PLANT HORMONES 



137 



and the peroxidases. Alloxazin or ribo- 

 flavin compounds occur as prosthetic 

 groups in another series of enzymes, some 

 of them differing from one another only 

 with respect to their protein parts. The 

 substrate of the ferment is determined by 

 the protein part, the general manner of ac- 

 tion by the prosthetic group. A flavin en- 

 zyme always carries hydrogen, and a hemin 

 ferment in most cases carries electrons, 

 whatever the substrate may be. It is ob- 

 vious that the interaction between the 

 prosthetic group and the protein part of 

 the enzymes is a question of the greatest 

 importance, and therefore I will explain 

 something about our present knowledge 

 of three different classes of enzymes with 

 respect to this particular point of view, 

 namely the proteolytic enzymes, the hemin 

 ferments, and the flavin enzymes. 



The hydrolytic enzymes have the ability 

 of loosening the linkages between carbon 

 and oxygen in the polysaccharides, or car- 

 bon and nitrogen in the peptide linkages. 

 Their special function is to break down the 

 foodstuffs in order to facilitate their absorp- 

 tion from the intestine and to make it pos- 

 sible for the cells to build up new substances. 

 These reactions do not deliver any energy. 

 The oxidation enzymes on the contrary 

 loosen the linkages between carbon and car- 

 bon, thus delivering the energy necessary 

 for life. 



Proteolytic Enzymes 



A great many proteolytic enzymes have 

 been crystallized — pepsin, pepsinogen, chy- 

 motrypsinogen, chymotrypsin, (3 - and y - 

 chymotrypsin, trypsinogen, trypsin inhibi- 

 tor, carboxypeptidase, ficase, and papain. 

 Of course further investigation was needed 

 to prove that the crystals really were the 

 enzymes themselves: there was a certain 

 possibility that the enzymes were only some 

 small part of the material, adsorbed to inac- 

 tive protein crystals. However, all attempts 

 to separate the enzyme activity from the 

 crystallized proteins have been unsuccess- 

 ful. Ultracentrifugation, electrophoresis, 

 and diffusion experiments showed, on the 

 whole, identity between protein and activ- 

 ity. The solubility in strong ammonium 



sulfate solutions, which is a very sensitive 

 test for the homogeneity of proteins, indi- 

 cated homogeneity for at least some of the 

 preparations. Irradiation with ultraviolet 

 light destroyed the activity in proportion 

 to the light absorption coefficient of the 

 proteins at different wave-lengths, and re- 

 crystallization showed no influence on the 

 absolute activity of the crystalline enzymes. 

 The most probable conclusion must be that 

 the crystallized proteins with hydrolytic 

 activity really are the enzymes themselves. 



The molecular weight of the enzymes is 

 high, for pepsin, 35,000. It seems unbe- 

 lievable to me that every atom of this big 

 molecule could be of equal importance, each 

 taking part directly in the hydrolysis of 

 the substrate. From analogy with the 

 oxidation enzymes one should expect even 

 the proteolytic enzymes to contain a pros- 

 thetic group, which goes through some cir- 

 cle of chemical reactions, i.e., taking up and 

 giving off hydrogen and hydroxyl ions. As 

 a matter of fact, we have no sufficient evi- 

 dence against such an assumption. I may 

 recall to your mind that no single high- 

 molecular protein hitherto has been com- 

 pletely analyzed with respect to its con- 

 tent of the various known amino acids. Let 

 us take an example : even if we could ac- 

 count for 99 per cent of the amino acid 

 composition of pepsin, which is certainly 

 far from the truth, there would still be 

 space for a prosthetic group of the molec- 

 ular weight of 350. The absence of par- 

 ticular light absorption in the ultraviolet, 

 besides the usual protein spectrum, cannot 

 show the absence of a prosthetic group. 

 Furthermore, all attempts to cleave the 

 proteolytic enzymes reversibly (in analogy 

 with the yellow oxidation enzymes) have 

 failed, which proves only that the linkage 

 between the prosthetic group and the pro- 

 tein may be stronger than the petide link- 

 ages ; and the same is true about one of the 

 oxidation enzymes, namely, cytochrome-c. 

 I will not especially argue in favor of some 

 prosthetic group being present in the hy- 

 drolytic enzymes.^ I only point out that 

 this possibility should still be kept in mind. 



1 The words ' ' enzyme ' ' and ' ' ferment ' ' for- 

 merly were used for two slightly different kinds of 



