CARBON METABOLISM 253 



Methylglyuxal hydrate, produced its in alcoholic fermenta- 

 tion (see Chapter XV), is partially converted into lactic 

 acid (type 3), partially broken down to give pjTuvic acid 

 and carbon dioxide and partially broken down to give 

 acetaldehyde and formic acid. The formic acid gives 

 hydrogen and carbon dioxide, as it does in the case of 

 Esch. coli. The pyruvic acid is decarboxylated (see 

 Chapter XV) to give acetaldeh^^de and carbon dioxide. 

 The molecules of acetaldehyde from this source and 

 directly from the methylglyoxal condense, under the 

 influence of the enzyme carboligase, to form acetoin. 

 The hydrogen evolved when methjdglyoxal hydrate 

 yields pyruvic acid is partly taken up in reducing some 

 acetaldehyde to alcohol and partly in reducing some of 

 the acetoin to 2 : 3-butylene glycol, which is almost 

 invariably found among the products of the Voges- 

 Proskauer positive organisms. The 2 : 1 ration of carbon 

 dioxide to hydrogen which is associated with the Voges- 

 Proskauer reaction follows from the mechanism suggested, 

 carbon dioxide arising from two sources and hydrogen 

 from one. The formation of acetoin is favoured by 

 conditions, such as aeration or the presence of other 

 hydrogen acceptors, which restrict the reduction of 

 acetaldehyde to alcohol. 



The formation of propionic acid from glycerol by 

 the propionic acid bacteria proceeds without any pro- 

 duction of gas. Propionaldehyde and pyruvic acid have 

 been detected in fermenting cultures. Wood and Work- 

 man suggest that the steps in the fermentation are : — 



- H3PO4 



CH2OH.CHOH.CH2OH — > CH2(O.P03H2).CHOH.CHoOH 



(glycerophosphate) 



-H3PO4 



— 2H 



CH2(O.P03H2)CHOH.CHO 



(methyl glyoxal) (phosphoglyceraldehyde) 



I + 2H 



\ — HoO + H2O 

 CH3.CHOH.CHO -"-> CH3CH2.CHO > CHj.CHo.COOH 



(lactic aldehyde) + 2H (propionaldehyde) — 2H (propionic acidj 



