VARIOUS METABOLIC PATHWAYS 889 



substrate, and a suspension density of 4 X 10^ cells/ml. Both respiration 

 and luminescence of these cells are inhibited potently by Hg++, the latter 

 being somewhat more sensitive (Fig. 7-35). The rate of inhibition is much 

 more rapid than with most cellular activities, half maximal inhibition being 

 reached in about 30-40 sec (Fig. 7-36). It is not immediately evident why 

 the inhibition is more potent in the rate experiments than in the studies 

 on the effect of concentration. The effects of cell density on the inhibitions 

 are very striking (Fig. 7-37). The initial suspension here contained 6 X 10^ 

 cells/ml and this was diluted as indicated in the graph. At 0.001 niM no 



lOOi 



LUMINESCENCE 



015 



0.02 



025 



0.03 



Fig. 7-35. Effects of Hg++ on the respiration and luminescence of 

 Achromobacter. (From Houck, 1942.) 



inhibition is observed until sufficient dilution is made and at low cell den- 

 sities the inhibition is complete. These curves illustrate very well what es- 

 sentially must occur in all cell or tissue preparations, whatever activity is 

 measured. The meaninglessness of statements that such and such a con- 

 centration of mercurial produces a certain degree of inhibition of some cel- 

 lular process is all too clear; in this case with 0.001 mM Hg++, one might 

 observe any inhibition from to 100% depending on the cell density chosen. 

 Inhibition was determined with 0.001 mM Hg++ at three values of the pH, 

 and was greatest at 5.3 and 8.4, and least at 7.3 (one can estimate the mean 

 per cent inhibitions at 1 min to be 91%, 60%, and 86% at pH 5.3, 7.3, 

 and 8.4, respectively). The light intensity is much greater at pH 7.3 and 

 this may possibly be related to the rate of glucose uptake. The effects of 

 temperature have already been illustrated (Fig. 1-15-9) and discussed (page 



