520 



PROTOPLASM 



The outstanding fact which particularly concerns us here is the 

 significant role that pigments have been shown to play as enzymes 

 (it was somewhat disturbing at the outset of these studies to call 

 a pigment an enzyme), and to this fact can be added the equally 

 important relationship shown to exist among the pigments, 

 respiratory enzymes, and vitamins. 



OTHER REGULATORY SUBSTANCES 



Needless to say, there are many regulatory substances con- 

 cerned with growth, health, and nutrition other than those men- 

 tioned in this chapter. Notable omissions are salts of the heavy 

 metals, of which there is only a trace in tissues; and certain car- 



Fig. 179. — A. Epithelium of normal adult mouse control. X950. B. 

 Basal membrane of epithelium of SH treated skin of adult mouse. X950. 

 {From F. Hammett.) 



bohydrates, fats, and proteins. Their function is referred to in 

 other chapters. But there is one among them that we may more 

 properly single out for discussion here; it is the growth-promoting 

 sulphhydryl group SH. 



Sulphhydryl, SH, has come into prominence of late through 

 the work of F. Hammett. It appears that a sulphur-containing 

 compound, attached to proteins or protein derivatives, augments 

 proliferative growth, but not sulphur in any form — not, for 

 example, as the disulphide but only as the sulphhydryl. Thio- 

 glycoUic acid and cysteine were used, and with them, as controls, 

 the corresponding compounds acetic acid and alanine, contain- 

 ing the identical elements in the same proportion but with sulphur 

 omitted. Sodium dithiodiglycollic acid and cystine were also 

 tried, and in every case the number of mitoses (dividing cells) 

 in the root tips of plants and the size of the colonies (number of 



