BIOGRAPHICAL MEMORANDA 



dation of disaccharides by yeasts that cannot ferment these sugars, 

 were, as has been mentioned earlier, largely attempts to negate the 

 occurrence of such phenomena which were contrary to the expecta- 

 tions based on the unitary theory. Only at a later stage, when experi- 

 ments in his own laboratory had incontrovertibly shown the reality of 

 such occurrences, was the situation investigated in somewhat greater 

 detail. In the publication with Custers [1940] on the suitability of 

 disaccharides as respiration and assimilation substrates for glucose fer- 

 menting yeasts that do not ferment these sugars the problem is intro- 

 duced in a manner calculated to emphasize the difference with which 

 it had been treated by others who had recorded similar observations : 

 'It is true that these authors do not offer any special comments to their 

 results' (p. 123), suggesting that they may have been unaware of the 

 fundamental problem that was raised by such experimental findings. 



When they had established that certain yeasts can oxidize a number 

 of disaccharides which they cannot ferment, Kluyver and Custers 

 next attempted to detect the presence in such yeasts of enzymes that 

 can hydrolyze these sugars to their constituent hexose units. The 

 positive results of these experiments indicated that the inability of the 

 yeasts to ferment the disaccharides could not be attributed to the ab- 

 sence of appropriate hydrolases per se. This made it unnecessary to 

 ascribe the oxidation of such sugars to the operation of a mechanism 

 that did not involve a preliminary hydrolysis, sometimes postulated 

 and designated as a 'direct' oxidation of disaccharides. Since the hy- 

 drolases were present, the only way to explain the non-fermentability 

 of the disaccharides seemed to call for the assumption that 'under 

 anaerobic conditions these hydrolases are inactivated either complete- 

 ly, or at least to such an extent that the fermentability of the disac- 

 charide is not detected by the relatively insensitive routine methods 

 for the determination of this property' (p. 159). 



Two possible reasons for the inactivity of the hydrolytic enzymes 

 under anaerobic conditions were briefly discussed. The first one im- 

 plicated a differential permeability of the disaccharides in the pres- 

 ence and absence of oxygen. After reviewing the results obtained by 

 various investigators in studies on cell permeability under different 

 conditions it became clear that in some cases permeability may be 

 increased, in others decreased by anaerobiosis. This situation led to 

 the following argument: 



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