is not sufficient, but a number of precipitation 

 and color reactions must be employed. 



For the quantitative estimation of coagulable proteids the determina- 

 tion by boiling with acetic acid can be performed with advantage, for 

 by operating carefully, it gives exact results. Treat the proteid solution 

 with a 1-2 per cent common-salt solution, or if the solution contains 

 large amounts of proteid dilute with the proper quantity of the above 

 salt solution, and then carefully neutralize with acetic acid. Now deter- 



1 Zeitschr. f. physiol. Chem., 72 and 85. 



2 Ber. d. d. chem. Gesellsch., 29 and 30. 



3 In regard to the precipitation and color reactions of proteids with aniline dyes 

 see Heidenhain, Pfliiger's Arch., 90, 96. 



