A CRITIQUE OF CYTOCHEMICAL METHODS 



223 



times the coiicentratioii of the nucleic acid to cause an equivalent ultra- 

 violet light loss. The distortion of the nucleic acid curve means therefore 

 that in the cell the nucleic acid is always accompanied by at least several 

 times as much protein. 



The intracellular nucleoprotein curves published by the Caspersson 

 group are of two distinctly different types 

 (I and II, Fig. 6-6). In type I the nucleic 

 acid peak is broadened, and the whole 

 right shoulder is shifted toward the longer 

 wave lengths. This shape is not unex- 

 pected for nucleoprotein; it seems to be 

 simply the summation of a nucleic acid 

 curve and that of a common protein type, 

 hke serum albumen (Fig. 6-3). On this 

 basis, cell regions showing this type ab- 

 sorption have been interpreted as sites 

 where nucleic acid and a typical acid pro- 

 tein ("globulin type") occur together. 

 The type II curves are very different and 

 puzzling; there is less broadening of the 

 nucleic acid peak, and within the long- 

 wave-length slope a second peak is indi- 

 cated by a distinct shoulder. For a variety 

 of reasons (see Caspersson and Thorell, 

 1941), curve II has been held to localize 

 nucleic acid accompanied by markedly 

 basic protein, called "histone type" or 

 "diamino-acid-rich" protein (Caspersson, 

 1940a, 1950). The strongest evidence for 

 this interpretation of the type II curves 

 was that certain nucleohistone prepara- 

 tions showed a protein peak apparently 

 shifted toward 2900 A. However, when 

 histones cjuite free of nucleic acid were 

 finally obtained, it was found that they did 

 not show such a shift of absorption, the 

 peak being near 2750 A as in typical pro- 

 teins (Mirsky and Pollister, 1943, 1946). 

 There remains therefore no certain expla- 

 nation of the peculiar shape of the type II curves. 



The nucleoprotein curves have been the basis for detailed speculations 

 concerning the roles of basic and acid proteins in cellular physiology 

 (Caspersson, 1950). For the general cytologist, perhaps the greatest 

 significance of these curves is that they emphasize unmistakably what 



2400 2600 2800 3000 

 WAVE LENGTH, A 



Fig. 6-6. Absorption curves of 

 the cytoplasm of cells of dif- 

 ferentiated (I) and undifferen- 

 tiated (II) renal tubules of the 

 chick embryo. Curves of the 

 I type are considered to repre- 

 sent ribonucleic acid with pro- 

 tein of the "globulin type"; the 

 type II curves are believed to 

 indicate the presence of ribonu- 

 cleic acid accompanied by con- 

 siderable basic protein, "histone 

 type." {After Caspersson and 

 Thorell, 1941.) 



