Organic Substances 85 



portant as they may be, are not the only factors at work.^^^ 

 Other structural properties (the presence of polar groups; 

 steric configuration), both of the adsorbing nucleoprotein 

 and of the adsorbed dye, may be more important than acid- 

 base aflfinity, just as in the case of cation exchangers^^^ (resins 

 and other substances). For instance, a compound as little 

 acidic as silica (pK of silicic acid, ±9.7) will quantitatively 

 remove methylene blue from a solution in 0.1 N sulfuric acid. 

 Many attempts have been made in the past to stain tissue 

 sections with basic dyes at graded pH levels and to draw 

 meaningful conclusions from the more or less abrupt changes 

 in staining intensity. ^^^"^^ The pH at which such sudden 

 changes occur has variously been called the isoelectric point 

 of the substance stained^^®' ^^^' ^^^' ^"^' ^^^ or the acidic dissoci- 

 ation constant of its basophilic group.^^*' ^^^ The fundamental 

 assumptions behind such experiments are that (a) acidic 

 substances will stain with increasing intensity above their 

 isoelectric point ( or pK of their acidic groups ) , while below 

 this point their staining intensity will decrease rapidly; and 



117. Michaelis, L.: Arch. f. mikr. Anat., 94:580, 1920. 



118. W^hitehom, J. C: J. Biol. Chem., 56:751, 1923; Walton, H. F.: Ion 

 exchange equilibria. In Ion exchange, theory, and application, ed. F. C. 

 Nachod (New York: Academic Press, 1949). 



119. Pischinger, A.: Ztschr. f. Zellforsch. u. mikr. Anat., 3:169, 1925-26. 



120. Pischinger, A.: ibid., 5:347, 1927. 



121. Hammarsten, E., Hammarsten, G., and TeoreU, T.: Acta med. Scan- 

 dinav., 68:219, 1928; Zeiger, K.: Ztschr. f. ZeUforsch. u. mikr. Anat., 

 10:481, 1930. 



122. Fautrez, J.: Bull, d'histol. appUq. a la physiol., 13:202, 1936. 



123. Levine, N. D.: Stain Technol., 15:91, 1940. 



124. Kelley, E. G.: J. Biol. Chem., 127:55, 1939. 



125. Kelley, E. G.: ibid., 127:73, 1939. 



126. Seki, M., and Kohashi, Y.: Okajima's FoHa anat. Jap., 19:47, 1940. 



127. McCaUa, T. M.: Stain Technol., 16:27, 1941. 



128. Hyden, H.: Acta physiol. Scandinav., Vol. 6, Suppl. 17, 1943. 



129. Dempsey, E. W., Bunting, H., Singer, M., and Wislocki, G. B.: Anat. 

 Rec, 98:417, 1947; Singer, M., and Morrison, P. R.: J. Biol. Chem., 

 175:133, 1948. 



