ENZYMES . 501 



SECTION 91. Enzymes. 



Plants frequently make use of enzymes or ferment substances capable of 

 inducing chemical metamorphoses of altogether disproportionate magnitude 

 relatively to the amount of ferment employed. Diastatic, inverting, pro- 

 teolytic (pepsin, trypsin, papain, papayotin), glucoside-splitting, and cytasic 

 ferments occur in many but not in all plants, and steapsins or fat-splitting 

 ferments are probably equally widely distributed. Rennet and urea ferments 

 have been detected in a few cases, and it is possible that many gum and 

 pectic ferments may also exist (Sect. 82). All the known enzymes appear 

 to be proteids 1 which, like toxalbumins, have powers altogether out of 

 proportion to their bulk. There can be no doubt that different forms 

 of diastase, invertin, &c., exist, and indeed each of the above terms 

 apparently includes a group of ferments varying in detail from one 

 another 2 , for certain diastatic extracts produce maltose, and others dextrose 

 when acting on starch. Similarly different proteolytic enzymes carry the 

 decomposition of proteids to varying degrees of disorganization, while 

 amygdalin may be decomposed with or without a production of prussic 

 acid, according to the ferment employed (Sect. 87). 



Each enzyme has a strictly limited sphere of action, which, however, 

 usually includes more than one substance : thus pepsin decomposes most 

 but not all proteids, while diastase not only hydrolyses starch but also 

 certain forms of cellulose, and a special invertin not only acts upon most 

 polysaccharides but also upon a few glucosides. It is possible that in 

 many cases the extracts used may contain a mixture of two ferments, but 

 the available experimental data suffice to prove that different varieties 

 of invertin, diastase, &c. do actually exist. It is not surprising to find that 

 a given ferment may only be able to decompose one of two stereo- isomeric 

 substances, and E. Fischer observed that when the sugar of a glucoside 

 was replaced by its optical antithesis, a previously active ferment was 

 unable to decompose the new glucoside. 



Many of the enzymes hitherto isolated are hydrolytic in character, 

 although a gas may be produced, as in the decomposition of urea by 

 urase (Sect. 102). Oxidizing ferments or oxidases also exist which, in 

 dead cells and perhaps in living ones also, may act as katalytic agents 

 by absorbing oxygen and transferring it to other substances (Sect. 101). 

 Similar katalytic actions may be exercised by acids as in the inversion 



1 A. Mayer, Die Lehre von den chemischen Fermenten, 1882, p. 19; Neumeister, Physiol. 

 Chemie, 1893, Bd. I, p. 81. A summary of the known ferments is given by Green, Annals of Botany, 

 1893, Vol. vn, p. 83. 



2 Cf. Beyerinck, Centralbl. f. Bact., 1895, Abth. ii, Bd. I, p. 229; E. Fischer, Ber. d. Chem. 

 Ges., 1894, p. 3481. 



