CELLULAR BIOCHEMISTRY 



known for mammalian cells (anaerobic lactic acid production 

 equivalent to 25 per cent of dry weight of cells per hour at 37° 

 G.) ; their rate of lactic acid production remains linear for 

 many hours under anaerobic conditions, and there is little if 

 any lag period when glucose is introduced at zero time, and ini- 

 tial acid production is measured titrimetrically with a glass elec- 

 trode assembly. The final distribution of various sugars (some 

 of which were utilizable and some of which were not), based on 

 the aqueous phase of medium and cells, was close to unity, and 

 equilibrium was reached within one minute at 37 ° G. over a 

 wide range of sugar concentrations. Only by lowering the 

 temperature to 20 ° G. could the actual rate of penetration be 

 measured, which was found to be rapid enough, even at that 

 temperature, to sustain the observed rate of lactic acid formation 

 at 37 ° C. Furthermore, rates of lactic acid formation, deter- 

 mined at different concentrations of glucose, gave typical Line- 

 weaver-Burk plots and K^ values which were of the same order 

 as iT^ determined on hexokinase extracted from the cells (A'^ was 

 close to 1 X \0~^ M per liter in both cases). These and other 

 observations make it quite certain that the rate of sugar metab- 

 olism of these cells is determined by the rate of activity of the 

 intracellular enzymes and not by the rate of penetration of the 

 sugars through the cell membrane. 



It is possible to decrease the rate of lactic acid formation in 

 these cells by means of sugars which, while not being phos- 

 phorylated, have sufficient affinity for a common transport sys- 

 tem to act as competitive inhibitors. Thus galactose and 3- 

 methyl glucose inhibit the rate of lactic acid formation from glu- 

 cose, fructose, and mannose, and the degree of inhibition, at 

 different concentrations, is determined by the relative affinities 

 of these sugars for the transport system. 



The question of the rate of penetration of sugars into the 

 tissues of the intact animal is not so easily analyzed. In a study 

 carried out some twenty years ago on rats it was noted that the 

 ratio, fermentable tissue sugar/plasma sugar, was lowest for 

 skeletal muscle, intermediate for diaphragm, and highest for 



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