CHAPTER $ 



THE CYTOCHEMISTRY OF PROTEINS 

 AND NUCLEIC ACIDS 



Introduction 



A common characteristic of the proteins and the nucleic acids 

 is that they both normally occur as macromolecules. It is, of 

 course, desirable if possible to characterize macromolecules by 

 their properties as a whole. But this is usually impossible, par- 

 ticularly in cytochemistry, and it is usually only by studying the 

 reactivity of certain chemical groups present in the macromole- 

 cules that these substances are identified. Consequently, there 

 is much similarity in the methods which are employed in the 

 study of the various proteins and nucleic acids, and in the diffi- 

 culties which are encountered in these studies. A convenient 

 classification of the groups present in proteins and nucleic acids 

 which are available for study from the cytochemical point of 

 view is: 



1. Carbohydrate groups. These may be found both in nucleic acids and 

 in proteins. Part of the cytochemistry of these groups has been discussed 

 in Chapter 4. 



2. Phosphate groups. Until recently phosphate groups have mainly 

 been studied in relation to nucleic acid. It is known, however, that in 

 addition to the phospho-proteins of yolk and of milk, other phospho- 

 proteins occur in nature. Myosin, for example, is probably a phosphoryl- 

 ated protein. 



3. Resonating ring structures. These are groups such as purines, pyrimi- 

 dines, tryptophane, tyrosine, and haem. These groups have the capacity 

 for absorbing light vigorously. Most of the naturally occurring resonating 

 ring groups of proteins and nucleic acids absorb in the ultraviolet region, 

 but a few, such as haem, also absorb light in the visible spectrum. 



4. Peptide linkages. These are found in proteins only, so far as is known. 



5. Reactive side-groups. These include such groups as NH2, CO2H, SH, 

 OH, tyrosine, histidine, tryptophane, etc. 



6. Enzyme activity. One method of characterizing a protein molecule 

 as a whole is by its enzyme activity. The study of alkaline phosphatase 

 discussed in Chapter 3 is, of course, an example of this. 



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