42 CYTOLOGICAL TECHNIQUE 



Polyglutamine is an artificial polypeptide consisting of nothing 

 but a group of glutamine molecules joined by peptide links. This 

 particular polypeptide, in striking contrast to polyglycine, poly- 

 glutamic acid, and silk fibroin, binds more formaldehyde than 

 any other macromolecule, so far as is known. "^ A glance at the 

 structural formulae shown here and on p. 19 suggests strongly 

 that the group that binds formaldehyde is the -NHo of poly- 

 glutamine. This conclusion is borne out by experiments made 

 with deaminized proteins; that is to say, with proteins in which 

 the -NH., groups of polyglutamine and other basic amino-acids 

 have been replaced by -OH. These deaminized proteins bind little 

 formaldehyde. 



Lysine is the most abundant amino-acid possessing the amino- 

 group, and is thought to be particularly important in fixation by 

 formaldehyde. The reaction might be as follows: 



-(CH2)4NH., 4- H,CO = -(CH.)iNH.H2COH 



lysine side- formaldehyde 



group 



The -OH group shown in this equation is reactive, and thus 

 a methylene bridge (-CHo-) can be formed between two protein 

 chains, each possessing an amino-group. 



-NH— CH2— NH- 



A methylene bridge 



One can readily imagine two lysine groups on neighbouring 

 protein chains being linked in this way, but analysis shows that 

 there is not a 1 :2 relation between the number of molecules of 

 formaldehyde taken up and the number of lysine groups linked to 

 them. It is not probable that two adjacent chains would have 

 many lysine radicles neatly arranged opposite one another, ready 

 to be linked by methylene bridges. Lysine may often in fact be 

 linked to glutamine, or may bind formaldehyde here and there 

 without the formation of a bridge. 



Bridge-formation will harden a protein gel, but formaldehyde, 

 as we have seen, is not a coagulant fixative, and it is evident that 

 many of the hydrophil groups retain their relation with water 

 molecules. It is a remarkable experience to transfer a living or 



