MECHANISM OF METABOLISM 203 



with quartz sand. They will then reduce nitrates to nitrites, sulphur 

 to hydrogen sulphide. The decolorization of litmus, methylene 

 blue, indigo, and other organic dyes is due in microbial cultures to 

 enzymes which are almost exclusively endo-enzymes. 



Additional Remarks on the Relation of Cells and Enzymes 



Enzymes are produced only by living cells. After they are once 

 formed, they act like chemical compounds, independent of the cell 

 which produces them. Even the endo-enzymes follow only the law 'of 

 enzyme-action and are not influenced by the cell which contains them. 

 The enzymes are mostly influenced by their own products, and when 

 a certain yeast ceases to ferment sugar at the concentration of 8.5 

 per cent of alcohol, this means that the alcoholase of this yeast cannot 

 tolerate more than 8.5 per cent of alcohol. The inability of the' cell 

 to regulate enzymic action may account for the fact that often a 

 culture produces an amount of fermentation products sufficient to 

 kill all cells. This is observed in the lactic, acetic and alcoholic fer- 

 mentations, and, perhaps, occurs in many others. 



Probably, all cells produce several, enzymes. Microorganisms 

 feeding upon various foods must form various enzymes. Frequently 

 several enzymes are necessary for the decomposition of one com- 

 pound. Rhizopus oryzcB uses three enzymes in order to form alcohol 

 from starch, first the diastase to change starch to maltose, then 

 maltase to change maltose to dextrose and finally alcbholase 

 to change dextrose to alcohol and carbon dioxide. The number of 

 enzymes formed by certain microorganisms is surprising. Asper- 

 gillus mger has the reputation of forming almost all enzymes which 

 have ever been found in microorganisms. Penicillium camemberti 

 produces (after Dox) erepsin, nuclease, amidase, lipase, emulsin, 

 amylase, inulase, raffinase, invertase, maltase and lactase. It has 

 been believed for a long time that certain enzymes are regular products 

 of the cell while others are formed only if the substance upon which 

 they act is present. According to Dox's investigations with Peni- 

 cillium camemberti, there is no evidence that enzymes not normally 

 formed by the organism in demonstrable quantities can be developed 

 by special methods of nutrition. The addition of a particular 

 food compound does not develop an entirely new enzyme, but stimu- 



