1 68 PHYSIOLOGY OF NUTRITION 



CH 2 OH; dihydroxyacetone is then changed by dihydroxyacetonase into alcohol 

 and carbon dioxide. The process of alcoholic fermentation may therefore be 

 represented in the following way: 



C 6 H 12 6 = 2С 3 Н 6 0з = 2C0H5OH + 2C0 2 



The parts of the plant that are rich in protoplasm usually contain appre- 

 ciable amounts of catalase, which splits hydrogen peroxide into molecular oxygen 

 and water. The physiological role of catalase is not well understood at present; 

 it is probably connected with anaerobic processes, to which the common reduc- 

 tion processes 1 seem also to be related. Whether the latter are caused by specific 

 enzymes (reductase, hydrogenase) is still undetermined. The process of reduc- 

 tion may be demonstrated if plant tissues are placed in a solution of methylene 

 blue or sodium selenite, in the absence of oxygen. Methylene blue is thus 

 bleached, while sodium selenite is decomposed with the formation of red metallic 

 selenium. While oxidase is to be conceived as a system consisting of peroxidase 

 with a peroxide-former (oxygenase), Bach 2 considers reductase as a combination 

 of an enzyme with a water-splitting substance. »' 



Only the most important of the enzymes thus far discovered have been de- 

 scribed in the preceding paragraphs, but it is probable that the living protoplasm 

 produces specific enzymes for most of the biochemical reactions. The same 

 organism may produce different enzymes according to the chemical nature of 

 the nutritive material at its disposal. Thus, Penicillium glaucum produces 

 saccharase when grown in a medium containing calcium lactate, casease when 

 cultivated in milk, and lipase when supplied with monobutyrin. 



Synthetic processes, as well as those of decomposition, can be brought about 

 by enzymes. Hill, 3 for example, has found that the inversion of maltose by mal- 

 tase is not complete, but proceeds to a definite equilibrium point as the velocity 

 of the process is reduced by the accumulation of glucose. From this it is at 

 least apparent that this is a reversible reaction. Hill proved that a concen- 

 trated glucose solution is actually transformed, in the presence of maltase, 

 into a maltose solution. It seems plausible to suppose, in the light of the studies 

 on this subject so far available, that enzymatic processes in general may be thus 

 reversible. 4 



It is now possible to produce death in plants without destroying the enzymes 

 of their tissues. Plants that have been so treated are not the same as those that 

 have been killed in such a way as to render their enzymes inactive. (Enzymes 



1 Ehrlich, Paul, Das Sauerstoff-Bedürfniss des Organismus. 167 p. Berlin, 1885. Palladin, W., 

 Beteiligung der Reduktase im Prozesse der Alkoholgärung. Zeitsch. physiol. Chem. 56: 81-88. 1908. 

 Zaleski, W., Ueber die Rolle der Reduktionsprozesse bei der Atmung der Pflanzen. (Vorläufige Mitteil- 

 ung.) Ber. Deutsch. Bot. Ges. 28: 319-329. 1910. [Apple.man, Charles O., Relation of oxidases and 

 catalase to respiration in plants. Amer. jour. bot. 5 : 223-233. 1916. (Other references are there given.)] 



2 Bach, A., Zur Kenntnis der Reduktionsfermente. I. Mitteilung. Ueber das Schardinger-Enzym 

 (Perhydridase). Biochem. Zeitsch. 31: 443-449. 191 1. 



3 Hill, Arthur Croft, Reversible zymohydrolysis. Jour. Chem. Soc. London 73: 634-658. 1898. 



4 Dietz, Wilhelm, Ueber eine umkehrbare Fermentreaktion im heterogenen System. Esterbildung und 

 Esterverseifung. Zeitsch. physiol. Chem. 52 : 279-325. 1907. Loeb, Jacques, The dynamics of living 

 matter. 233 p. New York, 1906. 



' The considerations of this paragraph receive more detailed attention in the following 

 chapter. — Ed. 



