146 



PROTEINS 



increase in SH groups after reduction is a measure of the S— S groups 

 present. The reagents commonly used in titrating are potassium ferri- 

 cyanide and a dye, porphydrindin blue. The end point of the former 

 is determined with sodium nitroprusside as indicator. It gives a red 

 color with SH groups. On reduction the dye becomes a colorless com- 

 pound. 



Phenol and indole groups have weak reducing properties in alkaline 

 solution toward ferricyanide and can be determined after previous oxida- 

 tion of the SH groups at a different pH. 



The question naturally arises as to what occurs in the structure of 

 the molecule when a protein is denatured. Denaturation is undoubtedly 

 a disruption of the highly complex and precisely organized folding of 

 the peptide chains and the interrelation of these chains to one another. 

 In the disruption of this organization, groups {e.g., SH) that were previ- 

 ously buried deep in the molecule become exposed and reactive. Bio- 

 logical activity, which depends upon very specific arrangements in the 

 chain structures, is lost with the disappearance of these arrangements. 

 The regeneration of the crystalline form and partial recovery of biological 

 activity that has been observed in some instances may be ascribed to an 

 only partial disorganization of the chains. If the pattern still exists, the 

 chains may refold themselves into the original structure. 



QUANTITATIVE DETERMINATION OF PROTEIN 

 (KJELDAHL METHOD) 



Crude protein 



The protein content of any food material is obtained by determining 

 the total nitrogen and multiplying the result by 6.25. This value is 

 called the crude protein content of the food material. Two assumptions 

 are made in determining protein in this way: 



1. All nitrogen is assumed to be present in the substance as protein. 

 This is not necessarily the case, as food materials frequently contain a 

 large proportion of the nitrogen in other than protein form. Examples 

 of compounds containing nitrogen that are not protein are amino acids, 

 amides, alkaloids, cyanates, purines, pyrimidines, creatine, and creatinine. 

 From this list it is seen that in foodstuffs there are many compounds that 

 contain nitrogen but are not proteins. In finished products such as seeds 

 the major portion of the nitrogen is in the form of protein, while in 

 actively metabolizing tissue such as partly formed seeds and vegetables, 

 particularly string beans and green peas, the larger part of the nitrogen 

 may be in other than protein form. In muscle and other animal tissue 

 there is a considerable proportion of so-called meat extractives that 

 contain nitrogen but are not protein. 



