50 FIXATION 



power to digest. As a rule enzymatic properties are destroyed, and 

 special fixatives must be chosen if they are to be displayed in 

 microscopical preparations. If the right denaturing agent is chosen, 

 it will coagulate the proteins in general but leave certain enzymes 

 more or less intact. Sections may then be cut and placed in a solu- 

 tion of a suitable substrate. The latter must be carefully chosen, 

 for it must leave microscopically visible evidence of the places in 

 the tissue in which it was attacked by the enzyme : the product of 

 the reaction must be immobile and either visible or capable of 

 being made visible. In this roundabout w^ay the original site of 

 the enzyme can be determined. It is chiefly for work of this sort 

 that acetone deserves to be listed as a fixative. Certain enzymes, 

 such as alkaline phosphatase, are resistant to its action. Acetone 

 distorts tissues seriously and would never be chosen for purely 

 morphological studies. It has, however, certain uses as a fixative 

 in other branches of histochemistry beside enzymology. 



An important eflfect of denaturation on globular proteins is 

 their elongation into fibres. This change can best be witnessed 

 when loss of solubility has not yet occurred. The extension of the 

 protein molecules renders the sol more viscous, and since they 

 tend to arrange themselves parallel with one another in a moving 

 fluid, the formerly isotropic sol now exhibits the birefringence of 

 flow. The protein never extends fully into a straight chain on 

 denaturation, but always remains to some extent folded. Remark- 

 ably enough, the fibrous proteins are aflFected in the opposite way: 

 the chain shortens somewhat by folding. There is thus an approxi- 

 mation of both kinds of proteins towards a similar structure, but 

 this is fibrous, not globular. A coagulum could not be formed if 

 the product of denaturation had not been fibrous. The firmness of 

 the coagulum must depend on the nature of the bonds that tie 

 the fibres together. 



A microscopical preparation of denatured protein generally 

 shows a network that appears to consist of interlacing fibres, fused 

 where they touch; there may or may not be swellings at these 

 points. The appearances are similar to those shown in fig. 4 (p. 

 41). The difference in scale between these microscopically visible 

 fibres on one hand and the polypeptide chains of the proteins on 

 the other is enormous; still, the latter underlie and make possible 

 the former. The visible network is an artifact, but it reminds us of 

 an important truth about the submicroscopical structure of pro- 

 teins. In protein gels we must imagine the polypeptide chains 



