100 PHYSIOLOGICAL CHEMISTRY. 



It must be remembered that the various commercial products 

 sold as " peptones " may contain many other substances, and 

 may be quite unfit for use as food or in medicine. While in 

 some cases a considerable portion of real peptone (with albu- 

 mose) is present, in others the main constituents are decompo- 

 sition products formed by too long digestion of the meat or 

 fibrin with the acid and pepsin mixture. Some of these 

 commercial peptones appear to be formed by digesting with 

 weak acid under pressure, which results in the formation of 

 bodies of little nutritive value; indeed, it is likely that such 

 products are distinctly harmful when taken into the stomach 

 of man. 



THE PROTEIDS. 



The term proteid, as already explained, is used to designate 

 a certain group of protein compounds. This use is a per- 

 fectly arbitrary one as the word was once employed to describe 

 all the bodies discussed in this chapter. According to the gen- 

 erally accepted modern classifications the bodies now called 

 proteids are compound substances in which a true or native 

 albumin is found in combination with some other group which 

 often may be separated as such. In the table given earlier in 

 the chapter three such combinations are mentioned : the nucleo- 

 proteids, the hemoglobins and the gluco-proteids. A brief de- 

 scription of each group will here follow. 



NUCLEO-PROTEIDS. 



These proteins are important as making up a large part of 

 the cell nucleus. In treating tissues rich in cells with the 

 pepsin-hydrochloric acid digestive mixture it was long ago 

 recognized that a certain portion went easily into solution, 

 while another portion was always left undissolved. This res- 

 idue was called nuclein and was found to contain all the phos- 

 phorus of the original protein. If in place of the pepsin 

 mixture some other hydrolyzing agent is used the general 

 result is similar; a separation into two component parts takes 



