THE BIOCHEMISTRY OF RESPIRATION 253 



Recent research has shown that the process of hydrolysis 

 is by no means so simple as that indicated in the above equation. 

 Harden and Young l find that the hexose interacts with the 

 phosphates always present in yeast juice to form a hexose- 

 phosphate of the type CeH^O^PO^^, simultaneously with C0 2 

 and alcohol. Subsequently the hexose-phosphate splits up into 

 free hexose and phosphate. Also they find that zymase is quite 

 unable to ferment sugar unless a "co-enzyme" is present. 

 This co-enzyme is a thermostable substance which can be 

 removed from yeast juice by dialysis. It can be hydrolysed 

 by lipase and so appears to be an ester, but it has not been 

 isolated and its chemical nature is unknown. 



The conversion of hexose into alcohol and C0 2 is probably 

 not a direct one. Buchner and Meisenheimer 2 formerly thought 

 that lactic acid is an intermediate product in the decomposition, 

 but they have now abandoned this view, and believe that the 

 evidence points rather to dihydroxyacetone as the chief inter- 

 mediary. Another product appears to be glycerin aldehyde, and 

 this may be converted into methyl glyoxal, CH 3 — CO — CHO, 

 which in turn is split up into pyruvic acid and pyruvic alcohol. 

 The pyruvic acid, CH 3 — CO — COOH, splits up into C0 2 and 

 acetaldehyde, the acetaldehyde being reduced to the final 

 product, ethyl alcohol. 3 Traces of lactic acid are formed as a 

 by-reaction, perhaps by the hydration of some of the methyl 

 glyoxal, for Neuberg * has shown that minced liver and muscle 

 can effect this conversion. In any case the lactic acid appears 

 to be an end-product, and is not an intermediate step in the 

 decomposition. 



In addition to forming C0 2 and alcohol, yeast juice converts 

 about 5 per cent, of the sugar decomposed into glycerol. Also 

 small quantities of acetic acid, formic acid, acetaldehyde, and 

 formaldehyde are produced, but the amyl alcohols and succinic 

 acid liberated by the action of living yeast cells do not appear 

 in yeast-juice fermentation. Yeast juice is likewise capable of 

 liberating C0 2 from certain organic acids. For instance, it can 

 convert pyruvic acid into C0 2 and acetaldehyde, and as much 

 as 40 per cent, of the theoretical quantity of the aldehyde has 



1 Cf. Harden, Alcoholic Fermentation, London, 191 1. 

 - Buchner and Meisenheimer, Rer. 43, p. 1773, 19 10. 

 * Cf. Oppenheimer, Die Fermente, p. 696, 1913. 

 4 Neuberg, Biochem. Zeit. 49, p. 502, 191 3. 



