F. G. GUSTAFSON 619 



oxidase in potato and apple juice was inhibited, when the solution 

 had had hydrogen ion concentrations of 5.5 X 10~^ (pH 3.26) and 

 7.4 X 10"'* (pH 3.15) respectively, and that in both cases the opti- 

 mum is near neutrality. BunzelU^ found that potato juice oxidase 

 was inhibited by pH 3.55 to pH 3.70, and that the activity increased 

 as the pH number increased up to 7. The oxidase of the tulip tree 

 was inhibited at pH 2.30 to pH 2.80, and increased in activity up to 

 pH 6. He also found that the oxidase of magnolia was inhibited at 

 pH 2.45 to pH 3.05 and increased with increasing pH value up to 

 pH 5.70. 



In the experiments to be described PenicilUum chrysogenum was 

 used. This plant was chosen because under the conditions of these 

 experiments it does not produce sufhcient alkali or acid (other than 

 carbonic) to interfere with the results. Another reason which made 

 it very good material to work with was its hardiness (it was originally 

 found growing on strong formalin solutions). Aspergillus niger, 

 which has been used by the writer in previous work on respiration,^^ 

 produces a small amount of a non-volatile acid, for which reason it 

 was not well suited to the method employed in the present investi- 

 gation. 



The way in which the fungus was grown, as well as the procedure 

 employed in measuring the production of CO2, and the method em- 

 ployed in calculating the rate are described in a previous paper by 

 the writer. ^^ 



The pH of the solution in which the fungus was placed to deter- 

 mine the effect of hydrogen ion concentration was determined by 

 means of indicators, using buffer solutions for comparison. For 

 determining the concentrations around pH 1 to 4 tropeolin 00 and 

 methyl red were used, with buffers containing HCl and sodium 

 citrate. 1* For the concentrations at pH 7 to 9 phenolsulfonephthalein 

 and phenolphthalein were employed and borate buffers were used for 

 comparison. 



The normal respiration was measured in 0.5 per cent dextrose 

 solution in distilled water. To a similar solution sufficient acid or 

 alkali was added to get the desired hydrogen ion concentration. 



i^Bunzell, H. H., /. Biol. Chem., 1916-17, xxviii, 315. 

 13 Gustafson, F. G., /. Gen. Physiol., 1919-20, ii, 17. 

 "^^Sorenson, S. P. L., Blochem. Z., 1909, xxi, 131. 



