191 1] John L. Kantor and William J. Gies 267 



control in doubtful cases, and supplies a stock Solution for con- 

 venient use in comparative tinctorial tests. We know of nothing 

 which interferes with successful application of the biuret reagent 

 that does not have the same or even more deleterious effect on the 

 biuret test as applied in the classical manner. 



In our second preliminary report on this subject we made the 

 following remark : "" The reagent is useful for the detection of 

 reducing substances."^ In this respect the biuret reagent provides 

 the conditions that prevail in the Trommer test. The biuret reagent 

 is not as satisfactory for this purpose as the Fehling-Benedict 

 reagent, evidentiy because the concentration of the alkali is too high 

 and the concentration of the copper is too low. (See the second 

 footnote, page 265.) 



Among our findings was the Observation that although ammo- 

 nium salts interfere with protein responses to the biuret reagent, 

 ammonium hydroxid fails to do so to any appreciable degree. 

 Ammonium salts react with the alkali of the reagent. By reducing 

 the alkalinity of the liquid not by introducing ammonia into it, 

 ammonium salts require the addition of a moderate excess of the 

 reagent for an elicitation of the test^ in any protein mixture con- 

 taining any of them in a fairly large proportion. 



During the past summer, in reflecting on the results with am- 

 monium salts and ammonium hydroxid to which we have just 

 referred, the senior author recalled his first quantitative experiences 

 with the Pavy reagent,'^ and thought of the possibility of using 

 decolorized Pavy reagent as a colorless biuret reagent. As is well 

 known, the Pavy reagent is, in effect, Fehling reagent rendered 

 strongly ammoniacal. By careful reduction, with dilute glucose 

 Solution for example, the resultant cuprous oxid may be kept in 

 Solution, the copper combination with ammonium hydroxid being 

 colorless although speedy oxidation to a blue Compound is difficult 

 to prevent. Experiments in this connection were performed imme- 

 diately after the idea came to mind. The very first test revealed the 



* Kantor and Gies: loc. cit. 



• The details of our work in all these connections will soon be published 

 elsewhere. 



■^ Asher and Gies : Zeitschrift für Biologie, 1900, xl, p. 180. 



